1----------------------------------------------------------------------------- 2This file contains a concatenation of the PCRE man pages, converted to plain 3text format for ease of searching with a text editor, or for use on systems 4that do not have a man page processor. The small individual files that give 5synopses of each function in the library have not been included. Neither has 6the pcredemo program. There are separate text files for the pcregrep and 7pcretest commands. 8----------------------------------------------------------------------------- 9 10 11PCRE(3) PCRE(3) 12 13 14NAME 15 PCRE - Perl-compatible regular expressions 16 17 18INTRODUCTION 19 20 The PCRE library is a set of functions that implement regular expres- 21 sion pattern matching using the same syntax and semantics as Perl, with 22 just a few differences. Some features that appeared in Python and PCRE 23 before they appeared in Perl are also available using the Python syn- 24 tax, there is some support for one or two .NET and Oniguruma syntax 25 items, and there is an option for requesting some minor changes that 26 give better JavaScript compatibility. 27 28 Starting with release 8.30, it is possible to compile two separate PCRE 29 libraries: the original, which supports 8-bit character strings 30 (including UTF-8 strings), and a second library that supports 16-bit 31 character strings (including UTF-16 strings). The build process allows 32 either one or both to be built. The majority of the work to make this 33 possible was done by Zoltan Herczeg. 34 35 The two libraries contain identical sets of functions, except that the 36 names in the 16-bit library start with pcre16_ instead of pcre_. To 37 avoid over-complication and reduce the documentation maintenance load, 38 most of the documentation describes the 8-bit library, with the differ- 39 ences for the 16-bit library described separately in the pcre16 page. 40 References to functions or structures of the form pcre[16]_xxx should 41 be read as meaning "pcre_xxx when using the 8-bit library and 42 pcre16_xxx when using the 16-bit library". 43 44 The current implementation of PCRE corresponds approximately with Perl 45 5.12, including support for UTF-8/16 encoded strings and Unicode gen- 46 eral category properties. However, UTF-8/16 and Unicode support has to 47 be explicitly enabled; it is not the default. The Unicode tables corre- 48 spond to Unicode release 6.0.0. 49 50 In addition to the Perl-compatible matching function, PCRE contains an 51 alternative function that matches the same compiled patterns in a dif- 52 ferent way. In certain circumstances, the alternative function has some 53 advantages. For a discussion of the two matching algorithms, see the 54 pcrematching page. 55 56 PCRE is written in C and released as a C library. A number of people 57 have written wrappers and interfaces of various kinds. In particular, 58 Google Inc. have provided a comprehensive C++ wrapper for the 8-bit 59 library. This is now included as part of the PCRE distribution. The 60 pcrecpp page has details of this interface. Other people's contribu- 61 tions can be found in the Contrib directory at the primary FTP site, 62 which is: 63 64 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre 65 66 Details of exactly which Perl regular expression features are and are 67 not supported by PCRE are given in separate documents. See the pcrepat- 68 tern and pcrecompat pages. There is a syntax summary in the pcresyntax 69 page. 70 71 Some features of PCRE can be included, excluded, or changed when the 72 library is built. The pcre_config() function makes it possible for a 73 client to discover which features are available. The features them- 74 selves are described in the pcrebuild page. Documentation about build- 75 ing PCRE for various operating systems can be found in the README and 76 NON-UNIX-USE files in the source distribution. 77 78 The libraries contains a number of undocumented internal functions and 79 data tables that are used by more than one of the exported external 80 functions, but which are not intended for use by external callers. 81 Their names all begin with "_pcre_" or "_pcre16_", which hopefully will 82 not provoke any name clashes. In some environments, it is possible to 83 control which external symbols are exported when a shared library is 84 built, and in these cases the undocumented symbols are not exported. 85 86 87USER DOCUMENTATION 88 89 The user documentation for PCRE comprises a number of different sec- 90 tions. In the "man" format, each of these is a separate "man page". In 91 the HTML format, each is a separate page, linked from the index page. 92 In the plain text format, all the sections, except the pcredemo sec- 93 tion, are concatenated, for ease of searching. The sections are as fol- 94 lows: 95 96 pcre this document 97 pcre16 details of the 16-bit library 98 pcre-config show PCRE installation configuration information 99 pcreapi details of PCRE's native C API 100 pcrebuild options for building PCRE 101 pcrecallout details of the callout feature 102 pcrecompat discussion of Perl compatibility 103 pcrecpp details of the C++ wrapper for the 8-bit library 104 pcredemo a demonstration C program that uses PCRE 105 pcregrep description of the pcregrep command (8-bit only) 106 pcrejit discussion of the just-in-time optimization support 107 pcrelimits details of size and other limits 108 pcrematching discussion of the two matching algorithms 109 pcrepartial details of the partial matching facility 110 pcrepattern syntax and semantics of supported 111 regular expressions 112 pcreperform discussion of performance issues 113 pcreposix the POSIX-compatible C API for the 8-bit library 114 pcreprecompile details of saving and re-using precompiled patterns 115 pcresample discussion of the pcredemo program 116 pcrestack discussion of stack usage 117 pcresyntax quick syntax reference 118 pcretest description of the pcretest testing command 119 pcreunicode discussion of Unicode and UTF-8/16 support 120 121 In addition, in the "man" and HTML formats, there is a short page for 122 each 8-bit C library function, listing its arguments and results. 123 124 125AUTHOR 126 127 Philip Hazel 128 University Computing Service 129 Cambridge CB2 3QH, England. 130 131 Putting an actual email address here seems to have been a spam magnet, 132 so I've taken it away. If you want to email me, use my two initials, 133 followed by the two digits 10, at the domain cam.ac.uk. 134 135 136REVISION 137 138 Last updated: 10 January 2012 139 Copyright (c) 1997-2012 University of Cambridge. 140------------------------------------------------------------------------------ 141 142 143PCRE(3) PCRE(3) 144 145 146NAME 147 PCRE - Perl-compatible regular expressions 148 149 #include <pcre.h> 150 151 152PCRE 16-BIT API BASIC FUNCTIONS 153 154 pcre16 *pcre16_compile(PCRE_SPTR16 pattern, int options, 155 const char **errptr, int *erroffset, 156 const unsigned char *tableptr); 157 158 pcre16 *pcre16_compile2(PCRE_SPTR16 pattern, int options, 159 int *errorcodeptr, 160 const char **errptr, int *erroffset, 161 const unsigned char *tableptr); 162 163 pcre16_extra *pcre16_study(const pcre16 *code, int options, 164 const char **errptr); 165 166 void pcre16_free_study(pcre16_extra *extra); 167 168 int pcre16_exec(const pcre16 *code, const pcre16_extra *extra, 169 PCRE_SPTR16 subject, int length, int startoffset, 170 int options, int *ovector, int ovecsize); 171 172 int pcre16_dfa_exec(const pcre16 *code, const pcre16_extra *extra, 173 PCRE_SPTR16 subject, int length, int startoffset, 174 int options, int *ovector, int ovecsize, 175 int *workspace, int wscount); 176 177 178PCRE 16-BIT API STRING EXTRACTION FUNCTIONS 179 180 int pcre16_copy_named_substring(const pcre16 *code, 181 PCRE_SPTR16 subject, int *ovector, 182 int stringcount, PCRE_SPTR16 stringname, 183 PCRE_UCHAR16 *buffer, int buffersize); 184 185 int pcre16_copy_substring(PCRE_SPTR16 subject, int *ovector, 186 int stringcount, int stringnumber, PCRE_UCHAR16 *buffer, 187 int buffersize); 188 189 int pcre16_get_named_substring(const pcre16 *code, 190 PCRE_SPTR16 subject, int *ovector, 191 int stringcount, PCRE_SPTR16 stringname, 192 PCRE_SPTR16 *stringptr); 193 194 int pcre16_get_stringnumber(const pcre16 *code, 195 PCRE_SPTR16 name); 196 197 int pcre16_get_stringtable_entries(const pcre16 *code, 198 PCRE_SPTR16 name, PCRE_UCHAR16 **first, PCRE_UCHAR16 **last); 199 200 int pcre16_get_substring(PCRE_SPTR16 subject, int *ovector, 201 int stringcount, int stringnumber, 202 PCRE_SPTR16 *stringptr); 203 204 int pcre16_get_substring_list(PCRE_SPTR16 subject, 205 int *ovector, int stringcount, PCRE_SPTR16 **listptr); 206 207 void pcre16_free_substring(PCRE_SPTR16 stringptr); 208 209 void pcre16_free_substring_list(PCRE_SPTR16 *stringptr); 210 211 212PCRE 16-BIT API AUXILIARY FUNCTIONS 213 214 pcre16_jit_stack *pcre16_jit_stack_alloc(int startsize, int maxsize); 215 216 void pcre16_jit_stack_free(pcre16_jit_stack *stack); 217 218 void pcre16_assign_jit_stack(pcre16_extra *extra, 219 pcre16_jit_callback callback, void *data); 220 221 const unsigned char *pcre16_maketables(void); 222 223 int pcre16_fullinfo(const pcre16 *code, const pcre16_extra *extra, 224 int what, void *where); 225 226 int pcre16_refcount(pcre16 *code, int adjust); 227 228 int pcre16_config(int what, void *where); 229 230 const char *pcre16_version(void); 231 232 int pcre16_pattern_to_host_byte_order(pcre16 *code, 233 pcre16_extra *extra, const unsigned char *tables); 234 235 236PCRE 16-BIT API INDIRECTED FUNCTIONS 237 238 void *(*pcre16_malloc)(size_t); 239 240 void (*pcre16_free)(void *); 241 242 void *(*pcre16_stack_malloc)(size_t); 243 244 void (*pcre16_stack_free)(void *); 245 246 int (*pcre16_callout)(pcre16_callout_block *); 247 248 249PCRE 16-BIT API 16-BIT-ONLY FUNCTION 250 251 int pcre16_utf16_to_host_byte_order(PCRE_UCHAR16 *output, 252 PCRE_SPTR16 input, int length, int *byte_order, 253 int keep_boms); 254 255 256THE PCRE 16-BIT LIBRARY 257 258 Starting with release 8.30, it is possible to compile a PCRE library 259 that supports 16-bit character strings, including UTF-16 strings, as 260 well as or instead of the original 8-bit library. The majority of the 261 work to make this possible was done by Zoltan Herczeg. The two 262 libraries contain identical sets of functions, used in exactly the same 263 way. Only the names of the functions and the data types of their argu- 264 ments and results are different. To avoid over-complication and reduce 265 the documentation maintenance load, most of the PCRE documentation 266 describes the 8-bit library, with only occasional references to the 267 16-bit library. This page describes what is different when you use the 268 16-bit library. 269 270 WARNING: A single application can be linked with both libraries, but 271 you must take care when processing any particular pattern to use func- 272 tions from just one library. For example, if you want to study a pat- 273 tern that was compiled with pcre16_compile(), you must do so with 274 pcre16_study(), not pcre_study(), and you must free the study data with 275 pcre16_free_study(). 276 277 278THE HEADER FILE 279 280 There is only one header file, pcre.h. It contains prototypes for all 281 the functions in both libraries, as well as definitions of flags, 282 structures, error codes, etc. 283 284 285THE LIBRARY NAME 286 287 In Unix-like systems, the 16-bit library is called libpcre16, and can 288 normally be accesss by adding -lpcre16 to the command for linking an 289 application that uses PCRE. 290 291 292STRING TYPES 293 294 In the 8-bit library, strings are passed to PCRE library functions as 295 vectors of bytes with the C type "char *". In the 16-bit library, 296 strings are passed as vectors of unsigned 16-bit quantities. The macro 297 PCRE_UCHAR16 specifies an appropriate data type, and PCRE_SPTR16 is 298 defined as "const PCRE_UCHAR16 *". In very many environments, "short 299 int" is a 16-bit data type. When PCRE is built, it defines PCRE_UCHAR16 300 as "short int", but checks that it really is a 16-bit data type. If it 301 is not, the build fails with an error message telling the maintainer to 302 modify the definition appropriately. 303 304 305STRUCTURE TYPES 306 307 The types of the opaque structures that are used for compiled 16-bit 308 patterns and JIT stacks are pcre16 and pcre16_jit_stack respectively. 309 The type of the user-accessible structure that is returned by 310 pcre16_study() is pcre16_extra, and the type of the structure that is 311 used for passing data to a callout function is pcre16_callout_block. 312 These structures contain the same fields, with the same names, as their 313 8-bit counterparts. The only difference is that pointers to character 314 strings are 16-bit instead of 8-bit types. 315 316 31716-BIT FUNCTIONS 318 319 For every function in the 8-bit library there is a corresponding func- 320 tion in the 16-bit library with a name that starts with pcre16_ instead 321 of pcre_. The prototypes are listed above. In addition, there is one 322 extra function, pcre16_utf16_to_host_byte_order(). This is a utility 323 function that converts a UTF-16 character string to host byte order if 324 necessary. The other 16-bit functions expect the strings they are 325 passed to be in host byte order. 326 327 The input and output arguments of pcre16_utf16_to_host_byte_order() may 328 point to the same address, that is, conversion in place is supported. 329 The output buffer must be at least as long as the input. 330 331 The length argument specifies the number of 16-bit data units in the 332 input string; a negative value specifies a zero-terminated string. 333 334 If byte_order is NULL, it is assumed that the string starts off in host 335 byte order. This may be changed by byte-order marks (BOMs) anywhere in 336 the string (commonly as the first character). 337 338 If byte_order is not NULL, a non-zero value of the integer to which it 339 points means that the input starts off in host byte order, otherwise 340 the opposite order is assumed. Again, BOMs in the string can change 341 this. The final byte order is passed back at the end of processing. 342 343 If keep_boms is not zero, byte-order mark characters (0xfeff) are 344 copied into the output string. Otherwise they are discarded. 345 346 The result of the function is the number of 16-bit units placed into 347 the output buffer, including the zero terminator if the string was 348 zero-terminated. 349 350 351SUBJECT STRING OFFSETS 352 353 The offsets within subject strings that are returned by the matching 354 functions are in 16-bit units rather than bytes. 355 356 357NAMED SUBPATTERNS 358 359 The name-to-number translation table that is maintained for named sub- 360 patterns uses 16-bit characters. The pcre16_get_stringtable_entries() 361 function returns the length of each entry in the table as the number of 362 16-bit data units. 363 364 365OPTION NAMES 366 367 There are two new general option names, PCRE_UTF16 and 368 PCRE_NO_UTF16_CHECK, which correspond to PCRE_UTF8 and 369 PCRE_NO_UTF8_CHECK in the 8-bit library. In fact, these new options 370 define the same bits in the options word. There is a discussion about 371 the validity of UTF-16 strings in the pcreunicode page. 372 373 For the pcre16_config() function there is an option PCRE_CONFIG_UTF16 374 that returns 1 if UTF-16 support is configured, otherwise 0. If this 375 option is given to pcre_config(), or if the PCRE_CONFIG_UTF8 option is 376 given to pcre16_config(), the result is the PCRE_ERROR_BADOPTION error. 377 378 379CHARACTER CODES 380 381 In 16-bit mode, when PCRE_UTF16 is not set, character values are 382 treated in the same way as in 8-bit, non UTF-8 mode, except, of course, 383 that they can range from 0 to 0xffff instead of 0 to 0xff. Character 384 types for characters less than 0xff can therefore be influenced by the 385 locale in the same way as before. Characters greater than 0xff have 386 only one case, and no "type" (such as letter or digit). 387 388 In UTF-16 mode, the character code is Unicode, in the range 0 to 389 0x10ffff, with the exception of values in the range 0xd800 to 0xdfff 390 because those are "surrogate" values that are used in pairs to encode 391 values greater than 0xffff. 392 393 A UTF-16 string can indicate its endianness by special code knows as a 394 byte-order mark (BOM). The PCRE functions do not handle this, expecting 395 strings to be in host byte order. A utility function called 396 pcre16_utf16_to_host_byte_order() is provided to help with this (see 397 above). 398 399 400ERROR NAMES 401 402 The errors PCRE_ERROR_BADUTF16_OFFSET and PCRE_ERROR_SHORTUTF16 corre- 403 spond to their 8-bit counterparts. The error PCRE_ERROR_BADMODE is 404 given when a compiled pattern is passed to a function that processes 405 patterns in the other mode, for example, if a pattern compiled with 406 pcre_compile() is passed to pcre16_exec(). 407 408 There are new error codes whose names begin with PCRE_UTF16_ERR for 409 invalid UTF-16 strings, corresponding to the PCRE_UTF8_ERR codes for 410 UTF-8 strings that are described in the section entitled "Reason codes 411 for invalid UTF-8 strings" in the main pcreapi page. The UTF-16 errors 412 are: 413 414 PCRE_UTF16_ERR1 Missing low surrogate at end of string 415 PCRE_UTF16_ERR2 Invalid low surrogate follows high surrogate 416 PCRE_UTF16_ERR3 Isolated low surrogate 417 PCRE_UTF16_ERR4 Invalid character 0xfffe 418 419 420ERROR TEXTS 421 422 If there is an error while compiling a pattern, the error text that is 423 passed back by pcre16_compile() or pcre16_compile2() is still an 8-bit 424 character string, zero-terminated. 425 426 427CALLOUTS 428 429 The subject and mark fields in the callout block that is passed to a 430 callout function point to 16-bit vectors. 431 432 433TESTING 434 435 The pcretest program continues to operate with 8-bit input and output 436 files, but it can be used for testing the 16-bit library. If it is run 437 with the command line option -16, patterns and subject strings are con- 438 verted from 8-bit to 16-bit before being passed to PCRE, and the 16-bit 439 library functions are used instead of the 8-bit ones. Returned 16-bit 440 strings are converted to 8-bit for output. If the 8-bit library was not 441 compiled, pcretest defaults to 16-bit and the -16 option is ignored. 442 443 When PCRE is being built, the RunTest script that is called by "make 444 check" uses the pcretest -C option to discover which of the 8-bit and 445 16-bit libraries has been built, and runs the tests appropriately. 446 447 448NOT SUPPORTED IN 16-BIT MODE 449 450 Not all the features of the 8-bit library are available with the 16-bit 451 library. The C++ and POSIX wrapper functions support only the 8-bit 452 library, and the pcregrep program is at present 8-bit only. 453 454 455AUTHOR 456 457 Philip Hazel 458 University Computing Service 459 Cambridge CB2 3QH, England. 460 461 462REVISION 463 464 Last updated: 14 April 2012 465 Copyright (c) 1997-2012 University of Cambridge. 466------------------------------------------------------------------------------ 467 468 469PCREBUILD(3) PCREBUILD(3) 470 471 472NAME 473 PCRE - Perl-compatible regular expressions 474 475 476PCRE BUILD-TIME OPTIONS 477 478 This document describes the optional features of PCRE that can be 479 selected when the library is compiled. It assumes use of the configure 480 script, where the optional features are selected or deselected by pro- 481 viding options to configure before running the make command. However, 482 the same options can be selected in both Unix-like and non-Unix-like 483 environments using the GUI facility of cmake-gui if you are using CMake 484 instead of configure to build PCRE. 485 486 There is a lot more information about building PCRE in non-Unix-like 487 environments in the file called NON_UNIX_USE, which is part of the PCRE 488 distribution. You should consult this file as well as the README file 489 if you are building in a non-Unix-like environment. 490 491 The complete list of options for configure (which includes the standard 492 ones such as the selection of the installation directory) can be 493 obtained by running 494 495 ./configure --help 496 497 The following sections include descriptions of options whose names 498 begin with --enable or --disable. These settings specify changes to the 499 defaults for the configure command. Because of the way that configure 500 works, --enable and --disable always come in pairs, so the complemen- 501 tary option always exists as well, but as it specifies the default, it 502 is not described. 503 504 505BUILDING 8-BIT and 16-BIT LIBRARIES 506 507 By default, a library called libpcre is built, containing functions 508 that take string arguments contained in vectors of bytes, either as 509 single-byte characters, or interpreted as UTF-8 strings. You can also 510 build a separate library, called libpcre16, in which strings are con- 511 tained in vectors of 16-bit data units and interpreted either as sin- 512 gle-unit characters or UTF-16 strings, by adding 513 514 --enable-pcre16 515 516 to the configure command. If you do not want the 8-bit library, add 517 518 --disable-pcre8 519 520 as well. At least one of the two libraries must be built. Note that the 521 C++ and POSIX wrappers are for the 8-bit library only, and that pcre- 522 grep is an 8-bit program. None of these are built if you select only 523 the 16-bit library. 524 525 526BUILDING SHARED AND STATIC LIBRARIES 527 528 The PCRE building process uses libtool to build both shared and static 529 Unix libraries by default. You can suppress one of these by adding one 530 of 531 532 --disable-shared 533 --disable-static 534 535 to the configure command, as required. 536 537 538C++ SUPPORT 539 540 By default, if the 8-bit library is being built, the configure script 541 will search for a C++ compiler and C++ header files. If it finds them, 542 it automatically builds the C++ wrapper library (which supports only 543 8-bit strings). You can disable this by adding 544 545 --disable-cpp 546 547 to the configure command. 548 549 550UTF-8 and UTF-16 SUPPORT 551 552 To build PCRE with support for UTF Unicode character strings, add 553 554 --enable-utf 555 556 to the configure command. This setting applies to both libraries, 557 adding support for UTF-8 to the 8-bit library and support for UTF-16 to 558 the 16-bit library. There are no separate options for enabling UTF-8 559 and UTF-16 independently because that would allow ridiculous settings 560 such as requesting UTF-16 support while building only the 8-bit 561 library. It is not possible to build one library with UTF support and 562 the other without in the same configuration. (For backwards compatibil- 563 ity, --enable-utf8 is a synonym of --enable-utf.) 564 565 Of itself, this setting does not make PCRE treat strings as UTF-8 or 566 UTF-16. As well as compiling PCRE with this option, you also have have 567 to set the PCRE_UTF8 or PCRE_UTF16 option when you call one of the pat- 568 tern compiling functions. 569 570 If you set --enable-utf when compiling in an EBCDIC environment, PCRE 571 expects its input to be either ASCII or UTF-8 (depending on the run- 572 time option). It is not possible to support both EBCDIC and UTF-8 codes 573 in the same version of the library. Consequently, --enable-utf and 574 --enable-ebcdic are mutually exclusive. 575 576 577UNICODE CHARACTER PROPERTY SUPPORT 578 579 UTF support allows the libraries to process character codepoints up to 580 0x10ffff in the strings that they handle. On its own, however, it does 581 not provide any facilities for accessing the properties of such charac- 582 ters. If you want to be able to use the pattern escapes \P, \p, and \X, 583 which refer to Unicode character properties, you must add 584 585 --enable-unicode-properties 586 587 to the configure command. This implies UTF support, even if you have 588 not explicitly requested it. 589 590 Including Unicode property support adds around 30K of tables to the 591 PCRE library. Only the general category properties such as Lu and Nd 592 are supported. Details are given in the pcrepattern documentation. 593 594 595JUST-IN-TIME COMPILER SUPPORT 596 597 Just-in-time compiler support is included in the build by specifying 598 599 --enable-jit 600 601 This support is available only for certain hardware architectures. If 602 this option is set for an unsupported architecture, a compile time 603 error occurs. See the pcrejit documentation for a discussion of JIT 604 usage. When JIT support is enabled, pcregrep automatically makes use of 605 it, unless you add 606 607 --disable-pcregrep-jit 608 609 to the "configure" command. 610 611 612CODE VALUE OF NEWLINE 613 614 By default, PCRE interprets the linefeed (LF) character as indicating 615 the end of a line. This is the normal newline character on Unix-like 616 systems. You can compile PCRE to use carriage return (CR) instead, by 617 adding 618 619 --enable-newline-is-cr 620 621 to the configure command. There is also a --enable-newline-is-lf 622 option, which explicitly specifies linefeed as the newline character. 623 624 Alternatively, you can specify that line endings are to be indicated by 625 the two character sequence CRLF. If you want this, add 626 627 --enable-newline-is-crlf 628 629 to the configure command. There is a fourth option, specified by 630 631 --enable-newline-is-anycrlf 632 633 which causes PCRE to recognize any of the three sequences CR, LF, or 634 CRLF as indicating a line ending. Finally, a fifth option, specified by 635 636 --enable-newline-is-any 637 638 causes PCRE to recognize any Unicode newline sequence. 639 640 Whatever line ending convention is selected when PCRE is built can be 641 overridden when the library functions are called. At build time it is 642 conventional to use the standard for your operating system. 643 644 645WHAT \R MATCHES 646 647 By default, the sequence \R in a pattern matches any Unicode newline 648 sequence, whatever has been selected as the line ending sequence. If 649 you specify 650 651 --enable-bsr-anycrlf 652 653 the default is changed so that \R matches only CR, LF, or CRLF. What- 654 ever is selected when PCRE is built can be overridden when the library 655 functions are called. 656 657 658POSIX MALLOC USAGE 659 660 When the 8-bit library is called through the POSIX interface (see the 661 pcreposix documentation), additional working storage is required for 662 holding the pointers to capturing substrings, because PCRE requires 663 three integers per substring, whereas the POSIX interface provides only 664 two. If the number of expected substrings is small, the wrapper func- 665 tion uses space on the stack, because this is faster than using mal- 666 loc() for each call. The default threshold above which the stack is no 667 longer used is 10; it can be changed by adding a setting such as 668 669 --with-posix-malloc-threshold=20 670 671 to the configure command. 672 673 674HANDLING VERY LARGE PATTERNS 675 676 Within a compiled pattern, offset values are used to point from one 677 part to another (for example, from an opening parenthesis to an alter- 678 nation metacharacter). By default, two-byte values are used for these 679 offsets, leading to a maximum size for a compiled pattern of around 680 64K. This is sufficient to handle all but the most gigantic patterns. 681 Nevertheless, some people do want to process truly enormous patterns, 682 so it is possible to compile PCRE to use three-byte or four-byte off- 683 sets by adding a setting such as 684 685 --with-link-size=3 686 687 to the configure command. The value given must be 2, 3, or 4. For the 688 16-bit library, a value of 3 is rounded up to 4. Using longer offsets 689 slows down the operation of PCRE because it has to load additional data 690 when handling them. 691 692 693AVOIDING EXCESSIVE STACK USAGE 694 695 When matching with the pcre_exec() function, PCRE implements backtrack- 696 ing by making recursive calls to an internal function called match(). 697 In environments where the size of the stack is limited, this can se- 698 verely limit PCRE's operation. (The Unix environment does not usually 699 suffer from this problem, but it may sometimes be necessary to increase 700 the maximum stack size. There is a discussion in the pcrestack docu- 701 mentation.) An alternative approach to recursion that uses memory from 702 the heap to remember data, instead of using recursive function calls, 703 has been implemented to work round the problem of limited stack size. 704 If you want to build a version of PCRE that works this way, add 705 706 --disable-stack-for-recursion 707 708 to the configure command. With this configuration, PCRE will use the 709 pcre_stack_malloc and pcre_stack_free variables to call memory manage- 710 ment functions. By default these point to malloc() and free(), but you 711 can replace the pointers so that your own functions are used instead. 712 713 Separate functions are provided rather than using pcre_malloc and 714 pcre_free because the usage is very predictable: the block sizes 715 requested are always the same, and the blocks are always freed in 716 reverse order. A calling program might be able to implement optimized 717 functions that perform better than malloc() and free(). PCRE runs 718 noticeably more slowly when built in this way. This option affects only 719 the pcre_exec() function; it is not relevant for pcre_dfa_exec(). 720 721 722LIMITING PCRE RESOURCE USAGE 723 724 Internally, PCRE has a function called match(), which it calls repeat- 725 edly (sometimes recursively) when matching a pattern with the 726 pcre_exec() function. By controlling the maximum number of times this 727 function may be called during a single matching operation, a limit can 728 be placed on the resources used by a single call to pcre_exec(). The 729 limit can be changed at run time, as described in the pcreapi documen- 730 tation. The default is 10 million, but this can be changed by adding a 731 setting such as 732 733 --with-match-limit=500000 734 735 to the configure command. This setting has no effect on the 736 pcre_dfa_exec() matching function. 737 738 In some environments it is desirable to limit the depth of recursive 739 calls of match() more strictly than the total number of calls, in order 740 to restrict the maximum amount of stack (or heap, if --disable-stack- 741 for-recursion is specified) that is used. A second limit controls this; 742 it defaults to the value that is set for --with-match-limit, which 743 imposes no additional constraints. However, you can set a lower limit 744 by adding, for example, 745 746 --with-match-limit-recursion=10000 747 748 to the configure command. This value can also be overridden at run 749 time. 750 751 752CREATING CHARACTER TABLES AT BUILD TIME 753 754 PCRE uses fixed tables for processing characters whose code values are 755 less than 256. By default, PCRE is built with a set of tables that are 756 distributed in the file pcre_chartables.c.dist. These tables are for 757 ASCII codes only. If you add 758 759 --enable-rebuild-chartables 760 761 to the configure command, the distributed tables are no longer used. 762 Instead, a program called dftables is compiled and run. This outputs 763 the source for new set of tables, created in the default locale of your 764 C run-time system. (This method of replacing the tables does not work 765 if you are cross compiling, because dftables is run on the local host. 766 If you need to create alternative tables when cross compiling, you will 767 have to do so "by hand".) 768 769 770USING EBCDIC CODE 771 772 PCRE assumes by default that it will run in an environment where the 773 character code is ASCII (or Unicode, which is a superset of ASCII). 774 This is the case for most computer operating systems. PCRE can, how- 775 ever, be compiled to run in an EBCDIC environment by adding 776 777 --enable-ebcdic 778 779 to the configure command. This setting implies --enable-rebuild-charta- 780 bles. You should only use it if you know that you are in an EBCDIC 781 environment (for example, an IBM mainframe operating system). The 782 --enable-ebcdic option is incompatible with --enable-utf. 783 784 785PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT 786 787 By default, pcregrep reads all files as plain text. You can build it so 788 that it recognizes files whose names end in .gz or .bz2, and reads them 789 with libz or libbz2, respectively, by adding one or both of 790 791 --enable-pcregrep-libz 792 --enable-pcregrep-libbz2 793 794 to the configure command. These options naturally require that the rel- 795 evant libraries are installed on your system. Configuration will fail 796 if they are not. 797 798 799PCREGREP BUFFER SIZE 800 801 pcregrep uses an internal buffer to hold a "window" on the file it is 802 scanning, in order to be able to output "before" and "after" lines when 803 it finds a match. The size of the buffer is controlled by a parameter 804 whose default value is 20K. The buffer itself is three times this size, 805 but because of the way it is used for holding "before" lines, the long- 806 est line that is guaranteed to be processable is the parameter size. 807 You can change the default parameter value by adding, for example, 808 809 --with-pcregrep-bufsize=50K 810 811 to the configure command. The caller of pcregrep can, however, override 812 this value by specifying a run-time option. 813 814 815PCRETEST OPTION FOR LIBREADLINE SUPPORT 816 817 If you add 818 819 --enable-pcretest-libreadline 820 821 to the configure command, pcretest is linked with the libreadline 822 library, and when its input is from a terminal, it reads it using the 823 readline() function. This provides line-editing and history facilities. 824 Note that libreadline is GPL-licensed, so if you distribute a binary of 825 pcretest linked in this way, there may be licensing issues. 826 827 Setting this option causes the -lreadline option to be added to the 828 pcretest build. In many operating environments with a sytem-installed 829 libreadline this is sufficient. However, in some environments (e.g. if 830 an unmodified distribution version of readline is in use), some extra 831 configuration may be necessary. The INSTALL file for libreadline says 832 this: 833 834 "Readline uses the termcap functions, but does not link with the 835 termcap or curses library itself, allowing applications which link 836 with readline the to choose an appropriate library." 837 838 If your environment has not been set up so that an appropriate library 839 is automatically included, you may need to add something like 840 841 LIBS="-ncurses" 842 843 immediately before the configure command. 844 845 846SEE ALSO 847 848 pcreapi(3), pcre16, pcre_config(3). 849 850 851AUTHOR 852 853 Philip Hazel 854 University Computing Service 855 Cambridge CB2 3QH, England. 856 857 858REVISION 859 860 Last updated: 07 January 2012 861 Copyright (c) 1997-2012 University of Cambridge. 862------------------------------------------------------------------------------ 863 864 865PCREMATCHING(3) PCREMATCHING(3) 866 867 868NAME 869 PCRE - Perl-compatible regular expressions 870 871 872PCRE MATCHING ALGORITHMS 873 874 This document describes the two different algorithms that are available 875 in PCRE for matching a compiled regular expression against a given sub- 876 ject string. The "standard" algorithm is the one provided by the 877 pcre_exec() and pcre16_exec() functions. These work in the same was as 878 Perl's matching function, and provide a Perl-compatible matching opera- 879 tion. The just-in-time (JIT) optimization that is described in the 880 pcrejit documentation is compatible with these functions. 881 882 An alternative algorithm is provided by the pcre_dfa_exec() and 883 pcre16_dfa_exec() functions; they operate in a different way, and are 884 not Perl-compatible. This alternative has advantages and disadvantages 885 compared with the standard algorithm, and these are described below. 886 887 When there is only one possible way in which a given subject string can 888 match a pattern, the two algorithms give the same answer. A difference 889 arises, however, when there are multiple possibilities. For example, if 890 the pattern 891 892 ^<.*> 893 894 is matched against the string 895 896 <something> <something else> <something further> 897 898 there are three possible answers. The standard algorithm finds only one 899 of them, whereas the alternative algorithm finds all three. 900 901 902REGULAR EXPRESSIONS AS TREES 903 904 The set of strings that are matched by a regular expression can be rep- 905 resented as a tree structure. An unlimited repetition in the pattern 906 makes the tree of infinite size, but it is still a tree. Matching the 907 pattern to a given subject string (from a given starting point) can be 908 thought of as a search of the tree. There are two ways to search a 909 tree: depth-first and breadth-first, and these correspond to the two 910 matching algorithms provided by PCRE. 911 912 913THE STANDARD MATCHING ALGORITHM 914 915 In the terminology of Jeffrey Friedl's book "Mastering Regular Expres- 916 sions", the standard algorithm is an "NFA algorithm". It conducts a 917 depth-first search of the pattern tree. That is, it proceeds along a 918 single path through the tree, checking that the subject matches what is 919 required. When there is a mismatch, the algorithm tries any alterna- 920 tives at the current point, and if they all fail, it backs up to the 921 previous branch point in the tree, and tries the next alternative 922 branch at that level. This often involves backing up (moving to the 923 left) in the subject string as well. The order in which repetition 924 branches are tried is controlled by the greedy or ungreedy nature of 925 the quantifier. 926 927 If a leaf node is reached, a matching string has been found, and at 928 that point the algorithm stops. Thus, if there is more than one possi- 929 ble match, this algorithm returns the first one that it finds. Whether 930 this is the shortest, the longest, or some intermediate length depends 931 on the way the greedy and ungreedy repetition quantifiers are specified 932 in the pattern. 933 934 Because it ends up with a single path through the tree, it is rela- 935 tively straightforward for this algorithm to keep track of the sub- 936 strings that are matched by portions of the pattern in parentheses. 937 This provides support for capturing parentheses and back references. 938 939 940THE ALTERNATIVE MATCHING ALGORITHM 941 942 This algorithm conducts a breadth-first search of the tree. Starting 943 from the first matching point in the subject, it scans the subject 944 string from left to right, once, character by character, and as it does 945 this, it remembers all the paths through the tree that represent valid 946 matches. In Friedl's terminology, this is a kind of "DFA algorithm", 947 though it is not implemented as a traditional finite state machine (it 948 keeps multiple states active simultaneously). 949 950 Although the general principle of this matching algorithm is that it 951 scans the subject string only once, without backtracking, there is one 952 exception: when a lookaround assertion is encountered, the characters 953 following or preceding the current point have to be independently 954 inspected. 955 956 The scan continues until either the end of the subject is reached, or 957 there are no more unterminated paths. At this point, terminated paths 958 represent the different matching possibilities (if there are none, the 959 match has failed). Thus, if there is more than one possible match, 960 this algorithm finds all of them, and in particular, it finds the long- 961 est. The matches are returned in decreasing order of length. There is 962 an option to stop the algorithm after the first match (which is neces- 963 sarily the shortest) is found. 964 965 Note that all the matches that are found start at the same point in the 966 subject. If the pattern 967 968 cat(er(pillar)?)? 969 970 is matched against the string "the caterpillar catchment", the result 971 will be the three strings "caterpillar", "cater", and "cat" that start 972 at the fifth character of the subject. The algorithm does not automati- 973 cally move on to find matches that start at later positions. 974 975 There are a number of features of PCRE regular expressions that are not 976 supported by the alternative matching algorithm. They are as follows: 977 978 1. Because the algorithm finds all possible matches, the greedy or 979 ungreedy nature of repetition quantifiers is not relevant. Greedy and 980 ungreedy quantifiers are treated in exactly the same way. However, pos- 981 sessive quantifiers can make a difference when what follows could also 982 match what is quantified, for example in a pattern like this: 983 984 ^a++\w! 985 986 This pattern matches "aaab!" but not "aaa!", which would be matched by 987 a non-possessive quantifier. Similarly, if an atomic group is present, 988 it is matched as if it were a standalone pattern at the current point, 989 and the longest match is then "locked in" for the rest of the overall 990 pattern. 991 992 2. When dealing with multiple paths through the tree simultaneously, it 993 is not straightforward to keep track of captured substrings for the 994 different matching possibilities, and PCRE's implementation of this 995 algorithm does not attempt to do this. This means that no captured sub- 996 strings are available. 997 998 3. Because no substrings are captured, back references within the pat- 999 tern are not supported, and cause errors if encountered. 1000 1001 4. For the same reason, conditional expressions that use a backrefer- 1002 ence as the condition or test for a specific group recursion are not 1003 supported. 1004 1005 5. Because many paths through the tree may be active, the \K escape 1006 sequence, which resets the start of the match when encountered (but may 1007 be on some paths and not on others), is not supported. It causes an 1008 error if encountered. 1009 1010 6. Callouts are supported, but the value of the capture_top field is 1011 always 1, and the value of the capture_last field is always -1. 1012 1013 7. The \C escape sequence, which (in the standard algorithm) always 1014 matches a single data unit, even in UTF-8 or UTF-16 modes, is not sup- 1015 ported in these modes, because the alternative algorithm moves through 1016 the subject string one character (not data unit) at a time, for all 1017 active paths through the tree. 1018 1019 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) 1020 are not supported. (*FAIL) is supported, and behaves like a failing 1021 negative assertion. 1022 1023 1024ADVANTAGES OF THE ALTERNATIVE ALGORITHM 1025 1026 Using the alternative matching algorithm provides the following advan- 1027 tages: 1028 1029 1. All possible matches (at a single point in the subject) are automat- 1030 ically found, and in particular, the longest match is found. To find 1031 more than one match using the standard algorithm, you have to do kludgy 1032 things with callouts. 1033 1034 2. Because the alternative algorithm scans the subject string just 1035 once, and never needs to backtrack (except for lookbehinds), it is pos- 1036 sible to pass very long subject strings to the matching function in 1037 several pieces, checking for partial matching each time. Although it is 1038 possible to do multi-segment matching using the standard algorithm by 1039 retaining partially matched substrings, it is more complicated. The 1040 pcrepartial documentation gives details of partial matching and dis- 1041 cusses multi-segment matching. 1042 1043 1044DISADVANTAGES OF THE ALTERNATIVE ALGORITHM 1045 1046 The alternative algorithm suffers from a number of disadvantages: 1047 1048 1. It is substantially slower than the standard algorithm. This is 1049 partly because it has to search for all possible matches, but is also 1050 because it is less susceptible to optimization. 1051 1052 2. Capturing parentheses and back references are not supported. 1053 1054 3. Although atomic groups are supported, their use does not provide the 1055 performance advantage that it does for the standard algorithm. 1056 1057 1058AUTHOR 1059 1060 Philip Hazel 1061 University Computing Service 1062 Cambridge CB2 3QH, England. 1063 1064 1065REVISION 1066 1067 Last updated: 08 January 2012 1068 Copyright (c) 1997-2012 University of Cambridge. 1069------------------------------------------------------------------------------ 1070 1071 1072PCREAPI(3) PCREAPI(3) 1073 1074 1075NAME 1076 PCRE - Perl-compatible regular expressions 1077 1078 #include <pcre.h> 1079 1080 1081PCRE NATIVE API BASIC FUNCTIONS 1082 1083 pcre *pcre_compile(const char *pattern, int options, 1084 const char **errptr, int *erroffset, 1085 const unsigned char *tableptr); 1086 1087 pcre *pcre_compile2(const char *pattern, int options, 1088 int *errorcodeptr, 1089 const char **errptr, int *erroffset, 1090 const unsigned char *tableptr); 1091 1092 pcre_extra *pcre_study(const pcre *code, int options, 1093 const char **errptr); 1094 1095 void pcre_free_study(pcre_extra *extra); 1096 1097 int pcre_exec(const pcre *code, const pcre_extra *extra, 1098 const char *subject, int length, int startoffset, 1099 int options, int *ovector, int ovecsize); 1100 1101 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra, 1102 const char *subject, int length, int startoffset, 1103 int options, int *ovector, int ovecsize, 1104 int *workspace, int wscount); 1105 1106 1107PCRE NATIVE API STRING EXTRACTION FUNCTIONS 1108 1109 int pcre_copy_named_substring(const pcre *code, 1110 const char *subject, int *ovector, 1111 int stringcount, const char *stringname, 1112 char *buffer, int buffersize); 1113 1114 int pcre_copy_substring(const char *subject, int *ovector, 1115 int stringcount, int stringnumber, char *buffer, 1116 int buffersize); 1117 1118 int pcre_get_named_substring(const pcre *code, 1119 const char *subject, int *ovector, 1120 int stringcount, const char *stringname, 1121 const char **stringptr); 1122 1123 int pcre_get_stringnumber(const pcre *code, 1124 const char *name); 1125 1126 int pcre_get_stringtable_entries(const pcre *code, 1127 const char *name, char **first, char **last); 1128 1129 int pcre_get_substring(const char *subject, int *ovector, 1130 int stringcount, int stringnumber, 1131 const char **stringptr); 1132 1133 int pcre_get_substring_list(const char *subject, 1134 int *ovector, int stringcount, const char ***listptr); 1135 1136 void pcre_free_substring(const char *stringptr); 1137 1138 void pcre_free_substring_list(const char **stringptr); 1139 1140 1141PCRE NATIVE API AUXILIARY FUNCTIONS 1142 1143 pcre_jit_stack *pcre_jit_stack_alloc(int startsize, int maxsize); 1144 1145 void pcre_jit_stack_free(pcre_jit_stack *stack); 1146 1147 void pcre_assign_jit_stack(pcre_extra *extra, 1148 pcre_jit_callback callback, void *data); 1149 1150 const unsigned char *pcre_maketables(void); 1151 1152 int pcre_fullinfo(const pcre *code, const pcre_extra *extra, 1153 int what, void *where); 1154 1155 int pcre_refcount(pcre *code, int adjust); 1156 1157 int pcre_config(int what, void *where); 1158 1159 const char *pcre_version(void); 1160 1161 int pcre_pattern_to_host_byte_order(pcre *code, 1162 pcre_extra *extra, const unsigned char *tables); 1163 1164 1165PCRE NATIVE API INDIRECTED FUNCTIONS 1166 1167 void *(*pcre_malloc)(size_t); 1168 1169 void (*pcre_free)(void *); 1170 1171 void *(*pcre_stack_malloc)(size_t); 1172 1173 void (*pcre_stack_free)(void *); 1174 1175 int (*pcre_callout)(pcre_callout_block *); 1176 1177 1178PCRE 8-BIT AND 16-BIT LIBRARIES 1179 1180 From release 8.30, PCRE can be compiled as a library for handling 1181 16-bit character strings as well as, or instead of, the original 1182 library that handles 8-bit character strings. To avoid too much compli- 1183 cation, this document describes the 8-bit versions of the functions, 1184 with only occasional references to the 16-bit library. 1185 1186 The 16-bit functions operate in the same way as their 8-bit counter- 1187 parts; they just use different data types for their arguments and 1188 results, and their names start with pcre16_ instead of pcre_. For every 1189 option that has UTF8 in its name (for example, PCRE_UTF8), there is a 1190 corresponding 16-bit name with UTF8 replaced by UTF16. This facility is 1191 in fact just cosmetic; the 16-bit option names define the same bit val- 1192 ues. 1193 1194 References to bytes and UTF-8 in this document should be read as refer- 1195 ences to 16-bit data quantities and UTF-16 when using the 16-bit 1196 library, unless specified otherwise. More details of the specific dif- 1197 ferences for the 16-bit library are given in the pcre16 page. 1198 1199 1200PCRE API OVERVIEW 1201 1202 PCRE has its own native API, which is described in this document. There 1203 are also some wrapper functions (for the 8-bit library only) that cor- 1204 respond to the POSIX regular expression API, but they do not give 1205 access to all the functionality. They are described in the pcreposix 1206 documentation. Both of these APIs define a set of C function calls. A 1207 C++ wrapper (again for the 8-bit library only) is also distributed with 1208 PCRE. It is documented in the pcrecpp page. 1209 1210 The native API C function prototypes are defined in the header file 1211 pcre.h, and on Unix-like systems the (8-bit) library itself is called 1212 libpcre. It can normally be accessed by adding -lpcre to the command 1213 for linking an application that uses PCRE. The header file defines the 1214 macros PCRE_MAJOR and PCRE_MINOR to contain the major and minor release 1215 numbers for the library. Applications can use these to include support 1216 for different releases of PCRE. 1217 1218 In a Windows environment, if you want to statically link an application 1219 program against a non-dll pcre.a file, you must define PCRE_STATIC 1220 before including pcre.h or pcrecpp.h, because otherwise the pcre_mal- 1221 loc() and pcre_free() exported functions will be declared 1222 __declspec(dllimport), with unwanted results. 1223 1224 The functions pcre_compile(), pcre_compile2(), pcre_study(), and 1225 pcre_exec() are used for compiling and matching regular expressions in 1226 a Perl-compatible manner. A sample program that demonstrates the sim- 1227 plest way of using them is provided in the file called pcredemo.c in 1228 the PCRE source distribution. A listing of this program is given in the 1229 pcredemo documentation, and the pcresample documentation describes how 1230 to compile and run it. 1231 1232 Just-in-time compiler support is an optional feature of PCRE that can 1233 be built in appropriate hardware environments. It greatly speeds up the 1234 matching performance of many patterns. Simple programs can easily 1235 request that it be used if available, by setting an option that is 1236 ignored when it is not relevant. More complicated programs might need 1237 to make use of the functions pcre_jit_stack_alloc(), 1238 pcre_jit_stack_free(), and pcre_assign_jit_stack() in order to control 1239 the JIT code's memory usage. These functions are discussed in the 1240 pcrejit documentation. 1241 1242 A second matching function, pcre_dfa_exec(), which is not Perl-compati- 1243 ble, is also provided. This uses a different algorithm for the match- 1244 ing. The alternative algorithm finds all possible matches (at a given 1245 point in the subject), and scans the subject just once (unless there 1246 are lookbehind assertions). However, this algorithm does not return 1247 captured substrings. A description of the two matching algorithms and 1248 their advantages and disadvantages is given in the pcrematching docu- 1249 mentation. 1250 1251 In addition to the main compiling and matching functions, there are 1252 convenience functions for extracting captured substrings from a subject 1253 string that is matched by pcre_exec(). They are: 1254 1255 pcre_copy_substring() 1256 pcre_copy_named_substring() 1257 pcre_get_substring() 1258 pcre_get_named_substring() 1259 pcre_get_substring_list() 1260 pcre_get_stringnumber() 1261 pcre_get_stringtable_entries() 1262 1263 pcre_free_substring() and pcre_free_substring_list() are also provided, 1264 to free the memory used for extracted strings. 1265 1266 The function pcre_maketables() is used to build a set of character 1267 tables in the current locale for passing to pcre_compile(), 1268 pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is 1269 provided for specialist use. Most commonly, no special tables are 1270 passed, in which case internal tables that are generated when PCRE is 1271 built are used. 1272 1273 The function pcre_fullinfo() is used to find out information about a 1274 compiled pattern. The function pcre_version() returns a pointer to a 1275 string containing the version of PCRE and its date of release. 1276 1277 The function pcre_refcount() maintains a reference count in a data 1278 block containing a compiled pattern. This is provided for the benefit 1279 of object-oriented applications. 1280 1281 The global variables pcre_malloc and pcre_free initially contain the 1282 entry points of the standard malloc() and free() functions, respec- 1283 tively. PCRE calls the memory management functions via these variables, 1284 so a calling program can replace them if it wishes to intercept the 1285 calls. This should be done before calling any PCRE functions. 1286 1287 The global variables pcre_stack_malloc and pcre_stack_free are also 1288 indirections to memory management functions. These special functions 1289 are used only when PCRE is compiled to use the heap for remembering 1290 data, instead of recursive function calls, when running the pcre_exec() 1291 function. See the pcrebuild documentation for details of how to do 1292 this. It is a non-standard way of building PCRE, for use in environ- 1293 ments that have limited stacks. Because of the greater use of memory 1294 management, it runs more slowly. Separate functions are provided so 1295 that special-purpose external code can be used for this case. When 1296 used, these functions are always called in a stack-like manner (last 1297 obtained, first freed), and always for memory blocks of the same size. 1298 There is a discussion about PCRE's stack usage in the pcrestack docu- 1299 mentation. 1300 1301 The global variable pcre_callout initially contains NULL. It can be set 1302 by the caller to a "callout" function, which PCRE will then call at 1303 specified points during a matching operation. Details are given in the 1304 pcrecallout documentation. 1305 1306 1307NEWLINES 1308 1309 PCRE supports five different conventions for indicating line breaks in 1310 strings: a single CR (carriage return) character, a single LF (line- 1311 feed) character, the two-character sequence CRLF, any of the three pre- 1312 ceding, or any Unicode newline sequence. The Unicode newline sequences 1313 are the three just mentioned, plus the single characters VT (vertical 1314 tab, U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line 1315 separator, U+2028), and PS (paragraph separator, U+2029). 1316 1317 Each of the first three conventions is used by at least one operating 1318 system as its standard newline sequence. When PCRE is built, a default 1319 can be specified. The default default is LF, which is the Unix stan- 1320 dard. When PCRE is run, the default can be overridden, either when a 1321 pattern is compiled, or when it is matched. 1322 1323 At compile time, the newline convention can be specified by the options 1324 argument of pcre_compile(), or it can be specified by special text at 1325 the start of the pattern itself; this overrides any other settings. See 1326 the pcrepattern page for details of the special character sequences. 1327 1328 In the PCRE documentation the word "newline" is used to mean "the char- 1329 acter or pair of characters that indicate a line break". The choice of 1330 newline convention affects the handling of the dot, circumflex, and 1331 dollar metacharacters, the handling of #-comments in /x mode, and, when 1332 CRLF is a recognized line ending sequence, the match position advance- 1333 ment for a non-anchored pattern. There is more detail about this in the 1334 section on pcre_exec() options below. 1335 1336 The choice of newline convention does not affect the interpretation of 1337 the \n or \r escape sequences, nor does it affect what \R matches, 1338 which is controlled in a similar way, but by separate options. 1339 1340 1341MULTITHREADING 1342 1343 The PCRE functions can be used in multi-threading applications, with 1344 the proviso that the memory management functions pointed to by 1345 pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the 1346 callout function pointed to by pcre_callout, are shared by all threads. 1347 1348 The compiled form of a regular expression is not altered during match- 1349 ing, so the same compiled pattern can safely be used by several threads 1350 at once. 1351 1352 If the just-in-time optimization feature is being used, it needs sepa- 1353 rate memory stack areas for each thread. See the pcrejit documentation 1354 for more details. 1355 1356 1357SAVING PRECOMPILED PATTERNS FOR LATER USE 1358 1359 The compiled form of a regular expression can be saved and re-used at a 1360 later time, possibly by a different program, and even on a host other 1361 than the one on which it was compiled. Details are given in the 1362 pcreprecompile documentation, which includes a description of the 1363 pcre_pattern_to_host_byte_order() function. However, compiling a regu- 1364 lar expression with one version of PCRE for use with a different ver- 1365 sion is not guaranteed to work and may cause crashes. 1366 1367 1368CHECKING BUILD-TIME OPTIONS 1369 1370 int pcre_config(int what, void *where); 1371 1372 The function pcre_config() makes it possible for a PCRE client to dis- 1373 cover which optional features have been compiled into the PCRE library. 1374 The pcrebuild documentation has more details about these optional fea- 1375 tures. 1376 1377 The first argument for pcre_config() is an integer, specifying which 1378 information is required; the second argument is a pointer to a variable 1379 into which the information is placed. The returned value is zero on 1380 success, or the negative error code PCRE_ERROR_BADOPTION if the value 1381 in the first argument is not recognized. The following information is 1382 available: 1383 1384 PCRE_CONFIG_UTF8 1385 1386 The output is an integer that is set to one if UTF-8 support is avail- 1387 able; otherwise it is set to zero. If this option is given to the 1388 16-bit version of this function, pcre16_config(), the result is 1389 PCRE_ERROR_BADOPTION. 1390 1391 PCRE_CONFIG_UTF16 1392 1393 The output is an integer that is set to one if UTF-16 support is avail- 1394 able; otherwise it is set to zero. This value should normally be given 1395 to the 16-bit version of this function, pcre16_config(). If it is given 1396 to the 8-bit version of this function, the result is PCRE_ERROR_BADOP- 1397 TION. 1398 1399 PCRE_CONFIG_UNICODE_PROPERTIES 1400 1401 The output is an integer that is set to one if support for Unicode 1402 character properties is available; otherwise it is set to zero. 1403 1404 PCRE_CONFIG_JIT 1405 1406 The output is an integer that is set to one if support for just-in-time 1407 compiling is available; otherwise it is set to zero. 1408 1409 PCRE_CONFIG_JITTARGET 1410 1411 The output is a pointer to a zero-terminated "const char *" string. If 1412 JIT support is available, the string contains the name of the architec- 1413 ture for which the JIT compiler is configured, for example "x86 32bit 1414 (little endian + unaligned)". If JIT support is not available, the 1415 result is NULL. 1416 1417 PCRE_CONFIG_NEWLINE 1418 1419 The output is an integer whose value specifies the default character 1420 sequence that is recognized as meaning "newline". The four values that 1421 are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF, 1422 and -1 for ANY. Though they are derived from ASCII, the same values 1423 are returned in EBCDIC environments. The default should normally corre- 1424 spond to the standard sequence for your operating system. 1425 1426 PCRE_CONFIG_BSR 1427 1428 The output is an integer whose value indicates what character sequences 1429 the \R escape sequence matches by default. A value of 0 means that \R 1430 matches any Unicode line ending sequence; a value of 1 means that \R 1431 matches only CR, LF, or CRLF. The default can be overridden when a pat- 1432 tern is compiled or matched. 1433 1434 PCRE_CONFIG_LINK_SIZE 1435 1436 The output is an integer that contains the number of bytes used for 1437 internal linkage in compiled regular expressions. For the 8-bit 1438 library, the value can be 2, 3, or 4. For the 16-bit library, the value 1439 is either 2 or 4 and is still a number of bytes. The default value of 2 1440 is sufficient for all but the most massive patterns, since it allows 1441 the compiled pattern to be up to 64K in size. Larger values allow 1442 larger regular expressions to be compiled, at the expense of slower 1443 matching. 1444 1445 PCRE_CONFIG_POSIX_MALLOC_THRESHOLD 1446 1447 The output is an integer that contains the threshold above which the 1448 POSIX interface uses malloc() for output vectors. Further details are 1449 given in the pcreposix documentation. 1450 1451 PCRE_CONFIG_MATCH_LIMIT 1452 1453 The output is a long integer that gives the default limit for the num- 1454 ber of internal matching function calls in a pcre_exec() execution. 1455 Further details are given with pcre_exec() below. 1456 1457 PCRE_CONFIG_MATCH_LIMIT_RECURSION 1458 1459 The output is a long integer that gives the default limit for the depth 1460 of recursion when calling the internal matching function in a 1461 pcre_exec() execution. Further details are given with pcre_exec() 1462 below. 1463 1464 PCRE_CONFIG_STACKRECURSE 1465 1466 The output is an integer that is set to one if internal recursion when 1467 running pcre_exec() is implemented by recursive function calls that use 1468 the stack to remember their state. This is the usual way that PCRE is 1469 compiled. The output is zero if PCRE was compiled to use blocks of data 1470 on the heap instead of recursive function calls. In this case, 1471 pcre_stack_malloc and pcre_stack_free are called to manage memory 1472 blocks on the heap, thus avoiding the use of the stack. 1473 1474 1475COMPILING A PATTERN 1476 1477 pcre *pcre_compile(const char *pattern, int options, 1478 const char **errptr, int *erroffset, 1479 const unsigned char *tableptr); 1480 1481 pcre *pcre_compile2(const char *pattern, int options, 1482 int *errorcodeptr, 1483 const char **errptr, int *erroffset, 1484 const unsigned char *tableptr); 1485 1486 Either of the functions pcre_compile() or pcre_compile2() can be called 1487 to compile a pattern into an internal form. The only difference between 1488 the two interfaces is that pcre_compile2() has an additional argument, 1489 errorcodeptr, via which a numerical error code can be returned. To 1490 avoid too much repetition, we refer just to pcre_compile() below, but 1491 the information applies equally to pcre_compile2(). 1492 1493 The pattern is a C string terminated by a binary zero, and is passed in 1494 the pattern argument. A pointer to a single block of memory that is 1495 obtained via pcre_malloc is returned. This contains the compiled code 1496 and related data. The pcre type is defined for the returned block; this 1497 is a typedef for a structure whose contents are not externally defined. 1498 It is up to the caller to free the memory (via pcre_free) when it is no 1499 longer required. 1500 1501 Although the compiled code of a PCRE regex is relocatable, that is, it 1502 does not depend on memory location, the complete pcre data block is not 1503 fully relocatable, because it may contain a copy of the tableptr argu- 1504 ment, which is an address (see below). 1505 1506 The options argument contains various bit settings that affect the com- 1507 pilation. It should be zero if no options are required. The available 1508 options are described below. Some of them (in particular, those that 1509 are compatible with Perl, but some others as well) can also be set and 1510 unset from within the pattern (see the detailed description in the 1511 pcrepattern documentation). For those options that can be different in 1512 different parts of the pattern, the contents of the options argument 1513 specifies their settings at the start of compilation and execution. The 1514 PCRE_ANCHORED, PCRE_BSR_xxx, PCRE_NEWLINE_xxx, PCRE_NO_UTF8_CHECK, and 1515 PCRE_NO_START_OPTIMIZE options can be set at the time of matching as 1516 well as at compile time. 1517 1518 If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise, 1519 if compilation of a pattern fails, pcre_compile() returns NULL, and 1520 sets the variable pointed to by errptr to point to a textual error mes- 1521 sage. This is a static string that is part of the library. You must not 1522 try to free it. Normally, the offset from the start of the pattern to 1523 the byte that was being processed when the error was discovered is 1524 placed in the variable pointed to by erroffset, which must not be NULL 1525 (if it is, an immediate error is given). However, for an invalid UTF-8 1526 string, the offset is that of the first byte of the failing character. 1527 1528 Some errors are not detected until the whole pattern has been scanned; 1529 in these cases, the offset passed back is the length of the pattern. 1530 Note that the offset is in bytes, not characters, even in UTF-8 mode. 1531 It may sometimes point into the middle of a UTF-8 character. 1532 1533 If pcre_compile2() is used instead of pcre_compile(), and the error- 1534 codeptr argument is not NULL, a non-zero error code number is returned 1535 via this argument in the event of an error. This is in addition to the 1536 textual error message. Error codes and messages are listed below. 1537 1538 If the final argument, tableptr, is NULL, PCRE uses a default set of 1539 character tables that are built when PCRE is compiled, using the 1540 default C locale. Otherwise, tableptr must be an address that is the 1541 result of a call to pcre_maketables(). This value is stored with the 1542 compiled pattern, and used again by pcre_exec(), unless another table 1543 pointer is passed to it. For more discussion, see the section on locale 1544 support below. 1545 1546 This code fragment shows a typical straightforward call to pcre_com- 1547 pile(): 1548 1549 pcre *re; 1550 const char *error; 1551 int erroffset; 1552 re = pcre_compile( 1553 "^A.*Z", /* the pattern */ 1554 0, /* default options */ 1555 &error, /* for error message */ 1556 &erroffset, /* for error offset */ 1557 NULL); /* use default character tables */ 1558 1559 The following names for option bits are defined in the pcre.h header 1560 file: 1561 1562 PCRE_ANCHORED 1563 1564 If this bit is set, the pattern is forced to be "anchored", that is, it 1565 is constrained to match only at the first matching point in the string 1566 that is being searched (the "subject string"). This effect can also be 1567 achieved by appropriate constructs in the pattern itself, which is the 1568 only way to do it in Perl. 1569 1570 PCRE_AUTO_CALLOUT 1571 1572 If this bit is set, pcre_compile() automatically inserts callout items, 1573 all with number 255, before each pattern item. For discussion of the 1574 callout facility, see the pcrecallout documentation. 1575 1576 PCRE_BSR_ANYCRLF 1577 PCRE_BSR_UNICODE 1578 1579 These options (which are mutually exclusive) control what the \R escape 1580 sequence matches. The choice is either to match only CR, LF, or CRLF, 1581 or to match any Unicode newline sequence. The default is specified when 1582 PCRE is built. It can be overridden from within the pattern, or by set- 1583 ting an option when a compiled pattern is matched. 1584 1585 PCRE_CASELESS 1586 1587 If this bit is set, letters in the pattern match both upper and lower 1588 case letters. It is equivalent to Perl's /i option, and it can be 1589 changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE 1590 always understands the concept of case for characters whose values are 1591 less than 128, so caseless matching is always possible. For characters 1592 with higher values, the concept of case is supported if PCRE is com- 1593 piled with Unicode property support, but not otherwise. If you want to 1594 use caseless matching for characters 128 and above, you must ensure 1595 that PCRE is compiled with Unicode property support as well as with 1596 UTF-8 support. 1597 1598 PCRE_DOLLAR_ENDONLY 1599 1600 If this bit is set, a dollar metacharacter in the pattern matches only 1601 at the end of the subject string. Without this option, a dollar also 1602 matches immediately before a newline at the end of the string (but not 1603 before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored 1604 if PCRE_MULTILINE is set. There is no equivalent to this option in 1605 Perl, and no way to set it within a pattern. 1606 1607 PCRE_DOTALL 1608 1609 If this bit is set, a dot metacharacter in the pattern matches a char- 1610 acter of any value, including one that indicates a newline. However, it 1611 only ever matches one character, even if newlines are coded as CRLF. 1612 Without this option, a dot does not match when the current position is 1613 at a newline. This option is equivalent to Perl's /s option, and it can 1614 be changed within a pattern by a (?s) option setting. A negative class 1615 such as [^a] always matches newline characters, independent of the set- 1616 ting of this option. 1617 1618 PCRE_DUPNAMES 1619 1620 If this bit is set, names used to identify capturing subpatterns need 1621 not be unique. This can be helpful for certain types of pattern when it 1622 is known that only one instance of the named subpattern can ever be 1623 matched. There are more details of named subpatterns below; see also 1624 the pcrepattern documentation. 1625 1626 PCRE_EXTENDED 1627 1628 If this bit is set, white space data characters in the pattern are 1629 totally ignored except when escaped or inside a character class. White 1630 space does not include the VT character (code 11). In addition, charac- 1631 ters between an unescaped # outside a character class and the next new- 1632 line, inclusive, are also ignored. This is equivalent to Perl's /x 1633 option, and it can be changed within a pattern by a (?x) option set- 1634 ting. 1635 1636 Which characters are interpreted as newlines is controlled by the 1637 options passed to pcre_compile() or by a special sequence at the start 1638 of the pattern, as described in the section entitled "Newline conven- 1639 tions" in the pcrepattern documentation. Note that the end of this type 1640 of comment is a literal newline sequence in the pattern; escape 1641 sequences that happen to represent a newline do not count. 1642 1643 This option makes it possible to include comments inside complicated 1644 patterns. Note, however, that this applies only to data characters. 1645 White space characters may never appear within special character 1646 sequences in a pattern, for example within the sequence (?( that intro- 1647 duces a conditional subpattern. 1648 1649 PCRE_EXTRA 1650 1651 This option was invented in order to turn on additional functionality 1652 of PCRE that is incompatible with Perl, but it is currently of very 1653 little use. When set, any backslash in a pattern that is followed by a 1654 letter that has no special meaning causes an error, thus reserving 1655 these combinations for future expansion. By default, as in Perl, a 1656 backslash followed by a letter with no special meaning is treated as a 1657 literal. (Perl can, however, be persuaded to give an error for this, by 1658 running it with the -w option.) There are at present no other features 1659 controlled by this option. It can also be set by a (?X) option setting 1660 within a pattern. 1661 1662 PCRE_FIRSTLINE 1663 1664 If this option is set, an unanchored pattern is required to match 1665 before or at the first newline in the subject string, though the 1666 matched text may continue over the newline. 1667 1668 PCRE_JAVASCRIPT_COMPAT 1669 1670 If this option is set, PCRE's behaviour is changed in some ways so that 1671 it is compatible with JavaScript rather than Perl. The changes are as 1672 follows: 1673 1674 (1) A lone closing square bracket in a pattern causes a compile-time 1675 error, because this is illegal in JavaScript (by default it is treated 1676 as a data character). Thus, the pattern AB]CD becomes illegal when this 1677 option is set. 1678 1679 (2) At run time, a back reference to an unset subpattern group matches 1680 an empty string (by default this causes the current matching alterna- 1681 tive to fail). A pattern such as (\1)(a) succeeds when this option is 1682 set (assuming it can find an "a" in the subject), whereas it fails by 1683 default, for Perl compatibility. 1684 1685 (3) \U matches an upper case "U" character; by default \U causes a com- 1686 pile time error (Perl uses \U to upper case subsequent characters). 1687 1688 (4) \u matches a lower case "u" character unless it is followed by four 1689 hexadecimal digits, in which case the hexadecimal number defines the 1690 code point to match. By default, \u causes a compile time error (Perl 1691 uses it to upper case the following character). 1692 1693 (5) \x matches a lower case "x" character unless it is followed by two 1694 hexadecimal digits, in which case the hexadecimal number defines the 1695 code point to match. By default, as in Perl, a hexadecimal number is 1696 always expected after \x, but it may have zero, one, or two digits (so, 1697 for example, \xz matches a binary zero character followed by z). 1698 1699 PCRE_MULTILINE 1700 1701 By default, PCRE treats the subject string as consisting of a single 1702 line of characters (even if it actually contains newlines). The "start 1703 of line" metacharacter (^) matches only at the start of the string, 1704 while the "end of line" metacharacter ($) matches only at the end of 1705 the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY 1706 is set). This is the same as Perl. 1707 1708 When PCRE_MULTILINE it is set, the "start of line" and "end of line" 1709 constructs match immediately following or immediately before internal 1710 newlines in the subject string, respectively, as well as at the very 1711 start and end. This is equivalent to Perl's /m option, and it can be 1712 changed within a pattern by a (?m) option setting. If there are no new- 1713 lines in a subject string, or no occurrences of ^ or $ in a pattern, 1714 setting PCRE_MULTILINE has no effect. 1715 1716 PCRE_NEWLINE_CR 1717 PCRE_NEWLINE_LF 1718 PCRE_NEWLINE_CRLF 1719 PCRE_NEWLINE_ANYCRLF 1720 PCRE_NEWLINE_ANY 1721 1722 These options override the default newline definition that was chosen 1723 when PCRE was built. Setting the first or the second specifies that a 1724 newline is indicated by a single character (CR or LF, respectively). 1725 Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the 1726 two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies 1727 that any of the three preceding sequences should be recognized. Setting 1728 PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be 1729 recognized. The Unicode newline sequences are the three just mentioned, 1730 plus the single characters VT (vertical tab, U+000B), FF (form feed, 1731 U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS 1732 (paragraph separator, U+2029). For the 8-bit library, the last two are 1733 recognized only in UTF-8 mode. 1734 1735 The newline setting in the options word uses three bits that are 1736 treated as a number, giving eight possibilities. Currently only six are 1737 used (default plus the five values above). This means that if you set 1738 more than one newline option, the combination may or may not be sensi- 1739 ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to 1740 PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and 1741 cause an error. 1742 1743 The only time that a line break in a pattern is specially recognized 1744 when compiling is when PCRE_EXTENDED is set. CR and LF are white space 1745 characters, and so are ignored in this mode. Also, an unescaped # out- 1746 side a character class indicates a comment that lasts until after the 1747 next line break sequence. In other circumstances, line break sequences 1748 in patterns are treated as literal data. 1749 1750 The newline option that is set at compile time becomes the default that 1751 is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden. 1752 1753 PCRE_NO_AUTO_CAPTURE 1754 1755 If this option is set, it disables the use of numbered capturing paren- 1756 theses in the pattern. Any opening parenthesis that is not followed by 1757 ? behaves as if it were followed by ?: but named parentheses can still 1758 be used for capturing (and they acquire numbers in the usual way). 1759 There is no equivalent of this option in Perl. 1760 1761 NO_START_OPTIMIZE 1762 1763 This is an option that acts at matching time; that is, it is really an 1764 option for pcre_exec() or pcre_dfa_exec(). If it is set at compile 1765 time, it is remembered with the compiled pattern and assumed at match- 1766 ing time. For details see the discussion of PCRE_NO_START_OPTIMIZE 1767 below. 1768 1769 PCRE_UCP 1770 1771 This option changes the way PCRE processes \B, \b, \D, \d, \S, \s, \W, 1772 \w, and some of the POSIX character classes. By default, only ASCII 1773 characters are recognized, but if PCRE_UCP is set, Unicode properties 1774 are used instead to classify characters. More details are given in the 1775 section on generic character types in the pcrepattern page. If you set 1776 PCRE_UCP, matching one of the items it affects takes much longer. The 1777 option is available only if PCRE has been compiled with Unicode prop- 1778 erty support. 1779 1780 PCRE_UNGREEDY 1781 1782 This option inverts the "greediness" of the quantifiers so that they 1783 are not greedy by default, but become greedy if followed by "?". It is 1784 not compatible with Perl. It can also be set by a (?U) option setting 1785 within the pattern. 1786 1787 PCRE_UTF8 1788 1789 This option causes PCRE to regard both the pattern and the subject as 1790 strings of UTF-8 characters instead of single-byte strings. However, it 1791 is available only when PCRE is built to include UTF support. If not, 1792 the use of this option provokes an error. Details of how this option 1793 changes the behaviour of PCRE are given in the pcreunicode page. 1794 1795 PCRE_NO_UTF8_CHECK 1796 1797 When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is 1798 automatically checked. There is a discussion about the validity of 1799 UTF-8 strings in the pcreunicode page. If an invalid UTF-8 sequence is 1800 found, pcre_compile() returns an error. If you already know that your 1801 pattern is valid, and you want to skip this check for performance rea- 1802 sons, you can set the PCRE_NO_UTF8_CHECK option. When it is set, the 1803 effect of passing an invalid UTF-8 string as a pattern is undefined. It 1804 may cause your program to crash. Note that this option can also be 1805 passed to pcre_exec() and pcre_dfa_exec(), to suppress the validity 1806 checking of subject strings. 1807 1808 1809COMPILATION ERROR CODES 1810 1811 The following table lists the error codes than may be returned by 1812 pcre_compile2(), along with the error messages that may be returned by 1813 both compiling functions. Note that error messages are always 8-bit 1814 ASCII strings, even in 16-bit mode. As PCRE has developed, some error 1815 codes have fallen out of use. To avoid confusion, they have not been 1816 re-used. 1817 1818 0 no error 1819 1 \ at end of pattern 1820 2 \c at end of pattern 1821 3 unrecognized character follows \ 1822 4 numbers out of order in {} quantifier 1823 5 number too big in {} quantifier 1824 6 missing terminating ] for character class 1825 7 invalid escape sequence in character class 1826 8 range out of order in character class 1827 9 nothing to repeat 1828 10 [this code is not in use] 1829 11 internal error: unexpected repeat 1830 12 unrecognized character after (? or (?- 1831 13 POSIX named classes are supported only within a class 1832 14 missing ) 1833 15 reference to non-existent subpattern 1834 16 erroffset passed as NULL 1835 17 unknown option bit(s) set 1836 18 missing ) after comment 1837 19 [this code is not in use] 1838 20 regular expression is too large 1839 21 failed to get memory 1840 22 unmatched parentheses 1841 23 internal error: code overflow 1842 24 unrecognized character after (?< 1843 25 lookbehind assertion is not fixed length 1844 26 malformed number or name after (?( 1845 27 conditional group contains more than two branches 1846 28 assertion expected after (?( 1847 29 (?R or (?[+-]digits must be followed by ) 1848 30 unknown POSIX class name 1849 31 POSIX collating elements are not supported 1850 32 this version of PCRE is compiled without UTF support 1851 33 [this code is not in use] 1852 34 character value in \x{...} sequence is too large 1853 35 invalid condition (?(0) 1854 36 \C not allowed in lookbehind assertion 1855 37 PCRE does not support \L, \l, \N{name}, \U, or \u 1856 38 number after (?C is > 255 1857 39 closing ) for (?C expected 1858 40 recursive call could loop indefinitely 1859 41 unrecognized character after (?P 1860 42 syntax error in subpattern name (missing terminator) 1861 43 two named subpatterns have the same name 1862 44 invalid UTF-8 string (specifically UTF-8) 1863 45 support for \P, \p, and \X has not been compiled 1864 46 malformed \P or \p sequence 1865 47 unknown property name after \P or \p 1866 48 subpattern name is too long (maximum 32 characters) 1867 49 too many named subpatterns (maximum 10000) 1868 50 [this code is not in use] 1869 51 octal value is greater than \377 in 8-bit non-UTF-8 mode 1870 52 internal error: overran compiling workspace 1871 53 internal error: previously-checked referenced subpattern 1872 not found 1873 54 DEFINE group contains more than one branch 1874 55 repeating a DEFINE group is not allowed 1875 56 inconsistent NEWLINE options 1876 57 \g is not followed by a braced, angle-bracketed, or quoted 1877 name/number or by a plain number 1878 58 a numbered reference must not be zero 1879 59 an argument is not allowed for (*ACCEPT), (*FAIL), or (*COMMIT) 1880 60 (*VERB) not recognized 1881 61 number is too big 1882 62 subpattern name expected 1883 63 digit expected after (?+ 1884 64 ] is an invalid data character in JavaScript compatibility mode 1885 65 different names for subpatterns of the same number are 1886 not allowed 1887 66 (*MARK) must have an argument 1888 67 this version of PCRE is not compiled with Unicode property 1889 support 1890 68 \c must be followed by an ASCII character 1891 69 \k is not followed by a braced, angle-bracketed, or quoted name 1892 70 internal error: unknown opcode in find_fixedlength() 1893 71 \N is not supported in a class 1894 72 too many forward references 1895 73 disallowed Unicode code point (>= 0xd800 && <= 0xdfff) 1896 74 invalid UTF-16 string (specifically UTF-16) 1897 75 name is too long in (*MARK), (*PRUNE), (*SKIP), or (*THEN) 1898 76 character value in \u.... sequence is too large 1899 1900 The numbers 32 and 10000 in errors 48 and 49 are defaults; different 1901 values may be used if the limits were changed when PCRE was built. 1902 1903 1904STUDYING A PATTERN 1905 1906 pcre_extra *pcre_study(const pcre *code, int options 1907 const char **errptr); 1908 1909 If a compiled pattern is going to be used several times, it is worth 1910 spending more time analyzing it in order to speed up the time taken for 1911 matching. The function pcre_study() takes a pointer to a compiled pat- 1912 tern as its first argument. If studying the pattern produces additional 1913 information that will help speed up matching, pcre_study() returns a 1914 pointer to a pcre_extra block, in which the study_data field points to 1915 the results of the study. 1916 1917 The returned value from pcre_study() can be passed directly to 1918 pcre_exec() or pcre_dfa_exec(). However, a pcre_extra block also con- 1919 tains other fields that can be set by the caller before the block is 1920 passed; these are described below in the section on matching a pattern. 1921 1922 If studying the pattern does not produce any useful information, 1923 pcre_study() returns NULL. In that circumstance, if the calling program 1924 wants to pass any of the other fields to pcre_exec() or 1925 pcre_dfa_exec(), it must set up its own pcre_extra block. 1926 1927 The second argument of pcre_study() contains option bits. There are 1928 three options: 1929 1930 PCRE_STUDY_JIT_COMPILE 1931 PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE 1932 PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE 1933 1934 If any of these are set, and the just-in-time compiler is available, 1935 the pattern is further compiled into machine code that executes much 1936 faster than the pcre_exec() interpretive matching function. If the 1937 just-in-time compiler is not available, these options are ignored. All 1938 other bits in the options argument must be zero. 1939 1940 JIT compilation is a heavyweight optimization. It can take some time 1941 for patterns to be analyzed, and for one-off matches and simple pat- 1942 terns the benefit of faster execution might be offset by a much slower 1943 study time. Not all patterns can be optimized by the JIT compiler. For 1944 those that cannot be handled, matching automatically falls back to the 1945 pcre_exec() interpreter. For more details, see the pcrejit documenta- 1946 tion. 1947 1948 The third argument for pcre_study() is a pointer for an error message. 1949 If studying succeeds (even if no data is returned), the variable it 1950 points to is set to NULL. Otherwise it is set to point to a textual 1951 error message. This is a static string that is part of the library. You 1952 must not try to free it. You should test the error pointer for NULL 1953 after calling pcre_study(), to be sure that it has run successfully. 1954 1955 When you are finished with a pattern, you can free the memory used for 1956 the study data by calling pcre_free_study(). This function was added to 1957 the API for release 8.20. For earlier versions, the memory could be 1958 freed with pcre_free(), just like the pattern itself. This will still 1959 work in cases where JIT optimization is not used, but it is advisable 1960 to change to the new function when convenient. 1961 1962 This is a typical way in which pcre_study() is used (except that in a 1963 real application there should be tests for errors): 1964 1965 int rc; 1966 pcre *re; 1967 pcre_extra *sd; 1968 re = pcre_compile("pattern", 0, &error, &erroroffset, NULL); 1969 sd = pcre_study( 1970 re, /* result of pcre_compile() */ 1971 0, /* no options */ 1972 &error); /* set to NULL or points to a message */ 1973 rc = pcre_exec( /* see below for details of pcre_exec() options */ 1974 re, sd, "subject", 7, 0, 0, ovector, 30); 1975 ... 1976 pcre_free_study(sd); 1977 pcre_free(re); 1978 1979 Studying a pattern does two things: first, a lower bound for the length 1980 of subject string that is needed to match the pattern is computed. This 1981 does not mean that there are any strings of that length that match, but 1982 it does guarantee that no shorter strings match. The value is used by 1983 pcre_exec() and pcre_dfa_exec() to avoid wasting time by trying to 1984 match strings that are shorter than the lower bound. You can find out 1985 the value in a calling program via the pcre_fullinfo() function. 1986 1987 Studying a pattern is also useful for non-anchored patterns that do not 1988 have a single fixed starting character. A bitmap of possible starting 1989 bytes is created. This speeds up finding a position in the subject at 1990 which to start matching. (In 16-bit mode, the bitmap is used for 16-bit 1991 values less than 256.) 1992 1993 These two optimizations apply to both pcre_exec() and pcre_dfa_exec(), 1994 and the information is also used by the JIT compiler. The optimiza- 1995 tions can be disabled by setting the PCRE_NO_START_OPTIMIZE option when 1996 calling pcre_exec() or pcre_dfa_exec(), but if this is done, JIT execu- 1997 tion is also disabled. You might want to do this if your pattern con- 1998 tains callouts or (*MARK) and you want to make use of these facilities 1999 in cases where matching fails. See the discussion of 2000 PCRE_NO_START_OPTIMIZE below. 2001 2002 2003LOCALE SUPPORT 2004 2005 PCRE handles caseless matching, and determines whether characters are 2006 letters, digits, or whatever, by reference to a set of tables, indexed 2007 by character value. When running in UTF-8 mode, this applies only to 2008 characters with codes less than 128. By default, higher-valued codes 2009 never match escapes such as \w or \d, but they can be tested with \p if 2010 PCRE is built with Unicode character property support. Alternatively, 2011 the PCRE_UCP option can be set at compile time; this causes \w and 2012 friends to use Unicode property support instead of built-in tables. The 2013 use of locales with Unicode is discouraged. If you are handling charac- 2014 ters with codes greater than 128, you should either use UTF-8 and Uni- 2015 code, or use locales, but not try to mix the two. 2016 2017 PCRE contains an internal set of tables that are used when the final 2018 argument of pcre_compile() is NULL. These are sufficient for many 2019 applications. Normally, the internal tables recognize only ASCII char- 2020 acters. However, when PCRE is built, it is possible to cause the inter- 2021 nal tables to be rebuilt in the default "C" locale of the local system, 2022 which may cause them to be different. 2023 2024 The internal tables can always be overridden by tables supplied by the 2025 application that calls PCRE. These may be created in a different locale 2026 from the default. As more and more applications change to using Uni- 2027 code, the need for this locale support is expected to die away. 2028 2029 External tables are built by calling the pcre_maketables() function, 2030 which has no arguments, in the relevant locale. The result can then be 2031 passed to pcre_compile() or pcre_exec() as often as necessary. For 2032 example, to build and use tables that are appropriate for the French 2033 locale (where accented characters with values greater than 128 are 2034 treated as letters), the following code could be used: 2035 2036 setlocale(LC_CTYPE, "fr_FR"); 2037 tables = pcre_maketables(); 2038 re = pcre_compile(..., tables); 2039 2040 The locale name "fr_FR" is used on Linux and other Unix-like systems; 2041 if you are using Windows, the name for the French locale is "french". 2042 2043 When pcre_maketables() runs, the tables are built in memory that is 2044 obtained via pcre_malloc. It is the caller's responsibility to ensure 2045 that the memory containing the tables remains available for as long as 2046 it is needed. 2047 2048 The pointer that is passed to pcre_compile() is saved with the compiled 2049 pattern, and the same tables are used via this pointer by pcre_study() 2050 and normally also by pcre_exec(). Thus, by default, for any single pat- 2051 tern, compilation, studying and matching all happen in the same locale, 2052 but different patterns can be compiled in different locales. 2053 2054 It is possible to pass a table pointer or NULL (indicating the use of 2055 the internal tables) to pcre_exec(). Although not intended for this 2056 purpose, this facility could be used to match a pattern in a different 2057 locale from the one in which it was compiled. Passing table pointers at 2058 run time is discussed below in the section on matching a pattern. 2059 2060 2061INFORMATION ABOUT A PATTERN 2062 2063 int pcre_fullinfo(const pcre *code, const pcre_extra *extra, 2064 int what, void *where); 2065 2066 The pcre_fullinfo() function returns information about a compiled pat- 2067 tern. It replaces the pcre_info() function, which was removed from the 2068 library at version 8.30, after more than 10 years of obsolescence. 2069 2070 The first argument for pcre_fullinfo() is a pointer to the compiled 2071 pattern. The second argument is the result of pcre_study(), or NULL if 2072 the pattern was not studied. The third argument specifies which piece 2073 of information is required, and the fourth argument is a pointer to a 2074 variable to receive the data. The yield of the function is zero for 2075 success, or one of the following negative numbers: 2076 2077 PCRE_ERROR_NULL the argument code was NULL 2078 the argument where was NULL 2079 PCRE_ERROR_BADMAGIC the "magic number" was not found 2080 PCRE_ERROR_BADENDIANNESS the pattern was compiled with different 2081 endianness 2082 PCRE_ERROR_BADOPTION the value of what was invalid 2083 2084 The "magic number" is placed at the start of each compiled pattern as 2085 an simple check against passing an arbitrary memory pointer. The endi- 2086 anness error can occur if a compiled pattern is saved and reloaded on a 2087 different host. Here is a typical call of pcre_fullinfo(), to obtain 2088 the length of the compiled pattern: 2089 2090 int rc; 2091 size_t length; 2092 rc = pcre_fullinfo( 2093 re, /* result of pcre_compile() */ 2094 sd, /* result of pcre_study(), or NULL */ 2095 PCRE_INFO_SIZE, /* what is required */ 2096 &length); /* where to put the data */ 2097 2098 The possible values for the third argument are defined in pcre.h, and 2099 are as follows: 2100 2101 PCRE_INFO_BACKREFMAX 2102 2103 Return the number of the highest back reference in the pattern. The 2104 fourth argument should point to an int variable. Zero is returned if 2105 there are no back references. 2106 2107 PCRE_INFO_CAPTURECOUNT 2108 2109 Return the number of capturing subpatterns in the pattern. The fourth 2110 argument should point to an int variable. 2111 2112 PCRE_INFO_DEFAULT_TABLES 2113 2114 Return a pointer to the internal default character tables within PCRE. 2115 The fourth argument should point to an unsigned char * variable. This 2116 information call is provided for internal use by the pcre_study() func- 2117 tion. External callers can cause PCRE to use its internal tables by 2118 passing a NULL table pointer. 2119 2120 PCRE_INFO_FIRSTBYTE 2121 2122 Return information about the first data unit of any matched string, for 2123 a non-anchored pattern. (The name of this option refers to the 8-bit 2124 library, where data units are bytes.) The fourth argument should point 2125 to an int variable. 2126 2127 If there is a fixed first value, for example, the letter "c" from a 2128 pattern such as (cat|cow|coyote), its value is returned. In the 8-bit 2129 library, the value is always less than 256; in the 16-bit library the 2130 value can be up to 0xffff. 2131 2132 If there is no fixed first value, and if either 2133 2134 (a) the pattern was compiled with the PCRE_MULTILINE option, and every 2135 branch starts with "^", or 2136 2137 (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not 2138 set (if it were set, the pattern would be anchored), 2139 2140 -1 is returned, indicating that the pattern matches only at the start 2141 of a subject string or after any newline within the string. Otherwise 2142 -2 is returned. For anchored patterns, -2 is returned. 2143 2144 PCRE_INFO_FIRSTTABLE 2145 2146 If the pattern was studied, and this resulted in the construction of a 2147 256-bit table indicating a fixed set of values for the first data unit 2148 in any matching string, a pointer to the table is returned. Otherwise 2149 NULL is returned. The fourth argument should point to an unsigned char 2150 * variable. 2151 2152 PCRE_INFO_HASCRORLF 2153 2154 Return 1 if the pattern contains any explicit matches for CR or LF 2155 characters, otherwise 0. The fourth argument should point to an int 2156 variable. An explicit match is either a literal CR or LF character, or 2157 \r or \n. 2158 2159 PCRE_INFO_JCHANGED 2160 2161 Return 1 if the (?J) or (?-J) option setting is used in the pattern, 2162 otherwise 0. The fourth argument should point to an int variable. (?J) 2163 and (?-J) set and unset the local PCRE_DUPNAMES option, respectively. 2164 2165 PCRE_INFO_JIT 2166 2167 Return 1 if the pattern was studied with one of the JIT options, and 2168 just-in-time compiling was successful. The fourth argument should point 2169 to an int variable. A return value of 0 means that JIT support is not 2170 available in this version of PCRE, or that the pattern was not studied 2171 with a JIT option, or that the JIT compiler could not handle this par- 2172 ticular pattern. See the pcrejit documentation for details of what can 2173 and cannot be handled. 2174 2175 PCRE_INFO_JITSIZE 2176 2177 If the pattern was successfully studied with a JIT option, return the 2178 size of the JIT compiled code, otherwise return zero. The fourth argu- 2179 ment should point to a size_t variable. 2180 2181 PCRE_INFO_LASTLITERAL 2182 2183 Return the value of the rightmost literal data unit that must exist in 2184 any matched string, other than at its start, if such a value has been 2185 recorded. The fourth argument should point to an int variable. If there 2186 is no such value, -1 is returned. For anchored patterns, a last literal 2187 value is recorded only if it follows something of variable length. For 2188 example, for the pattern /^a\d+z\d+/ the returned value is "z", but for 2189 /^a\dz\d/ the returned value is -1. 2190 2191 PCRE_INFO_MAXLOOKBEHIND 2192 2193 Return the number of characters (NB not bytes) in the longest lookbe- 2194 hind assertion in the pattern. Note that the simple assertions \b and 2195 \B require a one-character lookbehind. This information is useful when 2196 doing multi-segment matching using the partial matching facilities. 2197 2198 PCRE_INFO_MINLENGTH 2199 2200 If the pattern was studied and a minimum length for matching subject 2201 strings was computed, its value is returned. Otherwise the returned 2202 value is -1. The value is a number of characters, which in UTF-8 mode 2203 may be different from the number of bytes. The fourth argument should 2204 point to an int variable. A non-negative value is a lower bound to the 2205 length of any matching string. There may not be any strings of that 2206 length that do actually match, but every string that does match is at 2207 least that long. 2208 2209 PCRE_INFO_NAMECOUNT 2210 PCRE_INFO_NAMEENTRYSIZE 2211 PCRE_INFO_NAMETABLE 2212 2213 PCRE supports the use of named as well as numbered capturing parenthe- 2214 ses. The names are just an additional way of identifying the parenthe- 2215 ses, which still acquire numbers. Several convenience functions such as 2216 pcre_get_named_substring() are provided for extracting captured sub- 2217 strings by name. It is also possible to extract the data directly, by 2218 first converting the name to a number in order to access the correct 2219 pointers in the output vector (described with pcre_exec() below). To do 2220 the conversion, you need to use the name-to-number map, which is 2221 described by these three values. 2222 2223 The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT 2224 gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size 2225 of each entry; both of these return an int value. The entry size 2226 depends on the length of the longest name. PCRE_INFO_NAMETABLE returns 2227 a pointer to the first entry of the table. This is a pointer to char in 2228 the 8-bit library, where the first two bytes of each entry are the num- 2229 ber of the capturing parenthesis, most significant byte first. In the 2230 16-bit library, the pointer points to 16-bit data units, the first of 2231 which contains the parenthesis number. The rest of the entry is the 2232 corresponding name, zero terminated. 2233 2234 The names are in alphabetical order. Duplicate names may appear if (?| 2235 is used to create multiple groups with the same number, as described in 2236 the section on duplicate subpattern numbers in the pcrepattern page. 2237 Duplicate names for subpatterns with different numbers are permitted 2238 only if PCRE_DUPNAMES is set. In all cases of duplicate names, they 2239 appear in the table in the order in which they were found in the pat- 2240 tern. In the absence of (?| this is the order of increasing number; 2241 when (?| is used this is not necessarily the case because later subpat- 2242 terns may have lower numbers. 2243 2244 As a simple example of the name/number table, consider the following 2245 pattern after compilation by the 8-bit library (assume PCRE_EXTENDED is 2246 set, so white space - including newlines - is ignored): 2247 2248 (?<date> (?<year>(\d\d)?\d\d) - 2249 (?<month>\d\d) - (?<day>\d\d) ) 2250 2251 There are four named subpatterns, so the table has four entries, and 2252 each entry in the table is eight bytes long. The table is as follows, 2253 with non-printing bytes shows in hexadecimal, and undefined bytes shown 2254 as ??: 2255 2256 00 01 d a t e 00 ?? 2257 00 05 d a y 00 ?? ?? 2258 00 04 m o n t h 00 2259 00 02 y e a r 00 ?? 2260 2261 When writing code to extract data from named subpatterns using the 2262 name-to-number map, remember that the length of the entries is likely 2263 to be different for each compiled pattern. 2264 2265 PCRE_INFO_OKPARTIAL 2266 2267 Return 1 if the pattern can be used for partial matching with 2268 pcre_exec(), otherwise 0. The fourth argument should point to an int 2269 variable. From release 8.00, this always returns 1, because the 2270 restrictions that previously applied to partial matching have been 2271 lifted. The pcrepartial documentation gives details of partial match- 2272 ing. 2273 2274 PCRE_INFO_OPTIONS 2275 2276 Return a copy of the options with which the pattern was compiled. The 2277 fourth argument should point to an unsigned long int variable. These 2278 option bits are those specified in the call to pcre_compile(), modified 2279 by any top-level option settings at the start of the pattern itself. In 2280 other words, they are the options that will be in force when matching 2281 starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with 2282 the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE, 2283 and PCRE_EXTENDED. 2284 2285 A pattern is automatically anchored by PCRE if all of its top-level 2286 alternatives begin with one of the following: 2287 2288 ^ unless PCRE_MULTILINE is set 2289 \A always 2290 \G always 2291 .* if PCRE_DOTALL is set and there are no back 2292 references to the subpattern in which .* appears 2293 2294 For such patterns, the PCRE_ANCHORED bit is set in the options returned 2295 by pcre_fullinfo(). 2296 2297 PCRE_INFO_SIZE 2298 2299 Return the size of the compiled pattern in bytes (for both libraries). 2300 The fourth argument should point to a size_t variable. This value does 2301 not include the size of the pcre structure that is returned by 2302 pcre_compile(). The value that is passed as the argument to pcre_mal- 2303 loc() when pcre_compile() is getting memory in which to place the com- 2304 piled data is the value returned by this option plus the size of the 2305 pcre structure. Studying a compiled pattern, with or without JIT, does 2306 not alter the value returned by this option. 2307 2308 PCRE_INFO_STUDYSIZE 2309 2310 Return the size in bytes of the data block pointed to by the study_data 2311 field in a pcre_extra block. If pcre_extra is NULL, or there is no 2312 study data, zero is returned. The fourth argument should point to a 2313 size_t variable. The study_data field is set by pcre_study() to record 2314 information that will speed up matching (see the section entitled 2315 "Studying a pattern" above). The format of the study_data block is pri- 2316 vate, but its length is made available via this option so that it can 2317 be saved and restored (see the pcreprecompile documentation for 2318 details). 2319 2320 2321REFERENCE COUNTS 2322 2323 int pcre_refcount(pcre *code, int adjust); 2324 2325 The pcre_refcount() function is used to maintain a reference count in 2326 the data block that contains a compiled pattern. It is provided for the 2327 benefit of applications that operate in an object-oriented manner, 2328 where different parts of the application may be using the same compiled 2329 pattern, but you want to free the block when they are all done. 2330 2331 When a pattern is compiled, the reference count field is initialized to 2332 zero. It is changed only by calling this function, whose action is to 2333 add the adjust value (which may be positive or negative) to it. The 2334 yield of the function is the new value. However, the value of the count 2335 is constrained to lie between 0 and 65535, inclusive. If the new value 2336 is outside these limits, it is forced to the appropriate limit value. 2337 2338 Except when it is zero, the reference count is not correctly preserved 2339 if a pattern is compiled on one host and then transferred to a host 2340 whose byte-order is different. (This seems a highly unlikely scenario.) 2341 2342 2343MATCHING A PATTERN: THE TRADITIONAL FUNCTION 2344 2345 int pcre_exec(const pcre *code, const pcre_extra *extra, 2346 const char *subject, int length, int startoffset, 2347 int options, int *ovector, int ovecsize); 2348 2349 The function pcre_exec() is called to match a subject string against a 2350 compiled pattern, which is passed in the code argument. If the pattern 2351 was studied, the result of the study should be passed in the extra 2352 argument. You can call pcre_exec() with the same code and extra argu- 2353 ments as many times as you like, in order to match different subject 2354 strings with the same pattern. 2355 2356 This function is the main matching facility of the library, and it 2357 operates in a Perl-like manner. For specialist use there is also an 2358 alternative matching function, which is described below in the section 2359 about the pcre_dfa_exec() function. 2360 2361 In most applications, the pattern will have been compiled (and option- 2362 ally studied) in the same process that calls pcre_exec(). However, it 2363 is possible to save compiled patterns and study data, and then use them 2364 later in different processes, possibly even on different hosts. For a 2365 discussion about this, see the pcreprecompile documentation. 2366 2367 Here is an example of a simple call to pcre_exec(): 2368 2369 int rc; 2370 int ovector[30]; 2371 rc = pcre_exec( 2372 re, /* result of pcre_compile() */ 2373 NULL, /* we didn't study the pattern */ 2374 "some string", /* the subject string */ 2375 11, /* the length of the subject string */ 2376 0, /* start at offset 0 in the subject */ 2377 0, /* default options */ 2378 ovector, /* vector of integers for substring information */ 2379 30); /* number of elements (NOT size in bytes) */ 2380 2381 Extra data for pcre_exec() 2382 2383 If the extra argument is not NULL, it must point to a pcre_extra data 2384 block. The pcre_study() function returns such a block (when it doesn't 2385 return NULL), but you can also create one for yourself, and pass addi- 2386 tional information in it. The pcre_extra block contains the following 2387 fields (not necessarily in this order): 2388 2389 unsigned long int flags; 2390 void *study_data; 2391 void *executable_jit; 2392 unsigned long int match_limit; 2393 unsigned long int match_limit_recursion; 2394 void *callout_data; 2395 const unsigned char *tables; 2396 unsigned char **mark; 2397 2398 In the 16-bit version of this structure, the mark field has type 2399 "PCRE_UCHAR16 **". 2400 2401 The flags field is used to specify which of the other fields are set. 2402 The flag bits are: 2403 2404 PCRE_EXTRA_CALLOUT_DATA 2405 PCRE_EXTRA_EXECUTABLE_JIT 2406 PCRE_EXTRA_MARK 2407 PCRE_EXTRA_MATCH_LIMIT 2408 PCRE_EXTRA_MATCH_LIMIT_RECURSION 2409 PCRE_EXTRA_STUDY_DATA 2410 PCRE_EXTRA_TABLES 2411 2412 Other flag bits should be set to zero. The study_data field and some- 2413 times the executable_jit field are set in the pcre_extra block that is 2414 returned by pcre_study(), together with the appropriate flag bits. You 2415 should not set these yourself, but you may add to the block by setting 2416 other fields and their corresponding flag bits. 2417 2418 The match_limit field provides a means of preventing PCRE from using up 2419 a vast amount of resources when running patterns that are not going to 2420 match, but which have a very large number of possibilities in their 2421 search trees. The classic example is a pattern that uses nested unlim- 2422 ited repeats. 2423 2424 Internally, pcre_exec() uses a function called match(), which it calls 2425 repeatedly (sometimes recursively). The limit set by match_limit is 2426 imposed on the number of times this function is called during a match, 2427 which has the effect of limiting the amount of backtracking that can 2428 take place. For patterns that are not anchored, the count restarts from 2429 zero for each position in the subject string. 2430 2431 When pcre_exec() is called with a pattern that was successfully studied 2432 with a JIT option, the way that the matching is executed is entirely 2433 different. However, there is still the possibility of runaway matching 2434 that goes on for a very long time, and so the match_limit value is also 2435 used in this case (but in a different way) to limit how long the match- 2436 ing can continue. 2437 2438 The default value for the limit can be set when PCRE is built; the 2439 default default is 10 million, which handles all but the most extreme 2440 cases. You can override the default by suppling pcre_exec() with a 2441 pcre_extra block in which match_limit is set, and 2442 PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is 2443 exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT. 2444 2445 The match_limit_recursion field is similar to match_limit, but instead 2446 of limiting the total number of times that match() is called, it limits 2447 the depth of recursion. The recursion depth is a smaller number than 2448 the total number of calls, because not all calls to match() are recur- 2449 sive. This limit is of use only if it is set smaller than match_limit. 2450 2451 Limiting the recursion depth limits the amount of machine stack that 2452 can be used, or, when PCRE has been compiled to use memory on the heap 2453 instead of the stack, the amount of heap memory that can be used. This 2454 limit is not relevant, and is ignored, when matching is done using JIT 2455 compiled code. 2456 2457 The default value for match_limit_recursion can be set when PCRE is 2458 built; the default default is the same value as the default for 2459 match_limit. You can override the default by suppling pcre_exec() with 2460 a pcre_extra block in which match_limit_recursion is set, and 2461 PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the 2462 limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT. 2463 2464 The callout_data field is used in conjunction with the "callout" fea- 2465 ture, and is described in the pcrecallout documentation. 2466 2467 The tables field is used to pass a character tables pointer to 2468 pcre_exec(); this overrides the value that is stored with the compiled 2469 pattern. A non-NULL value is stored with the compiled pattern only if 2470 custom tables were supplied to pcre_compile() via its tableptr argu- 2471 ment. If NULL is passed to pcre_exec() using this mechanism, it forces 2472 PCRE's internal tables to be used. This facility is helpful when re- 2473 using patterns that have been saved after compiling with an external 2474 set of tables, because the external tables might be at a different 2475 address when pcre_exec() is called. See the pcreprecompile documenta- 2476 tion for a discussion of saving compiled patterns for later use. 2477 2478 If PCRE_EXTRA_MARK is set in the flags field, the mark field must be 2479 set to point to a suitable variable. If the pattern contains any back- 2480 tracking control verbs such as (*MARK:NAME), and the execution ends up 2481 with a name to pass back, a pointer to the name string (zero termi- 2482 nated) is placed in the variable pointed to by the mark field. The 2483 names are within the compiled pattern; if you wish to retain such a 2484 name you must copy it before freeing the memory of a compiled pattern. 2485 If there is no name to pass back, the variable pointed to by the mark 2486 field is set to NULL. For details of the backtracking control verbs, 2487 see the section entitled "Backtracking control" in the pcrepattern doc- 2488 umentation. 2489 2490 Option bits for pcre_exec() 2491 2492 The unused bits of the options argument for pcre_exec() must be zero. 2493 The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx, 2494 PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, 2495 PCRE_NO_START_OPTIMIZE, PCRE_NO_UTF8_CHECK, PCRE_PARTIAL_HARD, and 2496 PCRE_PARTIAL_SOFT. 2497 2498 If the pattern was successfully studied with one of the just-in-time 2499 (JIT) compile options, the only supported options for JIT execution are 2500 PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, 2501 PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and PCRE_PARTIAL_SOFT. If an 2502 unsupported option is used, JIT execution is disabled and the normal 2503 interpretive code in pcre_exec() is run. 2504 2505 PCRE_ANCHORED 2506 2507 The PCRE_ANCHORED option limits pcre_exec() to matching at the first 2508 matching position. If a pattern was compiled with PCRE_ANCHORED, or 2509 turned out to be anchored by virtue of its contents, it cannot be made 2510 unachored at matching time. 2511 2512 PCRE_BSR_ANYCRLF 2513 PCRE_BSR_UNICODE 2514 2515 These options (which are mutually exclusive) control what the \R escape 2516 sequence matches. The choice is either to match only CR, LF, or CRLF, 2517 or to match any Unicode newline sequence. These options override the 2518 choice that was made or defaulted when the pattern was compiled. 2519 2520 PCRE_NEWLINE_CR 2521 PCRE_NEWLINE_LF 2522 PCRE_NEWLINE_CRLF 2523 PCRE_NEWLINE_ANYCRLF 2524 PCRE_NEWLINE_ANY 2525 2526 These options override the newline definition that was chosen or 2527 defaulted when the pattern was compiled. For details, see the descrip- 2528 tion of pcre_compile() above. During matching, the newline choice 2529 affects the behaviour of the dot, circumflex, and dollar metacharac- 2530 ters. It may also alter the way the match position is advanced after a 2531 match failure for an unanchored pattern. 2532 2533 When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is 2534 set, and a match attempt for an unanchored pattern fails when the cur- 2535 rent position is at a CRLF sequence, and the pattern contains no 2536 explicit matches for CR or LF characters, the match position is 2537 advanced by two characters instead of one, in other words, to after the 2538 CRLF. 2539 2540 The above rule is a compromise that makes the most common cases work as 2541 expected. For example, if the pattern is .+A (and the PCRE_DOTALL 2542 option is not set), it does not match the string "\r\nA" because, after 2543 failing at the start, it skips both the CR and the LF before retrying. 2544 However, the pattern [\r\n]A does match that string, because it con- 2545 tains an explicit CR or LF reference, and so advances only by one char- 2546 acter after the first failure. 2547 2548 An explicit match for CR of LF is either a literal appearance of one of 2549 those characters, or one of the \r or \n escape sequences. Implicit 2550 matches such as [^X] do not count, nor does \s (which includes CR and 2551 LF in the characters that it matches). 2552 2553 Notwithstanding the above, anomalous effects may still occur when CRLF 2554 is a valid newline sequence and explicit \r or \n escapes appear in the 2555 pattern. 2556 2557 PCRE_NOTBOL 2558 2559 This option specifies that first character of the subject string is not 2560 the beginning of a line, so the circumflex metacharacter should not 2561 match before it. Setting this without PCRE_MULTILINE (at compile time) 2562 causes circumflex never to match. This option affects only the behav- 2563 iour of the circumflex metacharacter. It does not affect \A. 2564 2565 PCRE_NOTEOL 2566 2567 This option specifies that the end of the subject string is not the end 2568 of a line, so the dollar metacharacter should not match it nor (except 2569 in multiline mode) a newline immediately before it. Setting this with- 2570 out PCRE_MULTILINE (at compile time) causes dollar never to match. This 2571 option affects only the behaviour of the dollar metacharacter. It does 2572 not affect \Z or \z. 2573 2574 PCRE_NOTEMPTY 2575 2576 An empty string is not considered to be a valid match if this option is 2577 set. If there are alternatives in the pattern, they are tried. If all 2578 the alternatives match the empty string, the entire match fails. For 2579 example, if the pattern 2580 2581 a?b? 2582 2583 is applied to a string not beginning with "a" or "b", it matches an 2584 empty string at the start of the subject. With PCRE_NOTEMPTY set, this 2585 match is not valid, so PCRE searches further into the string for occur- 2586 rences of "a" or "b". 2587 2588 PCRE_NOTEMPTY_ATSTART 2589 2590 This is like PCRE_NOTEMPTY, except that an empty string match that is 2591 not at the start of the subject is permitted. If the pattern is 2592 anchored, such a match can occur only if the pattern contains \K. 2593 2594 Perl has no direct equivalent of PCRE_NOTEMPTY or 2595 PCRE_NOTEMPTY_ATSTART, but it does make a special case of a pattern 2596 match of the empty string within its split() function, and when using 2597 the /g modifier. It is possible to emulate Perl's behaviour after 2598 matching a null string by first trying the match again at the same off- 2599 set with PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED, and then if that 2600 fails, by advancing the starting offset (see below) and trying an ordi- 2601 nary match again. There is some code that demonstrates how to do this 2602 in the pcredemo sample program. In the most general case, you have to 2603 check to see if the newline convention recognizes CRLF as a newline, 2604 and if so, and the current character is CR followed by LF, advance the 2605 starting offset by two characters instead of one. 2606 2607 PCRE_NO_START_OPTIMIZE 2608 2609 There are a number of optimizations that pcre_exec() uses at the start 2610 of a match, in order to speed up the process. For example, if it is 2611 known that an unanchored match must start with a specific character, it 2612 searches the subject for that character, and fails immediately if it 2613 cannot find it, without actually running the main matching function. 2614 This means that a special item such as (*COMMIT) at the start of a pat- 2615 tern is not considered until after a suitable starting point for the 2616 match has been found. When callouts or (*MARK) items are in use, these 2617 "start-up" optimizations can cause them to be skipped if the pattern is 2618 never actually used. The start-up optimizations are in effect a pre- 2619 scan of the subject that takes place before the pattern is run. 2620 2621 The PCRE_NO_START_OPTIMIZE option disables the start-up optimizations, 2622 possibly causing performance to suffer, but ensuring that in cases 2623 where the result is "no match", the callouts do occur, and that items 2624 such as (*COMMIT) and (*MARK) are considered at every possible starting 2625 position in the subject string. If PCRE_NO_START_OPTIMIZE is set at 2626 compile time, it cannot be unset at matching time. The use of 2627 PCRE_NO_START_OPTIMIZE disables JIT execution; when it is set, matching 2628 is always done using interpretively. 2629 2630 Setting PCRE_NO_START_OPTIMIZE can change the outcome of a matching 2631 operation. Consider the pattern 2632 2633 (*COMMIT)ABC 2634 2635 When this is compiled, PCRE records the fact that a match must start 2636 with the character "A". Suppose the subject string is "DEFABC". The 2637 start-up optimization scans along the subject, finds "A" and runs the 2638 first match attempt from there. The (*COMMIT) item means that the pat- 2639 tern must match the current starting position, which in this case, it 2640 does. However, if the same match is run with PCRE_NO_START_OPTIMIZE 2641 set, the initial scan along the subject string does not happen. The 2642 first match attempt is run starting from "D" and when this fails, 2643 (*COMMIT) prevents any further matches being tried, so the overall 2644 result is "no match". If the pattern is studied, more start-up opti- 2645 mizations may be used. For example, a minimum length for the subject 2646 may be recorded. Consider the pattern 2647 2648 (*MARK:A)(X|Y) 2649 2650 The minimum length for a match is one character. If the subject is 2651 "ABC", there will be attempts to match "ABC", "BC", "C", and then 2652 finally an empty string. If the pattern is studied, the final attempt 2653 does not take place, because PCRE knows that the subject is too short, 2654 and so the (*MARK) is never encountered. In this case, studying the 2655 pattern does not affect the overall match result, which is still "no 2656 match", but it does affect the auxiliary information that is returned. 2657 2658 PCRE_NO_UTF8_CHECK 2659 2660 When PCRE_UTF8 is set at compile time, the validity of the subject as a 2661 UTF-8 string is automatically checked when pcre_exec() is subsequently 2662 called. The entire string is checked before any other processing takes 2663 place. The value of startoffset is also checked to ensure that it 2664 points to the start of a UTF-8 character. There is a discussion about 2665 the validity of UTF-8 strings in the pcreunicode page. If an invalid 2666 sequence of bytes is found, pcre_exec() returns the error 2667 PCRE_ERROR_BADUTF8 or, if PCRE_PARTIAL_HARD is set and the problem is a 2668 truncated character at the end of the subject, PCRE_ERROR_SHORTUTF8. In 2669 both cases, information about the precise nature of the error may also 2670 be returned (see the descriptions of these errors in the section enti- 2671 tled Error return values from pcre_exec() below). If startoffset con- 2672 tains a value that does not point to the start of a UTF-8 character (or 2673 to the end of the subject), PCRE_ERROR_BADUTF8_OFFSET is returned. 2674 2675 If you already know that your subject is valid, and you want to skip 2676 these checks for performance reasons, you can set the 2677 PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to 2678 do this for the second and subsequent calls to pcre_exec() if you are 2679 making repeated calls to find all the matches in a single subject 2680 string. However, you should be sure that the value of startoffset 2681 points to the start of a character (or the end of the subject). When 2682 PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid string as a 2683 subject or an invalid value of startoffset is undefined. Your program 2684 may crash. 2685 2686 PCRE_PARTIAL_HARD 2687 PCRE_PARTIAL_SOFT 2688 2689 These options turn on the partial matching feature. For backwards com- 2690 patibility, PCRE_PARTIAL is a synonym for PCRE_PARTIAL_SOFT. A partial 2691 match occurs if the end of the subject string is reached successfully, 2692 but there are not enough subject characters to complete the match. If 2693 this happens when PCRE_PARTIAL_SOFT (but not PCRE_PARTIAL_HARD) is set, 2694 matching continues by testing any remaining alternatives. Only if no 2695 complete match can be found is PCRE_ERROR_PARTIAL returned instead of 2696 PCRE_ERROR_NOMATCH. In other words, PCRE_PARTIAL_SOFT says that the 2697 caller is prepared to handle a partial match, but only if no complete 2698 match can be found. 2699 2700 If PCRE_PARTIAL_HARD is set, it overrides PCRE_PARTIAL_SOFT. In this 2701 case, if a partial match is found, pcre_exec() immediately returns 2702 PCRE_ERROR_PARTIAL, without considering any other alternatives. In 2703 other words, when PCRE_PARTIAL_HARD is set, a partial match is consid- 2704 ered to be more important that an alternative complete match. 2705 2706 In both cases, the portion of the string that was inspected when the 2707 partial match was found is set as the first matching string. There is a 2708 more detailed discussion of partial and multi-segment matching, with 2709 examples, in the pcrepartial documentation. 2710 2711 The string to be matched by pcre_exec() 2712 2713 The subject string is passed to pcre_exec() as a pointer in subject, a 2714 length in bytes in length, and a starting byte offset in startoffset. 2715 If this is negative or greater than the length of the subject, 2716 pcre_exec() returns PCRE_ERROR_BADOFFSET. When the starting offset is 2717 zero, the search for a match starts at the beginning of the subject, 2718 and this is by far the most common case. In UTF-8 mode, the byte offset 2719 must point to the start of a UTF-8 character (or the end of the sub- 2720 ject). Unlike the pattern string, the subject may contain binary zero 2721 bytes. 2722 2723 A non-zero starting offset is useful when searching for another match 2724 in the same subject by calling pcre_exec() again after a previous suc- 2725 cess. Setting startoffset differs from just passing over a shortened 2726 string and setting PCRE_NOTBOL in the case of a pattern that begins 2727 with any kind of lookbehind. For example, consider the pattern 2728 2729 \Biss\B 2730 2731 which finds occurrences of "iss" in the middle of words. (\B matches 2732 only if the current position in the subject is not a word boundary.) 2733 When applied to the string "Mississipi" the first call to pcre_exec() 2734 finds the first occurrence. If pcre_exec() is called again with just 2735 the remainder of the subject, namely "issipi", it does not match, 2736 because \B is always false at the start of the subject, which is deemed 2737 to be a word boundary. However, if pcre_exec() is passed the entire 2738 string again, but with startoffset set to 4, it finds the second occur- 2739 rence of "iss" because it is able to look behind the starting point to 2740 discover that it is preceded by a letter. 2741 2742 Finding all the matches in a subject is tricky when the pattern can 2743 match an empty string. It is possible to emulate Perl's /g behaviour by 2744 first trying the match again at the same offset, with the 2745 PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED options, and then if that 2746 fails, advancing the starting offset and trying an ordinary match 2747 again. There is some code that demonstrates how to do this in the pcre- 2748 demo sample program. In the most general case, you have to check to see 2749 if the newline convention recognizes CRLF as a newline, and if so, and 2750 the current character is CR followed by LF, advance the starting offset 2751 by two characters instead of one. 2752 2753 If a non-zero starting offset is passed when the pattern is anchored, 2754 one attempt to match at the given offset is made. This can only succeed 2755 if the pattern does not require the match to be at the start of the 2756 subject. 2757 2758 How pcre_exec() returns captured substrings 2759 2760 In general, a pattern matches a certain portion of the subject, and in 2761 addition, further substrings from the subject may be picked out by 2762 parts of the pattern. Following the usage in Jeffrey Friedl's book, 2763 this is called "capturing" in what follows, and the phrase "capturing 2764 subpattern" is used for a fragment of a pattern that picks out a sub- 2765 string. PCRE supports several other kinds of parenthesized subpattern 2766 that do not cause substrings to be captured. 2767 2768 Captured substrings are returned to the caller via a vector of integers 2769 whose address is passed in ovector. The number of elements in the vec- 2770 tor is passed in ovecsize, which must be a non-negative number. Note: 2771 this argument is NOT the size of ovector in bytes. 2772 2773 The first two-thirds of the vector is used to pass back captured sub- 2774 strings, each substring using a pair of integers. The remaining third 2775 of the vector is used as workspace by pcre_exec() while matching cap- 2776 turing subpatterns, and is not available for passing back information. 2777 The number passed in ovecsize should always be a multiple of three. If 2778 it is not, it is rounded down. 2779 2780 When a match is successful, information about captured substrings is 2781 returned in pairs of integers, starting at the beginning of ovector, 2782 and continuing up to two-thirds of its length at the most. The first 2783 element of each pair is set to the byte offset of the first character 2784 in a substring, and the second is set to the byte offset of the first 2785 character after the end of a substring. Note: these values are always 2786 byte offsets, even in UTF-8 mode. They are not character counts. 2787 2788 The first pair of integers, ovector[0] and ovector[1], identify the 2789 portion of the subject string matched by the entire pattern. The next 2790 pair is used for the first capturing subpattern, and so on. The value 2791 returned by pcre_exec() is one more than the highest numbered pair that 2792 has been set. For example, if two substrings have been captured, the 2793 returned value is 3. If there are no capturing subpatterns, the return 2794 value from a successful match is 1, indicating that just the first pair 2795 of offsets has been set. 2796 2797 If a capturing subpattern is matched repeatedly, it is the last portion 2798 of the string that it matched that is returned. 2799 2800 If the vector is too small to hold all the captured substring offsets, 2801 it is used as far as possible (up to two-thirds of its length), and the 2802 function returns a value of zero. If neither the actual string matched 2803 nor any captured substrings are of interest, pcre_exec() may be called 2804 with ovector passed as NULL and ovecsize as zero. However, if the pat- 2805 tern contains back references and the ovector is not big enough to 2806 remember the related substrings, PCRE has to get additional memory for 2807 use during matching. Thus it is usually advisable to supply an ovector 2808 of reasonable size. 2809 2810 There are some cases where zero is returned (indicating vector over- 2811 flow) when in fact the vector is exactly the right size for the final 2812 match. For example, consider the pattern 2813 2814 (a)(?:(b)c|bd) 2815 2816 If a vector of 6 elements (allowing for only 1 captured substring) is 2817 given with subject string "abd", pcre_exec() will try to set the second 2818 captured string, thereby recording a vector overflow, before failing to 2819 match "c" and backing up to try the second alternative. The zero 2820 return, however, does correctly indicate that the maximum number of 2821 slots (namely 2) have been filled. In similar cases where there is tem- 2822 porary overflow, but the final number of used slots is actually less 2823 than the maximum, a non-zero value is returned. 2824 2825 The pcre_fullinfo() function can be used to find out how many capturing 2826 subpatterns there are in a compiled pattern. The smallest size for 2827 ovector that will allow for n captured substrings, in addition to the 2828 offsets of the substring matched by the whole pattern, is (n+1)*3. 2829 2830 It is possible for capturing subpattern number n+1 to match some part 2831 of the subject when subpattern n has not been used at all. For example, 2832 if the string "abc" is matched against the pattern (a|(z))(bc) the 2833 return from the function is 4, and subpatterns 1 and 3 are matched, but 2834 2 is not. When this happens, both values in the offset pairs corre- 2835 sponding to unused subpatterns are set to -1. 2836 2837 Offset values that correspond to unused subpatterns at the end of the 2838 expression are also set to -1. For example, if the string "abc" is 2839 matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not 2840 matched. The return from the function is 2, because the highest used 2841 capturing subpattern number is 1, and the offsets for for the second 2842 and third capturing subpatterns (assuming the vector is large enough, 2843 of course) are set to -1. 2844 2845 Note: Elements in the first two-thirds of ovector that do not corre- 2846 spond to capturing parentheses in the pattern are never changed. That 2847 is, if a pattern contains n capturing parentheses, no more than ovec- 2848 tor[0] to ovector[2n+1] are set by pcre_exec(). The other elements (in 2849 the first two-thirds) retain whatever values they previously had. 2850 2851 Some convenience functions are provided for extracting the captured 2852 substrings as separate strings. These are described below. 2853 2854 Error return values from pcre_exec() 2855 2856 If pcre_exec() fails, it returns a negative number. The following are 2857 defined in the header file: 2858 2859 PCRE_ERROR_NOMATCH (-1) 2860 2861 The subject string did not match the pattern. 2862 2863 PCRE_ERROR_NULL (-2) 2864 2865 Either code or subject was passed as NULL, or ovector was NULL and 2866 ovecsize was not zero. 2867 2868 PCRE_ERROR_BADOPTION (-3) 2869 2870 An unrecognized bit was set in the options argument. 2871 2872 PCRE_ERROR_BADMAGIC (-4) 2873 2874 PCRE stores a 4-byte "magic number" at the start of the compiled code, 2875 to catch the case when it is passed a junk pointer and to detect when a 2876 pattern that was compiled in an environment of one endianness is run in 2877 an environment with the other endianness. This is the error that PCRE 2878 gives when the magic number is not present. 2879 2880 PCRE_ERROR_UNKNOWN_OPCODE (-5) 2881 2882 While running the pattern match, an unknown item was encountered in the 2883 compiled pattern. This error could be caused by a bug in PCRE or by 2884 overwriting of the compiled pattern. 2885 2886 PCRE_ERROR_NOMEMORY (-6) 2887 2888 If a pattern contains back references, but the ovector that is passed 2889 to pcre_exec() is not big enough to remember the referenced substrings, 2890 PCRE gets a block of memory at the start of matching to use for this 2891 purpose. If the call via pcre_malloc() fails, this error is given. The 2892 memory is automatically freed at the end of matching. 2893 2894 This error is also given if pcre_stack_malloc() fails in pcre_exec(). 2895 This can happen only when PCRE has been compiled with --disable-stack- 2896 for-recursion. 2897 2898 PCRE_ERROR_NOSUBSTRING (-7) 2899 2900 This error is used by the pcre_copy_substring(), pcre_get_substring(), 2901 and pcre_get_substring_list() functions (see below). It is never 2902 returned by pcre_exec(). 2903 2904 PCRE_ERROR_MATCHLIMIT (-8) 2905 2906 The backtracking limit, as specified by the match_limit field in a 2907 pcre_extra structure (or defaulted) was reached. See the description 2908 above. 2909 2910 PCRE_ERROR_CALLOUT (-9) 2911 2912 This error is never generated by pcre_exec() itself. It is provided for 2913 use by callout functions that want to yield a distinctive error code. 2914 See the pcrecallout documentation for details. 2915 2916 PCRE_ERROR_BADUTF8 (-10) 2917 2918 A string that contains an invalid UTF-8 byte sequence was passed as a 2919 subject, and the PCRE_NO_UTF8_CHECK option was not set. If the size of 2920 the output vector (ovecsize) is at least 2, the byte offset to the 2921 start of the the invalid UTF-8 character is placed in the first ele- 2922 ment, and a reason code is placed in the second element. The reason 2923 codes are listed in the following section. For backward compatibility, 2924 if PCRE_PARTIAL_HARD is set and the problem is a truncated UTF-8 char- 2925 acter at the end of the subject (reason codes 1 to 5), 2926 PCRE_ERROR_SHORTUTF8 is returned instead of PCRE_ERROR_BADUTF8. 2927 2928 PCRE_ERROR_BADUTF8_OFFSET (-11) 2929 2930 The UTF-8 byte sequence that was passed as a subject was checked and 2931 found to be valid (the PCRE_NO_UTF8_CHECK option was not set), but the 2932 value of startoffset did not point to the beginning of a UTF-8 charac- 2933 ter or the end of the subject. 2934 2935 PCRE_ERROR_PARTIAL (-12) 2936 2937 The subject string did not match, but it did match partially. See the 2938 pcrepartial documentation for details of partial matching. 2939 2940 PCRE_ERROR_BADPARTIAL (-13) 2941 2942 This code is no longer in use. It was formerly returned when the 2943 PCRE_PARTIAL option was used with a compiled pattern containing items 2944 that were not supported for partial matching. From release 8.00 2945 onwards, there are no restrictions on partial matching. 2946 2947 PCRE_ERROR_INTERNAL (-14) 2948 2949 An unexpected internal error has occurred. This error could be caused 2950 by a bug in PCRE or by overwriting of the compiled pattern. 2951 2952 PCRE_ERROR_BADCOUNT (-15) 2953 2954 This error is given if the value of the ovecsize argument is negative. 2955 2956 PCRE_ERROR_RECURSIONLIMIT (-21) 2957 2958 The internal recursion limit, as specified by the match_limit_recursion 2959 field in a pcre_extra structure (or defaulted) was reached. See the 2960 description above. 2961 2962 PCRE_ERROR_BADNEWLINE (-23) 2963 2964 An invalid combination of PCRE_NEWLINE_xxx options was given. 2965 2966 PCRE_ERROR_BADOFFSET (-24) 2967 2968 The value of startoffset was negative or greater than the length of the 2969 subject, that is, the value in length. 2970 2971 PCRE_ERROR_SHORTUTF8 (-25) 2972 2973 This error is returned instead of PCRE_ERROR_BADUTF8 when the subject 2974 string ends with a truncated UTF-8 character and the PCRE_PARTIAL_HARD 2975 option is set. Information about the failure is returned as for 2976 PCRE_ERROR_BADUTF8. It is in fact sufficient to detect this case, but 2977 this special error code for PCRE_PARTIAL_HARD precedes the implementa- 2978 tion of returned information; it is retained for backwards compatibil- 2979 ity. 2980 2981 PCRE_ERROR_RECURSELOOP (-26) 2982 2983 This error is returned when pcre_exec() detects a recursion loop within 2984 the pattern. Specifically, it means that either the whole pattern or a 2985 subpattern has been called recursively for the second time at the same 2986 position in the subject string. Some simple patterns that might do this 2987 are detected and faulted at compile time, but more complicated cases, 2988 in particular mutual recursions between two different subpatterns, can- 2989 not be detected until run time. 2990 2991 PCRE_ERROR_JIT_STACKLIMIT (-27) 2992 2993 This error is returned when a pattern that was successfully studied 2994 using a JIT compile option is being matched, but the memory available 2995 for the just-in-time processing stack is not large enough. See the 2996 pcrejit documentation for more details. 2997 2998 PCRE_ERROR_BADMODE (-28) 2999 3000 This error is given if a pattern that was compiled by the 8-bit library 3001 is passed to a 16-bit library function, or vice versa. 3002 3003 PCRE_ERROR_BADENDIANNESS (-29) 3004 3005 This error is given if a pattern that was compiled and saved is 3006 reloaded on a host with different endianness. The utility function 3007 pcre_pattern_to_host_byte_order() can be used to convert such a pattern 3008 so that it runs on the new host. 3009 3010 Error numbers -16 to -20, -22, and -30 are not used by pcre_exec(). 3011 3012 Reason codes for invalid UTF-8 strings 3013 3014 This section applies only to the 8-bit library. The corresponding 3015 information for the 16-bit library is given in the pcre16 page. 3016 3017 When pcre_exec() returns either PCRE_ERROR_BADUTF8 or PCRE_ERROR_SHORT- 3018 UTF8, and the size of the output vector (ovecsize) is at least 2, the 3019 offset of the start of the invalid UTF-8 character is placed in the 3020 first output vector element (ovector[0]) and a reason code is placed in 3021 the second element (ovector[1]). The reason codes are given names in 3022 the pcre.h header file: 3023 3024 PCRE_UTF8_ERR1 3025 PCRE_UTF8_ERR2 3026 PCRE_UTF8_ERR3 3027 PCRE_UTF8_ERR4 3028 PCRE_UTF8_ERR5 3029 3030 The string ends with a truncated UTF-8 character; the code specifies 3031 how many bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8 3032 characters to be no longer than 4 bytes, the encoding scheme (origi- 3033 nally defined by RFC 2279) allows for up to 6 bytes, and this is 3034 checked first; hence the possibility of 4 or 5 missing bytes. 3035 3036 PCRE_UTF8_ERR6 3037 PCRE_UTF8_ERR7 3038 PCRE_UTF8_ERR8 3039 PCRE_UTF8_ERR9 3040 PCRE_UTF8_ERR10 3041 3042 The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of 3043 the character do not have the binary value 0b10 (that is, either the 3044 most significant bit is 0, or the next bit is 1). 3045 3046 PCRE_UTF8_ERR11 3047 PCRE_UTF8_ERR12 3048 3049 A character that is valid by the RFC 2279 rules is either 5 or 6 bytes 3050 long; these code points are excluded by RFC 3629. 3051 3052 PCRE_UTF8_ERR13 3053 3054 A 4-byte character has a value greater than 0x10fff; these code points 3055 are excluded by RFC 3629. 3056 3057 PCRE_UTF8_ERR14 3058 3059 A 3-byte character has a value in the range 0xd800 to 0xdfff; this 3060 range of code points are reserved by RFC 3629 for use with UTF-16, and 3061 so are excluded from UTF-8. 3062 3063 PCRE_UTF8_ERR15 3064 PCRE_UTF8_ERR16 3065 PCRE_UTF8_ERR17 3066 PCRE_UTF8_ERR18 3067 PCRE_UTF8_ERR19 3068 3069 A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes 3070 for a value that can be represented by fewer bytes, which is invalid. 3071 For example, the two bytes 0xc0, 0xae give the value 0x2e, whose cor- 3072 rect coding uses just one byte. 3073 3074 PCRE_UTF8_ERR20 3075 3076 The two most significant bits of the first byte of a character have the 3077 binary value 0b10 (that is, the most significant bit is 1 and the sec- 3078 ond is 0). Such a byte can only validly occur as the second or subse- 3079 quent byte of a multi-byte character. 3080 3081 PCRE_UTF8_ERR21 3082 3083 The first byte of a character has the value 0xfe or 0xff. These values 3084 can never occur in a valid UTF-8 string. 3085 3086 3087EXTRACTING CAPTURED SUBSTRINGS BY NUMBER 3088 3089 int pcre_copy_substring(const char *subject, int *ovector, 3090 int stringcount, int stringnumber, char *buffer, 3091 int buffersize); 3092 3093 int pcre_get_substring(const char *subject, int *ovector, 3094 int stringcount, int stringnumber, 3095 const char **stringptr); 3096 3097 int pcre_get_substring_list(const char *subject, 3098 int *ovector, int stringcount, const char ***listptr); 3099 3100 Captured substrings can be accessed directly by using the offsets 3101 returned by pcre_exec() in ovector. For convenience, the functions 3102 pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub- 3103 string_list() are provided for extracting captured substrings as new, 3104 separate, zero-terminated strings. These functions identify substrings 3105 by number. The next section describes functions for extracting named 3106 substrings. 3107 3108 A substring that contains a binary zero is correctly extracted and has 3109 a further zero added on the end, but the result is not, of course, a C 3110 string. However, you can process such a string by referring to the 3111 length that is returned by pcre_copy_substring() and pcre_get_sub- 3112 string(). Unfortunately, the interface to pcre_get_substring_list() is 3113 not adequate for handling strings containing binary zeros, because the 3114 end of the final string is not independently indicated. 3115 3116 The first three arguments are the same for all three of these func- 3117 tions: subject is the subject string that has just been successfully 3118 matched, ovector is a pointer to the vector of integer offsets that was 3119 passed to pcre_exec(), and stringcount is the number of substrings that 3120 were captured by the match, including the substring that matched the 3121 entire regular expression. This is the value returned by pcre_exec() if 3122 it is greater than zero. If pcre_exec() returned zero, indicating that 3123 it ran out of space in ovector, the value passed as stringcount should 3124 be the number of elements in the vector divided by three. 3125 3126 The functions pcre_copy_substring() and pcre_get_substring() extract a 3127 single substring, whose number is given as stringnumber. A value of 3128 zero extracts the substring that matched the entire pattern, whereas 3129 higher values extract the captured substrings. For pcre_copy_sub- 3130 string(), the string is placed in buffer, whose length is given by 3131 buffersize, while for pcre_get_substring() a new block of memory is 3132 obtained via pcre_malloc, and its address is returned via stringptr. 3133 The yield of the function is the length of the string, not including 3134 the terminating zero, or one of these error codes: 3135 3136 PCRE_ERROR_NOMEMORY (-6) 3137 3138 The buffer was too small for pcre_copy_substring(), or the attempt to 3139 get memory failed for pcre_get_substring(). 3140 3141 PCRE_ERROR_NOSUBSTRING (-7) 3142 3143 There is no substring whose number is stringnumber. 3144 3145 The pcre_get_substring_list() function extracts all available sub- 3146 strings and builds a list of pointers to them. All this is done in a 3147 single block of memory that is obtained via pcre_malloc. The address of 3148 the memory block is returned via listptr, which is also the start of 3149 the list of string pointers. The end of the list is marked by a NULL 3150 pointer. The yield of the function is zero if all went well, or the 3151 error code 3152 3153 PCRE_ERROR_NOMEMORY (-6) 3154 3155 if the attempt to get the memory block failed. 3156 3157 When any of these functions encounter a substring that is unset, which 3158 can happen when capturing subpattern number n+1 matches some part of 3159 the subject, but subpattern n has not been used at all, they return an 3160 empty string. This can be distinguished from a genuine zero-length sub- 3161 string by inspecting the appropriate offset in ovector, which is nega- 3162 tive for unset substrings. 3163 3164 The two convenience functions pcre_free_substring() and pcre_free_sub- 3165 string_list() can be used to free the memory returned by a previous 3166 call of pcre_get_substring() or pcre_get_substring_list(), respec- 3167 tively. They do nothing more than call the function pointed to by 3168 pcre_free, which of course could be called directly from a C program. 3169 However, PCRE is used in some situations where it is linked via a spe- 3170 cial interface to another programming language that cannot use 3171 pcre_free directly; it is for these cases that the functions are pro- 3172 vided. 3173 3174 3175EXTRACTING CAPTURED SUBSTRINGS BY NAME 3176 3177 int pcre_get_stringnumber(const pcre *code, 3178 const char *name); 3179 3180 int pcre_copy_named_substring(const pcre *code, 3181 const char *subject, int *ovector, 3182 int stringcount, const char *stringname, 3183 char *buffer, int buffersize); 3184 3185 int pcre_get_named_substring(const pcre *code, 3186 const char *subject, int *ovector, 3187 int stringcount, const char *stringname, 3188 const char **stringptr); 3189 3190 To extract a substring by name, you first have to find associated num- 3191 ber. For example, for this pattern 3192 3193 (a+)b(?<xxx>\d+)... 3194 3195 the number of the subpattern called "xxx" is 2. If the name is known to 3196 be unique (PCRE_DUPNAMES was not set), you can find the number from the 3197 name by calling pcre_get_stringnumber(). The first argument is the com- 3198 piled pattern, and the second is the name. The yield of the function is 3199 the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no 3200 subpattern of that name. 3201 3202 Given the number, you can extract the substring directly, or use one of 3203 the functions described in the previous section. For convenience, there 3204 are also two functions that do the whole job. 3205 3206 Most of the arguments of pcre_copy_named_substring() and 3207 pcre_get_named_substring() are the same as those for the similarly 3208 named functions that extract by number. As these are described in the 3209 previous section, they are not re-described here. There are just two 3210 differences: 3211 3212 First, instead of a substring number, a substring name is given. Sec- 3213 ond, there is an extra argument, given at the start, which is a pointer 3214 to the compiled pattern. This is needed in order to gain access to the 3215 name-to-number translation table. 3216 3217 These functions call pcre_get_stringnumber(), and if it succeeds, they 3218 then call pcre_copy_substring() or pcre_get_substring(), as appropri- 3219 ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the 3220 behaviour may not be what you want (see the next section). 3221 3222 Warning: If the pattern uses the (?| feature to set up multiple subpat- 3223 terns with the same number, as described in the section on duplicate 3224 subpattern numbers in the pcrepattern page, you cannot use names to 3225 distinguish the different subpatterns, because names are not included 3226 in the compiled code. The matching process uses only numbers. For this 3227 reason, the use of different names for subpatterns of the same number 3228 causes an error at compile time. 3229 3230 3231DUPLICATE SUBPATTERN NAMES 3232 3233 int pcre_get_stringtable_entries(const pcre *code, 3234 const char *name, char **first, char **last); 3235 3236 When a pattern is compiled with the PCRE_DUPNAMES option, names for 3237 subpatterns are not required to be unique. (Duplicate names are always 3238 allowed for subpatterns with the same number, created by using the (?| 3239 feature. Indeed, if such subpatterns are named, they are required to 3240 use the same names.) 3241 3242 Normally, patterns with duplicate names are such that in any one match, 3243 only one of the named subpatterns participates. An example is shown in 3244 the pcrepattern documentation. 3245 3246 When duplicates are present, pcre_copy_named_substring() and 3247 pcre_get_named_substring() return the first substring corresponding to 3248 the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING 3249 (-7) is returned; no data is returned. The pcre_get_stringnumber() 3250 function returns one of the numbers that are associated with the name, 3251 but it is not defined which it is. 3252 3253 If you want to get full details of all captured substrings for a given 3254 name, you must use the pcre_get_stringtable_entries() function. The 3255 first argument is the compiled pattern, and the second is the name. The 3256 third and fourth are pointers to variables which are updated by the 3257 function. After it has run, they point to the first and last entries in 3258 the name-to-number table for the given name. The function itself 3259 returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if 3260 there are none. The format of the table is described above in the sec- 3261 tion entitled Information about a pattern above. Given all the rele- 3262 vant entries for the name, you can extract each of their numbers, and 3263 hence the captured data, if any. 3264 3265 3266FINDING ALL POSSIBLE MATCHES 3267 3268 The traditional matching function uses a similar algorithm to Perl, 3269 which stops when it finds the first match, starting at a given point in 3270 the subject. If you want to find all possible matches, or the longest 3271 possible match, consider using the alternative matching function (see 3272 below) instead. If you cannot use the alternative function, but still 3273 need to find all possible matches, you can kludge it up by making use 3274 of the callout facility, which is described in the pcrecallout documen- 3275 tation. 3276 3277 What you have to do is to insert a callout right at the end of the pat- 3278 tern. When your callout function is called, extract and save the cur- 3279 rent matched substring. Then return 1, which forces pcre_exec() to 3280 backtrack and try other alternatives. Ultimately, when it runs out of 3281 matches, pcre_exec() will yield PCRE_ERROR_NOMATCH. 3282 3283 3284OBTAINING AN ESTIMATE OF STACK USAGE 3285 3286 Matching certain patterns using pcre_exec() can use a lot of process 3287 stack, which in certain environments can be rather limited in size. 3288 Some users find it helpful to have an estimate of the amount of stack 3289 that is used by pcre_exec(), to help them set recursion limits, as 3290 described in the pcrestack documentation. The estimate that is output 3291 by pcretest when called with the -m and -C options is obtained by call- 3292 ing pcre_exec with the values NULL, NULL, NULL, -999, and -999 for its 3293 first five arguments. 3294 3295 Normally, if its first argument is NULL, pcre_exec() immediately 3296 returns the negative error code PCRE_ERROR_NULL, but with this special 3297 combination of arguments, it returns instead a negative number whose 3298 absolute value is the approximate stack frame size in bytes. (A nega- 3299 tive number is used so that it is clear that no match has happened.) 3300 The value is approximate because in some cases, recursive calls to 3301 pcre_exec() occur when there are one or two additional variables on the 3302 stack. 3303 3304 If PCRE has been compiled to use the heap instead of the stack for 3305 recursion, the value returned is the size of each block that is 3306 obtained from the heap. 3307 3308 3309MATCHING A PATTERN: THE ALTERNATIVE FUNCTION 3310 3311 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra, 3312 const char *subject, int length, int startoffset, 3313 int options, int *ovector, int ovecsize, 3314 int *workspace, int wscount); 3315 3316 The function pcre_dfa_exec() is called to match a subject string 3317 against a compiled pattern, using a matching algorithm that scans the 3318 subject string just once, and does not backtrack. This has different 3319 characteristics to the normal algorithm, and is not compatible with 3320 Perl. Some of the features of PCRE patterns are not supported. Never- 3321 theless, there are times when this kind of matching can be useful. For 3322 a discussion of the two matching algorithms, and a list of features 3323 that pcre_dfa_exec() does not support, see the pcrematching documenta- 3324 tion. 3325 3326 The arguments for the pcre_dfa_exec() function are the same as for 3327 pcre_exec(), plus two extras. The ovector argument is used in a differ- 3328 ent way, and this is described below. The other common arguments are 3329 used in the same way as for pcre_exec(), so their description is not 3330 repeated here. 3331 3332 The two additional arguments provide workspace for the function. The 3333 workspace vector should contain at least 20 elements. It is used for 3334 keeping track of multiple paths through the pattern tree. More 3335 workspace will be needed for patterns and subjects where there are a 3336 lot of potential matches. 3337 3338 Here is an example of a simple call to pcre_dfa_exec(): 3339 3340 int rc; 3341 int ovector[10]; 3342 int wspace[20]; 3343 rc = pcre_dfa_exec( 3344 re, /* result of pcre_compile() */ 3345 NULL, /* we didn't study the pattern */ 3346 "some string", /* the subject string */ 3347 11, /* the length of the subject string */ 3348 0, /* start at offset 0 in the subject */ 3349 0, /* default options */ 3350 ovector, /* vector of integers for substring information */ 3351 10, /* number of elements (NOT size in bytes) */ 3352 wspace, /* working space vector */ 3353 20); /* number of elements (NOT size in bytes) */ 3354 3355 Option bits for pcre_dfa_exec() 3356 3357 The unused bits of the options argument for pcre_dfa_exec() must be 3358 zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW- 3359 LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, 3360 PCRE_NOTEMPTY_ATSTART, PCRE_NO_UTF8_CHECK, PCRE_BSR_ANYCRLF, 3361 PCRE_BSR_UNICODE, PCRE_NO_START_OPTIMIZE, PCRE_PARTIAL_HARD, PCRE_PAR- 3362 TIAL_SOFT, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last 3363 four of these are exactly the same as for pcre_exec(), so their 3364 description is not repeated here. 3365 3366 PCRE_PARTIAL_HARD 3367 PCRE_PARTIAL_SOFT 3368 3369 These have the same general effect as they do for pcre_exec(), but the 3370 details are slightly different. When PCRE_PARTIAL_HARD is set for 3371 pcre_dfa_exec(), it returns PCRE_ERROR_PARTIAL if the end of the sub- 3372 ject is reached and there is still at least one matching possibility 3373 that requires additional characters. This happens even if some complete 3374 matches have also been found. When PCRE_PARTIAL_SOFT is set, the return 3375 code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end 3376 of the subject is reached, there have been no complete matches, but 3377 there is still at least one matching possibility. The portion of the 3378 string that was inspected when the longest partial match was found is 3379 set as the first matching string in both cases. There is a more 3380 detailed discussion of partial and multi-segment matching, with exam- 3381 ples, in the pcrepartial documentation. 3382 3383 PCRE_DFA_SHORTEST 3384 3385 Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to 3386 stop as soon as it has found one match. Because of the way the alterna- 3387 tive algorithm works, this is necessarily the shortest possible match 3388 at the first possible matching point in the subject string. 3389 3390 PCRE_DFA_RESTART 3391 3392 When pcre_dfa_exec() returns a partial match, it is possible to call it 3393 again, with additional subject characters, and have it continue with 3394 the same match. The PCRE_DFA_RESTART option requests this action; when 3395 it is set, the workspace and wscount options must reference the same 3396 vector as before because data about the match so far is left in them 3397 after a partial match. There is more discussion of this facility in the 3398 pcrepartial documentation. 3399 3400 Successful returns from pcre_dfa_exec() 3401 3402 When pcre_dfa_exec() succeeds, it may have matched more than one sub- 3403 string in the subject. Note, however, that all the matches from one run 3404 of the function start at the same point in the subject. The shorter 3405 matches are all initial substrings of the longer matches. For example, 3406 if the pattern 3407 3408 <.*> 3409 3410 is matched against the string 3411 3412 This is <something> <something else> <something further> no more 3413 3414 the three matched strings are 3415 3416 <something> 3417 <something> <something else> 3418 <something> <something else> <something further> 3419 3420 On success, the yield of the function is a number greater than zero, 3421 which is the number of matched substrings. The substrings themselves 3422 are returned in ovector. Each string uses two elements; the first is 3423 the offset to the start, and the second is the offset to the end. In 3424 fact, all the strings have the same start offset. (Space could have 3425 been saved by giving this only once, but it was decided to retain some 3426 compatibility with the way pcre_exec() returns data, even though the 3427 meaning of the strings is different.) 3428 3429 The strings are returned in reverse order of length; that is, the long- 3430 est matching string is given first. If there were too many matches to 3431 fit into ovector, the yield of the function is zero, and the vector is 3432 filled with the longest matches. Unlike pcre_exec(), pcre_dfa_exec() 3433 can use the entire ovector for returning matched strings. 3434 3435 Error returns from pcre_dfa_exec() 3436 3437 The pcre_dfa_exec() function returns a negative number when it fails. 3438 Many of the errors are the same as for pcre_exec(), and these are 3439 described above. There are in addition the following errors that are 3440 specific to pcre_dfa_exec(): 3441 3442 PCRE_ERROR_DFA_UITEM (-16) 3443 3444 This return is given if pcre_dfa_exec() encounters an item in the pat- 3445 tern that it does not support, for instance, the use of \C or a back 3446 reference. 3447 3448 PCRE_ERROR_DFA_UCOND (-17) 3449 3450 This return is given if pcre_dfa_exec() encounters a condition item 3451 that uses a back reference for the condition, or a test for recursion 3452 in a specific group. These are not supported. 3453 3454 PCRE_ERROR_DFA_UMLIMIT (-18) 3455 3456 This return is given if pcre_dfa_exec() is called with an extra block 3457 that contains a setting of the match_limit or match_limit_recursion 3458 fields. This is not supported (these fields are meaningless for DFA 3459 matching). 3460 3461 PCRE_ERROR_DFA_WSSIZE (-19) 3462 3463 This return is given if pcre_dfa_exec() runs out of space in the 3464 workspace vector. 3465 3466 PCRE_ERROR_DFA_RECURSE (-20) 3467 3468 When a recursive subpattern is processed, the matching function calls 3469 itself recursively, using private vectors for ovector and workspace. 3470 This error is given if the output vector is not large enough. This 3471 should be extremely rare, as a vector of size 1000 is used. 3472 3473 PCRE_ERROR_DFA_BADRESTART (-30) 3474 3475 When pcre_dfa_exec() is called with the PCRE_DFA_RESTART option, some 3476 plausibility checks are made on the contents of the workspace, which 3477 should contain data about the previous partial match. If any of these 3478 checks fail, this error is given. 3479 3480 3481SEE ALSO 3482 3483 pcre16(3), pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematch- 3484 ing(3), pcrepartial(3), pcreposix(3), pcreprecompile(3), pcresample(3), 3485 pcrestack(3). 3486 3487 3488AUTHOR 3489 3490 Philip Hazel 3491 University Computing Service 3492 Cambridge CB2 3QH, England. 3493 3494 3495REVISION 3496 3497 Last updated: 17 June 2012 3498 Copyright (c) 1997-2012 University of Cambridge. 3499------------------------------------------------------------------------------ 3500 3501 3502PCRECALLOUT(3) PCRECALLOUT(3) 3503 3504 3505NAME 3506 PCRE - Perl-compatible regular expressions 3507 3508 3509PCRE CALLOUTS 3510 3511 int (*pcre_callout)(pcre_callout_block *); 3512 3513 int (*pcre16_callout)(pcre16_callout_block *); 3514 3515 PCRE provides a feature called "callout", which is a means of temporar- 3516 ily passing control to the caller of PCRE in the middle of pattern 3517 matching. The caller of PCRE provides an external function by putting 3518 its entry point in the global variable pcre_callout (pcre16_callout for 3519 the 16-bit library). By default, this variable contains NULL, which 3520 disables all calling out. 3521 3522 Within a regular expression, (?C) indicates the points at which the 3523 external function is to be called. Different callout points can be 3524 identified by putting a number less than 256 after the letter C. The 3525 default value is zero. For example, this pattern has two callout 3526 points: 3527 3528 (?C1)abc(?C2)def 3529 3530 If the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled, 3531 PCRE automatically inserts callouts, all with number 255, before each 3532 item in the pattern. For example, if PCRE_AUTO_CALLOUT is used with the 3533 pattern 3534 3535 A(\d{2}|--) 3536 3537 it is processed as if it were 3538 3539 (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255) 3540 3541 Notice that there is a callout before and after each parenthesis and 3542 alternation bar. Automatic callouts can be used for tracking the 3543 progress of pattern matching. The pcretest command has an option that 3544 sets automatic callouts; when it is used, the output indicates how the 3545 pattern is matched. This is useful information when you are trying to 3546 optimize the performance of a particular pattern. 3547 3548 The use of callouts in a pattern makes it ineligible for optimization 3549 by the just-in-time compiler. Studying such a pattern with the 3550 PCRE_STUDY_JIT_COMPILE option always fails. 3551 3552 3553MISSING CALLOUTS 3554 3555 You should be aware that, because of optimizations in the way PCRE 3556 matches patterns by default, callouts sometimes do not happen. For 3557 example, if the pattern is 3558 3559 ab(?C4)cd 3560 3561 PCRE knows that any matching string must contain the letter "d". If the 3562 subject string is "abyz", the lack of "d" means that matching doesn't 3563 ever start, and the callout is never reached. However, with "abyd", 3564 though the result is still no match, the callout is obeyed. 3565 3566 If the pattern is studied, PCRE knows the minimum length of a matching 3567 string, and will immediately give a "no match" return without actually 3568 running a match if the subject is not long enough, or, for unanchored 3569 patterns, if it has been scanned far enough. 3570 3571 You can disable these optimizations by passing the PCRE_NO_START_OPTI- 3572 MIZE option to the matching function, or by starting the pattern with 3573 (*NO_START_OPT). This slows down the matching process, but does ensure 3574 that callouts such as the example above are obeyed. 3575 3576 3577THE CALLOUT INTERFACE 3578 3579 During matching, when PCRE reaches a callout point, the external func- 3580 tion defined by pcre_callout or pcre16_callout is called (if it is 3581 set). This applies to both normal and DFA matching. The only argument 3582 to the callout function is a pointer to a pcre_callout or pcre16_call- 3583 out block. These structures contains the following fields: 3584 3585 int version; 3586 int callout_number; 3587 int *offset_vector; 3588 const char *subject; (8-bit version) 3589 PCRE_SPTR16 subject; (16-bit version) 3590 int subject_length; 3591 int start_match; 3592 int current_position; 3593 int capture_top; 3594 int capture_last; 3595 void *callout_data; 3596 int pattern_position; 3597 int next_item_length; 3598 const unsigned char *mark; (8-bit version) 3599 const PCRE_UCHAR16 *mark; (16-bit version) 3600 3601 The version field is an integer containing the version number of the 3602 block format. The initial version was 0; the current version is 2. The 3603 version number will change again in future if additional fields are 3604 added, but the intention is never to remove any of the existing fields. 3605 3606 The callout_number field contains the number of the callout, as com- 3607 piled into the pattern (that is, the number after ?C for manual call- 3608 outs, and 255 for automatically generated callouts). 3609 3610 The offset_vector field is a pointer to the vector of offsets that was 3611 passed by the caller to the matching function. When pcre_exec() or 3612 pcre16_exec() is used, the contents can be inspected, in order to 3613 extract substrings that have been matched so far, in the same way as 3614 for extracting substrings after a match has completed. For the DFA 3615 matching functions, this field is not useful. 3616 3617 The subject and subject_length fields contain copies of the values that 3618 were passed to the matching function. 3619 3620 The start_match field normally contains the offset within the subject 3621 at which the current match attempt started. However, if the escape 3622 sequence \K has been encountered, this value is changed to reflect the 3623 modified starting point. If the pattern is not anchored, the callout 3624 function may be called several times from the same point in the pattern 3625 for different starting points in the subject. 3626 3627 The current_position field contains the offset within the subject of 3628 the current match pointer. 3629 3630 When the pcre_exec() or pcre16_exec() is used, the capture_top field 3631 contains one more than the number of the highest numbered captured sub- 3632 string so far. If no substrings have been captured, the value of cap- 3633 ture_top is one. This is always the case when the DFA functions are 3634 used, because they do not support captured substrings. 3635 3636 The capture_last field contains the number of the most recently cap- 3637 tured substring. If no substrings have been captured, its value is -1. 3638 This is always the case for the DFA matching functions. 3639 3640 The callout_data field contains a value that is passed to a matching 3641 function specifically so that it can be passed back in callouts. It is 3642 passed in the callout_data field of a pcre_extra or pcre16_extra data 3643 structure. If no such data was passed, the value of callout_data in a 3644 callout block is NULL. There is a description of the pcre_extra struc- 3645 ture in the pcreapi documentation. 3646 3647 The pattern_position field is present from version 1 of the callout 3648 structure. It contains the offset to the next item to be matched in the 3649 pattern string. 3650 3651 The next_item_length field is present from version 1 of the callout 3652 structure. It contains the length of the next item to be matched in the 3653 pattern string. When the callout immediately precedes an alternation 3654 bar, a closing parenthesis, or the end of the pattern, the length is 3655 zero. When the callout precedes an opening parenthesis, the length is 3656 that of the entire subpattern. 3657 3658 The pattern_position and next_item_length fields are intended to help 3659 in distinguishing between different automatic callouts, which all have 3660 the same callout number. However, they are set for all callouts. 3661 3662 The mark field is present from version 2 of the callout structure. In 3663 callouts from pcre_exec() or pcre16_exec() it contains a pointer to the 3664 zero-terminated name of the most recently passed (*MARK), (*PRUNE), or 3665 (*THEN) item in the match, or NULL if no such items have been passed. 3666 Instances of (*PRUNE) or (*THEN) without a name do not obliterate a 3667 previous (*MARK). In callouts from the DFA matching functions this 3668 field always contains NULL. 3669 3670 3671RETURN VALUES 3672 3673 The external callout function returns an integer to PCRE. If the value 3674 is zero, matching proceeds as normal. If the value is greater than 3675 zero, matching fails at the current point, but the testing of other 3676 matching possibilities goes ahead, just as if a lookahead assertion had 3677 failed. If the value is less than zero, the match is abandoned, the 3678 matching function returns the negative value. 3679 3680 Negative values should normally be chosen from the set of 3681 PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan- 3682 dard "no match" failure. The error number PCRE_ERROR_CALLOUT is 3683 reserved for use by callout functions; it will never be used by PCRE 3684 itself. 3685 3686 3687AUTHOR 3688 3689 Philip Hazel 3690 University Computing Service 3691 Cambridge CB2 3QH, England. 3692 3693 3694REVISION 3695 3696 Last updated: 08 Janurary 2012 3697 Copyright (c) 1997-2012 University of Cambridge. 3698------------------------------------------------------------------------------ 3699 3700 3701PCRECOMPAT(3) PCRECOMPAT(3) 3702 3703 3704NAME 3705 PCRE - Perl-compatible regular expressions 3706 3707 3708DIFFERENCES BETWEEN PCRE AND PERL 3709 3710 This document describes the differences in the ways that PCRE and Perl 3711 handle regular expressions. The differences described here are with 3712 respect to Perl versions 5.10 and above. 3713 3714 1. PCRE has only a subset of Perl's Unicode support. Details of what it 3715 does have are given in the pcreunicode page. 3716 3717 2. PCRE allows repeat quantifiers only on parenthesized assertions, but 3718 they do not mean what you might think. For example, (?!a){3} does not 3719 assert that the next three characters are not "a". It just asserts that 3720 the next character is not "a" three times (in principle: PCRE optimizes 3721 this to run the assertion just once). Perl allows repeat quantifiers on 3722 other assertions such as \b, but these do not seem to have any use. 3723 3724 3. Capturing subpatterns that occur inside negative lookahead asser- 3725 tions are counted, but their entries in the offsets vector are never 3726 set. Perl sets its numerical variables from any such patterns that are 3727 matched before the assertion fails to match something (thereby succeed- 3728 ing), but only if the negative lookahead assertion contains just one 3729 branch. 3730 3731 4. Though binary zero characters are supported in the subject string, 3732 they are not allowed in a pattern string because it is passed as a nor- 3733 mal C string, terminated by zero. The escape sequence \0 can be used in 3734 the pattern to represent a binary zero. 3735 3736 5. The following Perl escape sequences are not supported: \l, \u, \L, 3737 \U, and \N when followed by a character name or Unicode value. (\N on 3738 its own, matching a non-newline character, is supported.) In fact these 3739 are implemented by Perl's general string-handling and are not part of 3740 its pattern matching engine. If any of these are encountered by PCRE, 3741 an error is generated by default. However, if the PCRE_JAVASCRIPT_COM- 3742 PAT option is set, \U and \u are interpreted as JavaScript interprets 3743 them. 3744 3745 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE 3746 is built with Unicode character property support. The properties that 3747 can be tested with \p and \P are limited to the general category prop- 3748 erties such as Lu and Nd, script names such as Greek or Han, and the 3749 derived properties Any and L&. PCRE does support the Cs (surrogate) 3750 property, which Perl does not; the Perl documentation says "Because 3751 Perl hides the need for the user to understand the internal representa- 3752 tion of Unicode characters, there is no need to implement the somewhat 3753 messy concept of surrogates." 3754 3755 7. PCRE implements a simpler version of \X than Perl, which changed to 3756 make \X match what Unicode calls an "extended grapheme cluster". This 3757 is more complicated than an extended Unicode sequence, which is what 3758 PCRE matches. 3759 3760 8. PCRE does support the \Q...\E escape for quoting substrings. Charac- 3761 ters in between are treated as literals. This is slightly different 3762 from Perl in that $ and @ are also handled as literals inside the 3763 quotes. In Perl, they cause variable interpolation (but of course PCRE 3764 does not have variables). Note the following examples: 3765 3766 Pattern PCRE matches Perl matches 3767 3768 \Qabc$xyz\E abc$xyz abc followed by the 3769 contents of $xyz 3770 \Qabc\$xyz\E abc\$xyz abc\$xyz 3771 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz 3772 3773 The \Q...\E sequence is recognized both inside and outside character 3774 classes. 3775 3776 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code}) 3777 constructions. However, there is support for recursive patterns. This 3778 is not available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE 3779 "callout" feature allows an external function to be called during pat- 3780 tern matching. See the pcrecallout documentation for details. 3781 3782 10. Subpatterns that are called as subroutines (whether or not recur- 3783 sively) are always treated as atomic groups in PCRE. This is like 3784 Python, but unlike Perl. Captured values that are set outside a sub- 3785 routine call can be reference from inside in PCRE, but not in Perl. 3786 There is a discussion that explains these differences in more detail in 3787 the section on recursion differences from Perl in the pcrepattern page. 3788 3789 11. If any of the backtracking control verbs are used in an assertion 3790 or in a subpattern that is called as a subroutine (whether or not 3791 recursively), their effect is confined to that subpattern; it does not 3792 extend to the surrounding pattern. This is not always the case in Perl. 3793 In particular, if (*THEN) is present in a group that is called as a 3794 subroutine, its action is limited to that group, even if the group does 3795 not contain any | characters. There is one exception to this: the name 3796 from a *(MARK), (*PRUNE), or (*THEN) that is encountered in a success- 3797 ful positive assertion is passed back when a match succeeds (compare 3798 capturing parentheses in assertions). Note that such subpatterns are 3799 processed as anchored at the point where they are tested. 3800 3801 12. There are some differences that are concerned with the settings of 3802 captured strings when part of a pattern is repeated. For example, 3803 matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2 3804 unset, but in PCRE it is set to "b". 3805 3806 13. PCRE's handling of duplicate subpattern numbers and duplicate sub- 3807 pattern names is not as general as Perl's. This is a consequence of the 3808 fact the PCRE works internally just with numbers, using an external ta- 3809 ble to translate between numbers and names. In particular, a pattern 3810 such as (?|(?<a>A)|(?<b)B), where the two capturing parentheses have 3811 the same number but different names, is not supported, and causes an 3812 error at compile time. If it were allowed, it would not be possible to 3813 distinguish which parentheses matched, because both names map to cap- 3814 turing subpattern number 1. To avoid this confusing situation, an error 3815 is given at compile time. 3816 3817 14. Perl recognizes comments in some places that PCRE does not, for 3818 example, between the ( and ? at the start of a subpattern. If the /x 3819 modifier is set, Perl allows white space between ( and ? but PCRE never 3820 does, even if the PCRE_EXTENDED option is set. 3821 3822 15. PCRE provides some extensions to the Perl regular expression facil- 3823 ities. Perl 5.10 includes new features that are not in earlier ver- 3824 sions of Perl, some of which (such as named parentheses) have been in 3825 PCRE for some time. This list is with respect to Perl 5.10: 3826 3827 (a) Although lookbehind assertions in PCRE must match fixed length 3828 strings, each alternative branch of a lookbehind assertion can match a 3829 different length of string. Perl requires them all to have the same 3830 length. 3831 3832 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ 3833 meta-character matches only at the very end of the string. 3834 3835 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe- 3836 cial meaning is faulted. Otherwise, like Perl, the backslash is quietly 3837 ignored. (Perl can be made to issue a warning.) 3838 3839 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti- 3840 fiers is inverted, that is, by default they are not greedy, but if fol- 3841 lowed by a question mark they are. 3842 3843 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be 3844 tried only at the first matching position in the subject string. 3845 3846 (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, 3847 and PCRE_NO_AUTO_CAPTURE options for pcre_exec() have no Perl equiva- 3848 lents. 3849 3850 (g) The \R escape sequence can be restricted to match only CR, LF, or 3851 CRLF by the PCRE_BSR_ANYCRLF option. 3852 3853 (h) The callout facility is PCRE-specific. 3854 3855 (i) The partial matching facility is PCRE-specific. 3856 3857 (j) Patterns compiled by PCRE can be saved and re-used at a later time, 3858 even on different hosts that have the other endianness. However, this 3859 does not apply to optimized data created by the just-in-time compiler. 3860 3861 (k) The alternative matching functions (pcre_dfa_exec() and 3862 pcre16_dfa_exec()) match in a different way and are not Perl-compati- 3863 ble. 3864 3865 (l) PCRE recognizes some special sequences such as (*CR) at the start 3866 of a pattern that set overall options that cannot be changed within the 3867 pattern. 3868 3869 3870AUTHOR 3871 3872 Philip Hazel 3873 University Computing Service 3874 Cambridge CB2 3QH, England. 3875 3876 3877REVISION 3878 3879 Last updated: 01 June 2012 3880 Copyright (c) 1997-2012 University of Cambridge. 3881------------------------------------------------------------------------------ 3882 3883 3884PCREPATTERN(3) PCREPATTERN(3) 3885 3886 3887NAME 3888 PCRE - Perl-compatible regular expressions 3889 3890 3891PCRE REGULAR EXPRESSION DETAILS 3892 3893 The syntax and semantics of the regular expressions that are supported 3894 by PCRE are described in detail below. There is a quick-reference syn- 3895 tax summary in the pcresyntax page. PCRE tries to match Perl syntax and 3896 semantics as closely as it can. PCRE also supports some alternative 3897 regular expression syntax (which does not conflict with the Perl syn- 3898 tax) in order to provide some compatibility with regular expressions in 3899 Python, .NET, and Oniguruma. 3900 3901 Perl's regular expressions are described in its own documentation, and 3902 regular expressions in general are covered in a number of books, some 3903 of which have copious examples. Jeffrey Friedl's "Mastering Regular 3904 Expressions", published by O'Reilly, covers regular expressions in 3905 great detail. This description of PCRE's regular expressions is 3906 intended as reference material. 3907 3908 The original operation of PCRE was on strings of one-byte characters. 3909 However, there is now also support for UTF-8 strings in the original 3910 library, and a second library that supports 16-bit and UTF-16 character 3911 strings. To use these features, PCRE must be built to include appropri- 3912 ate support. When using UTF strings you must either call the compiling 3913 function with the PCRE_UTF8 or PCRE_UTF16 option, or the pattern must 3914 start with one of these special sequences: 3915 3916 (*UTF8) 3917 (*UTF16) 3918 3919 Starting a pattern with such a sequence is equivalent to setting the 3920 relevant option. This feature is not Perl-compatible. How setting a UTF 3921 mode affects pattern matching is mentioned in several places below. 3922 There is also a summary of features in the pcreunicode page. 3923 3924 Another special sequence that may appear at the start of a pattern or 3925 in combination with (*UTF8) or (*UTF16) is: 3926 3927 (*UCP) 3928 3929 This has the same effect as setting the PCRE_UCP option: it causes 3930 sequences such as \d and \w to use Unicode properties to determine 3931 character types, instead of recognizing only characters with codes less 3932 than 128 via a lookup table. 3933 3934 If a pattern starts with (*NO_START_OPT), it has the same effect as 3935 setting the PCRE_NO_START_OPTIMIZE option either at compile or matching 3936 time. There are also some more of these special sequences that are con- 3937 cerned with the handling of newlines; they are described below. 3938 3939 The remainder of this document discusses the patterns that are sup- 3940 ported by PCRE when one its main matching functions, pcre_exec() 3941 (8-bit) or pcre16_exec() (16-bit), is used. PCRE also has alternative 3942 matching functions, pcre_dfa_exec() and pcre16_dfa_exec(), which match 3943 using a different algorithm that is not Perl-compatible. Some of the 3944 features discussed below are not available when DFA matching is used. 3945 The advantages and disadvantages of the alternative functions, and how 3946 they differ from the normal functions, are discussed in the pcrematch- 3947 ing page. 3948 3949 3950NEWLINE CONVENTIONS 3951 3952 PCRE supports five different conventions for indicating line breaks in 3953 strings: a single CR (carriage return) character, a single LF (line- 3954 feed) character, the two-character sequence CRLF, any of the three pre- 3955 ceding, or any Unicode newline sequence. The pcreapi page has further 3956 discussion about newlines, and shows how to set the newline convention 3957 in the options arguments for the compiling and matching functions. 3958 3959 It is also possible to specify a newline convention by starting a pat- 3960 tern string with one of the following five sequences: 3961 3962 (*CR) carriage return 3963 (*LF) linefeed 3964 (*CRLF) carriage return, followed by linefeed 3965 (*ANYCRLF) any of the three above 3966 (*ANY) all Unicode newline sequences 3967 3968 These override the default and the options given to the compiling func- 3969 tion. For example, on a Unix system where LF is the default newline 3970 sequence, the pattern 3971 3972 (*CR)a.b 3973 3974 changes the convention to CR. That pattern matches "a\nb" because LF is 3975 no longer a newline. Note that these special settings, which are not 3976 Perl-compatible, are recognized only at the very start of a pattern, 3977 and that they must be in upper case. If more than one of them is 3978 present, the last one is used. 3979 3980 The newline convention affects the interpretation of the dot metachar- 3981 acter when PCRE_DOTALL is not set, and also the behaviour of \N. How- 3982 ever, it does not affect what the \R escape sequence matches. By 3983 default, this is any Unicode newline sequence, for Perl compatibility. 3984 However, this can be changed; see the description of \R in the section 3985 entitled "Newline sequences" below. A change of \R setting can be com- 3986 bined with a change of newline convention. 3987 3988 3989CHARACTERS AND METACHARACTERS 3990 3991 A regular expression is a pattern that is matched against a subject 3992 string from left to right. Most characters stand for themselves in a 3993 pattern, and match the corresponding characters in the subject. As a 3994 trivial example, the pattern 3995 3996 The quick brown fox 3997 3998 matches a portion of a subject string that is identical to itself. When 3999 caseless matching is specified (the PCRE_CASELESS option), letters are 4000 matched independently of case. In a UTF mode, PCRE always understands 4001 the concept of case for characters whose values are less than 128, so 4002 caseless matching is always possible. For characters with higher val- 4003 ues, the concept of case is supported if PCRE is compiled with Unicode 4004 property support, but not otherwise. If you want to use caseless 4005 matching for characters 128 and above, you must ensure that PCRE is 4006 compiled with Unicode property support as well as with UTF support. 4007 4008 The power of regular expressions comes from the ability to include 4009 alternatives and repetitions in the pattern. These are encoded in the 4010 pattern by the use of metacharacters, which do not stand for themselves 4011 but instead are interpreted in some special way. 4012 4013 There are two different sets of metacharacters: those that are recog- 4014 nized anywhere in the pattern except within square brackets, and those 4015 that are recognized within square brackets. Outside square brackets, 4016 the metacharacters are as follows: 4017 4018 \ general escape character with several uses 4019 ^ assert start of string (or line, in multiline mode) 4020 $ assert end of string (or line, in multiline mode) 4021 . match any character except newline (by default) 4022 [ start character class definition 4023 | start of alternative branch 4024 ( start subpattern 4025 ) end subpattern 4026 ? extends the meaning of ( 4027 also 0 or 1 quantifier 4028 also quantifier minimizer 4029 * 0 or more quantifier 4030 + 1 or more quantifier 4031 also "possessive quantifier" 4032 { start min/max quantifier 4033 4034 Part of a pattern that is in square brackets is called a "character 4035 class". In a character class the only metacharacters are: 4036 4037 \ general escape character 4038 ^ negate the class, but only if the first character 4039 - indicates character range 4040 [ POSIX character class (only if followed by POSIX 4041 syntax) 4042 ] terminates the character class 4043 4044 The following sections describe the use of each of the metacharacters. 4045 4046 4047BACKSLASH 4048 4049 The backslash character has several uses. Firstly, if it is followed by 4050 a character that is not a number or a letter, it takes away any special 4051 meaning that character may have. This use of backslash as an escape 4052 character applies both inside and outside character classes. 4053 4054 For example, if you want to match a * character, you write \* in the 4055 pattern. This escaping action applies whether or not the following 4056 character would otherwise be interpreted as a metacharacter, so it is 4057 always safe to precede a non-alphanumeric with backslash to specify 4058 that it stands for itself. In particular, if you want to match a back- 4059 slash, you write \\. 4060 4061 In a UTF mode, only ASCII numbers and letters have any special meaning 4062 after a backslash. All other characters (in particular, those whose 4063 codepoints are greater than 127) are treated as literals. 4064 4065 If a pattern is compiled with the PCRE_EXTENDED option, white space in 4066 the pattern (other than in a character class) and characters between a 4067 # outside a character class and the next newline are ignored. An escap- 4068 ing backslash can be used to include a white space or # character as 4069 part of the pattern. 4070 4071 If you want to remove the special meaning from a sequence of charac- 4072 ters, you can do so by putting them between \Q and \E. This is differ- 4073 ent from Perl in that $ and @ are handled as literals in \Q...\E 4074 sequences in PCRE, whereas in Perl, $ and @ cause variable interpola- 4075 tion. Note the following examples: 4076 4077 Pattern PCRE matches Perl matches 4078 4079 \Qabc$xyz\E abc$xyz abc followed by the 4080 contents of $xyz 4081 \Qabc\$xyz\E abc\$xyz abc\$xyz 4082 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz 4083 4084 The \Q...\E sequence is recognized both inside and outside character 4085 classes. An isolated \E that is not preceded by \Q is ignored. If \Q 4086 is not followed by \E later in the pattern, the literal interpretation 4087 continues to the end of the pattern (that is, \E is assumed at the 4088 end). If the isolated \Q is inside a character class, this causes an 4089 error, because the character class is not terminated. 4090 4091 Non-printing characters 4092 4093 A second use of backslash provides a way of encoding non-printing char- 4094 acters in patterns in a visible manner. There is no restriction on the 4095 appearance of non-printing characters, apart from the binary zero that 4096 terminates a pattern, but when a pattern is being prepared by text 4097 editing, it is often easier to use one of the following escape 4098 sequences than the binary character it represents: 4099 4100 \a alarm, that is, the BEL character (hex 07) 4101 \cx "control-x", where x is any ASCII character 4102 \e escape (hex 1B) 4103 \f form feed (hex 0C) 4104 \n linefeed (hex 0A) 4105 \r carriage return (hex 0D) 4106 \t tab (hex 09) 4107 \ddd character with octal code ddd, or back reference 4108 \xhh character with hex code hh 4109 \x{hhh..} character with hex code hhh.. (non-JavaScript mode) 4110 \uhhhh character with hex code hhhh (JavaScript mode only) 4111 4112 The precise effect of \cx is as follows: if x is a lower case letter, 4113 it is converted to upper case. Then bit 6 of the character (hex 40) is 4114 inverted. Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({ 4115 is 7B), while \c; becomes hex 7B (; is 3B). If the byte following \c 4116 has a value greater than 127, a compile-time error occurs. This locks 4117 out non-ASCII characters in all modes. (When PCRE is compiled in EBCDIC 4118 mode, all byte values are valid. A lower case letter is converted to 4119 upper case, and then the 0xc0 bits are flipped.) 4120 4121 By default, after \x, from zero to two hexadecimal digits are read 4122 (letters can be in upper or lower case). Any number of hexadecimal dig- 4123 its may appear between \x{ and }, but the character code is constrained 4124 as follows: 4125 4126 8-bit non-UTF mode less than 0x100 4127 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint 4128 16-bit non-UTF mode less than 0x10000 4129 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint 4130 4131 Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so- 4132 called "surrogate" codepoints). 4133 4134 If characters other than hexadecimal digits appear between \x{ and }, 4135 or if there is no terminating }, this form of escape is not recognized. 4136 Instead, the initial \x will be interpreted as a basic hexadecimal 4137 escape, with no following digits, giving a character whose value is 4138 zero. 4139 4140 If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x 4141 is as just described only when it is followed by two hexadecimal dig- 4142 its. Otherwise, it matches a literal "x" character. In JavaScript 4143 mode, support for code points greater than 256 is provided by \u, which 4144 must be followed by four hexadecimal digits; otherwise it matches a 4145 literal "u" character. Character codes specified by \u in JavaScript 4146 mode are constrained in the same was as those specified by \x in non- 4147 JavaScript mode. 4148 4149 Characters whose value is less than 256 can be defined by either of the 4150 two syntaxes for \x (or by \u in JavaScript mode). There is no differ- 4151 ence in the way they are handled. For example, \xdc is exactly the same 4152 as \x{dc} (or \u00dc in JavaScript mode). 4153 4154 After \0 up to two further octal digits are read. If there are fewer 4155 than two digits, just those that are present are used. Thus the 4156 sequence \0\x\07 specifies two binary zeros followed by a BEL character 4157 (code value 7). Make sure you supply two digits after the initial zero 4158 if the pattern character that follows is itself an octal digit. 4159 4160 The handling of a backslash followed by a digit other than 0 is compli- 4161 cated. Outside a character class, PCRE reads it and any following dig- 4162 its as a decimal number. If the number is less than 10, or if there 4163 have been at least that many previous capturing left parentheses in the 4164 expression, the entire sequence is taken as a back reference. A 4165 description of how this works is given later, following the discussion 4166 of parenthesized subpatterns. 4167 4168 Inside a character class, or if the decimal number is greater than 9 4169 and there have not been that many capturing subpatterns, PCRE re-reads 4170 up to three octal digits following the backslash, and uses them to gen- 4171 erate a data character. Any subsequent digits stand for themselves. The 4172 value of the character is constrained in the same way as characters 4173 specified in hexadecimal. For example: 4174 4175 \040 is another way of writing a space 4176 \40 is the same, provided there are fewer than 40 4177 previous capturing subpatterns 4178 \7 is always a back reference 4179 \11 might be a back reference, or another way of 4180 writing a tab 4181 \011 is always a tab 4182 \0113 is a tab followed by the character "3" 4183 \113 might be a back reference, otherwise the 4184 character with octal code 113 4185 \377 might be a back reference, otherwise 4186 the value 255 (decimal) 4187 \81 is either a back reference, or a binary zero 4188 followed by the two characters "8" and "1" 4189 4190 Note that octal values of 100 or greater must not be introduced by a 4191 leading zero, because no more than three octal digits are ever read. 4192 4193 All the sequences that define a single character value can be used both 4194 inside and outside character classes. In addition, inside a character 4195 class, \b is interpreted as the backspace character (hex 08). 4196 4197 \N is not allowed in a character class. \B, \R, and \X are not special 4198 inside a character class. Like other unrecognized escape sequences, 4199 they are treated as the literal characters "B", "R", and "X" by 4200 default, but cause an error if the PCRE_EXTRA option is set. Outside a 4201 character class, these sequences have different meanings. 4202 4203 Unsupported escape sequences 4204 4205 In Perl, the sequences \l, \L, \u, and \U are recognized by its string 4206 handler and used to modify the case of following characters. By 4207 default, PCRE does not support these escape sequences. However, if the 4208 PCRE_JAVASCRIPT_COMPAT option is set, \U matches a "U" character, and 4209 \u can be used to define a character by code point, as described in the 4210 previous section. 4211 4212 Absolute and relative back references 4213 4214 The sequence \g followed by an unsigned or a negative number, option- 4215 ally enclosed in braces, is an absolute or relative back reference. A 4216 named back reference can be coded as \g{name}. Back references are dis- 4217 cussed later, following the discussion of parenthesized subpatterns. 4218 4219 Absolute and relative subroutine calls 4220 4221 For compatibility with Oniguruma, the non-Perl syntax \g followed by a 4222 name or a number enclosed either in angle brackets or single quotes, is 4223 an alternative syntax for referencing a subpattern as a "subroutine". 4224 Details are discussed later. Note that \g{...} (Perl syntax) and 4225 \g<...> (Oniguruma syntax) are not synonymous. The former is a back 4226 reference; the latter is a subroutine call. 4227 4228 Generic character types 4229 4230 Another use of backslash is for specifying generic character types: 4231 4232 \d any decimal digit 4233 \D any character that is not a decimal digit 4234 \h any horizontal white space character 4235 \H any character that is not a horizontal white space character 4236 \s any white space character 4237 \S any character that is not a white space character 4238 \v any vertical white space character 4239 \V any character that is not a vertical white space character 4240 \w any "word" character 4241 \W any "non-word" character 4242 4243 There is also the single sequence \N, which matches a non-newline char- 4244 acter. This is the same as the "." metacharacter when PCRE_DOTALL is 4245 not set. Perl also uses \N to match characters by name; PCRE does not 4246 support this. 4247 4248 Each pair of lower and upper case escape sequences partitions the com- 4249 plete set of characters into two disjoint sets. Any given character 4250 matches one, and only one, of each pair. The sequences can appear both 4251 inside and outside character classes. They each match one character of 4252 the appropriate type. If the current matching point is at the end of 4253 the subject string, all of them fail, because there is no character to 4254 match. 4255 4256 For compatibility with Perl, \s does not match the VT character (code 4257 11). This makes it different from the the POSIX "space" class. The \s 4258 characters are HT (9), LF (10), FF (12), CR (13), and space (32). If 4259 "use locale;" is included in a Perl script, \s may match the VT charac- 4260 ter. In PCRE, it never does. 4261 4262 A "word" character is an underscore or any character that is a letter 4263 or digit. By default, the definition of letters and digits is con- 4264 trolled by PCRE's low-valued character tables, and may vary if locale- 4265 specific matching is taking place (see "Locale support" in the pcreapi 4266 page). For example, in a French locale such as "fr_FR" in Unix-like 4267 systems, or "french" in Windows, some character codes greater than 128 4268 are used for accented letters, and these are then matched by \w. The 4269 use of locales with Unicode is discouraged. 4270 4271 By default, in a UTF mode, characters with values greater than 128 4272 never match \d, \s, or \w, and always match \D, \S, and \W. These 4273 sequences retain their original meanings from before UTF support was 4274 available, mainly for efficiency reasons. However, if PCRE is compiled 4275 with Unicode property support, and the PCRE_UCP option is set, the be- 4276 haviour is changed so that Unicode properties are used to determine 4277 character types, as follows: 4278 4279 \d any character that \p{Nd} matches (decimal digit) 4280 \s any character that \p{Z} matches, plus HT, LF, FF, CR 4281 \w any character that \p{L} or \p{N} matches, plus underscore 4282 4283 The upper case escapes match the inverse sets of characters. Note that 4284 \d matches only decimal digits, whereas \w matches any Unicode digit, 4285 as well as any Unicode letter, and underscore. Note also that PCRE_UCP 4286 affects \b, and \B because they are defined in terms of \w and \W. 4287 Matching these sequences is noticeably slower when PCRE_UCP is set. 4288 4289 The sequences \h, \H, \v, and \V are features that were added to Perl 4290 at release 5.10. In contrast to the other sequences, which match only 4291 ASCII characters by default, these always match certain high-valued 4292 codepoints, whether or not PCRE_UCP is set. The horizontal space char- 4293 acters are: 4294 4295 U+0009 Horizontal tab 4296 U+0020 Space 4297 U+00A0 Non-break space 4298 U+1680 Ogham space mark 4299 U+180E Mongolian vowel separator 4300 U+2000 En quad 4301 U+2001 Em quad 4302 U+2002 En space 4303 U+2003 Em space 4304 U+2004 Three-per-em space 4305 U+2005 Four-per-em space 4306 U+2006 Six-per-em space 4307 U+2007 Figure space 4308 U+2008 Punctuation space 4309 U+2009 Thin space 4310 U+200A Hair space 4311 U+202F Narrow no-break space 4312 U+205F Medium mathematical space 4313 U+3000 Ideographic space 4314 4315 The vertical space characters are: 4316 4317 U+000A Linefeed 4318 U+000B Vertical tab 4319 U+000C Form feed 4320 U+000D Carriage return 4321 U+0085 Next line 4322 U+2028 Line separator 4323 U+2029 Paragraph separator 4324 4325 In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 4326 256 are relevant. 4327 4328 Newline sequences 4329 4330 Outside a character class, by default, the escape sequence \R matches 4331 any Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent 4332 to the following: 4333 4334 (?>\r\n|\n|\x0b|\f|\r|\x85) 4335 4336 This is an example of an "atomic group", details of which are given 4337 below. This particular group matches either the two-character sequence 4338 CR followed by LF, or one of the single characters LF (linefeed, 4339 U+000A), VT (vertical tab, U+000B), FF (form feed, U+000C), CR (car- 4340 riage return, U+000D), or NEL (next line, U+0085). The two-character 4341 sequence is treated as a single unit that cannot be split. 4342 4343 In other modes, two additional characters whose codepoints are greater 4344 than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa- 4345 rator, U+2029). Unicode character property support is not needed for 4346 these characters to be recognized. 4347 4348 It is possible to restrict \R to match only CR, LF, or CRLF (instead of 4349 the complete set of Unicode line endings) by setting the option 4350 PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched. 4351 (BSR is an abbrevation for "backslash R".) This can be made the default 4352 when PCRE is built; if this is the case, the other behaviour can be 4353 requested via the PCRE_BSR_UNICODE option. It is also possible to 4354 specify these settings by starting a pattern string with one of the 4355 following sequences: 4356 4357 (*BSR_ANYCRLF) CR, LF, or CRLF only 4358 (*BSR_UNICODE) any Unicode newline sequence 4359 4360 These override the default and the options given to the compiling func- 4361 tion, but they can themselves be overridden by options given to a 4362 matching function. Note that these special settings, which are not 4363 Perl-compatible, are recognized only at the very start of a pattern, 4364 and that they must be in upper case. If more than one of them is 4365 present, the last one is used. They can be combined with a change of 4366 newline convention; for example, a pattern can start with: 4367 4368 (*ANY)(*BSR_ANYCRLF) 4369 4370 They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special 4371 sequences. Inside a character class, \R is treated as an unrecognized 4372 escape sequence, and so matches the letter "R" by default, but causes 4373 an error if PCRE_EXTRA is set. 4374 4375 Unicode character properties 4376 4377 When PCRE is built with Unicode character property support, three addi- 4378 tional escape sequences that match characters with specific properties 4379 are available. When in 8-bit non-UTF-8 mode, these sequences are of 4380 course limited to testing characters whose codepoints are less than 4381 256, but they do work in this mode. The extra escape sequences are: 4382 4383 \p{xx} a character with the xx property 4384 \P{xx} a character without the xx property 4385 \X an extended Unicode sequence 4386 4387 The property names represented by xx above are limited to the Unicode 4388 script names, the general category properties, "Any", which matches any 4389 character (including newline), and some special PCRE properties 4390 (described in the next section). Other Perl properties such as "InMu- 4391 sicalSymbols" are not currently supported by PCRE. Note that \P{Any} 4392 does not match any characters, so always causes a match failure. 4393 4394 Sets of Unicode characters are defined as belonging to certain scripts. 4395 A character from one of these sets can be matched using a script name. 4396 For example: 4397 4398 \p{Greek} 4399 \P{Han} 4400 4401 Those that are not part of an identified script are lumped together as 4402 "Common". The current list of scripts is: 4403 4404 Arabic, Armenian, Avestan, Balinese, Bamum, Batak, Bengali, Bopomofo, 4405 Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Carian, Chakma, 4406 Cham, Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, 4407 Devanagari, Egyptian_Hieroglyphs, Ethiopic, Georgian, Glagolitic, 4408 Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira- 4409 gana, Imperial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscrip- 4410 tional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li, 4411 Kharoshthi, Khmer, Lao, Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian, 4412 Lydian, Malayalam, Mandaic, Meetei_Mayek, Meroitic_Cursive, 4413 Meroitic_Hieroglyphs, Miao, Mongolian, Myanmar, New_Tai_Lue, Nko, 4414 Ogham, Old_Italic, Old_Persian, Old_South_Arabian, Old_Turkic, 4415 Ol_Chiki, Oriya, Osmanya, Phags_Pa, Phoenician, Rejang, Runic, Samari- 4416 tan, Saurashtra, Sharada, Shavian, Sinhala, Sora_Sompeng, Sundanese, 4417 Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, 4418 Takri, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Vai, 4419 Yi. 4420 4421 Each character has exactly one Unicode general category property, spec- 4422 ified by a two-letter abbreviation. For compatibility with Perl, nega- 4423 tion can be specified by including a circumflex between the opening 4424 brace and the property name. For example, \p{^Lu} is the same as 4425 \P{Lu}. 4426 4427 If only one letter is specified with \p or \P, it includes all the gen- 4428 eral category properties that start with that letter. In this case, in 4429 the absence of negation, the curly brackets in the escape sequence are 4430 optional; these two examples have the same effect: 4431 4432 \p{L} 4433 \pL 4434 4435 The following general category property codes are supported: 4436 4437 C Other 4438 Cc Control 4439 Cf Format 4440 Cn Unassigned 4441 Co Private use 4442 Cs Surrogate 4443 4444 L Letter 4445 Ll Lower case letter 4446 Lm Modifier letter 4447 Lo Other letter 4448 Lt Title case letter 4449 Lu Upper case letter 4450 4451 M Mark 4452 Mc Spacing mark 4453 Me Enclosing mark 4454 Mn Non-spacing mark 4455 4456 N Number 4457 Nd Decimal number 4458 Nl Letter number 4459 No Other number 4460 4461 P Punctuation 4462 Pc Connector punctuation 4463 Pd Dash punctuation 4464 Pe Close punctuation 4465 Pf Final punctuation 4466 Pi Initial punctuation 4467 Po Other punctuation 4468 Ps Open punctuation 4469 4470 S Symbol 4471 Sc Currency symbol 4472 Sk Modifier symbol 4473 Sm Mathematical symbol 4474 So Other symbol 4475 4476 Z Separator 4477 Zl Line separator 4478 Zp Paragraph separator 4479 Zs Space separator 4480 4481 The special property L& is also supported: it matches a character that 4482 has the Lu, Ll, or Lt property, in other words, a letter that is not 4483 classified as a modifier or "other". 4484 4485 The Cs (Surrogate) property applies only to characters in the range 4486 U+D800 to U+DFFF. Such characters are not valid in Unicode strings and 4487 so cannot be tested by PCRE, unless UTF validity checking has been 4488 turned off (see the discussion of PCRE_NO_UTF8_CHECK and 4489 PCRE_NO_UTF16_CHECK in the pcreapi page). Perl does not support the Cs 4490 property. 4491 4492 The long synonyms for property names that Perl supports (such as 4493 \p{Letter}) are not supported by PCRE, nor is it permitted to prefix 4494 any of these properties with "Is". 4495 4496 No character that is in the Unicode table has the Cn (unassigned) prop- 4497 erty. Instead, this property is assumed for any code point that is not 4498 in the Unicode table. 4499 4500 Specifying caseless matching does not affect these escape sequences. 4501 For example, \p{Lu} always matches only upper case letters. 4502 4503 The \X escape matches any number of Unicode characters that form an 4504 extended Unicode sequence. \X is equivalent to 4505 4506 (?>\PM\pM*) 4507 4508 That is, it matches a character without the "mark" property, followed 4509 by zero or more characters with the "mark" property, and treats the 4510 sequence as an atomic group (see below). Characters with the "mark" 4511 property are typically accents that affect the preceding character. 4512 None of them have codepoints less than 256, so in 8-bit non-UTF-8 mode 4513 \X matches any one character. 4514 4515 Note that recent versions of Perl have changed \X to match what Unicode 4516 calls an "extended grapheme cluster", which has a more complicated def- 4517 inition. 4518 4519 Matching characters by Unicode property is not fast, because PCRE has 4520 to search a structure that contains data for over fifteen thousand 4521 characters. That is why the traditional escape sequences such as \d and 4522 \w do not use Unicode properties in PCRE by default, though you can 4523 make them do so by setting the PCRE_UCP option or by starting the pat- 4524 tern with (*UCP). 4525 4526 PCRE's additional properties 4527 4528 As well as the standard Unicode properties described in the previous 4529 section, PCRE supports four more that make it possible to convert tra- 4530 ditional escape sequences such as \w and \s and POSIX character classes 4531 to use Unicode properties. PCRE uses these non-standard, non-Perl prop- 4532 erties internally when PCRE_UCP is set. They are: 4533 4534 Xan Any alphanumeric character 4535 Xps Any POSIX space character 4536 Xsp Any Perl space character 4537 Xwd Any Perl "word" character 4538 4539 Xan matches characters that have either the L (letter) or the N (num- 4540 ber) property. Xps matches the characters tab, linefeed, vertical tab, 4541 form feed, or carriage return, and any other character that has the Z 4542 (separator) property. Xsp is the same as Xps, except that vertical tab 4543 is excluded. Xwd matches the same characters as Xan, plus underscore. 4544 4545 Resetting the match start 4546 4547 The escape sequence \K causes any previously matched characters not to 4548 be included in the final matched sequence. For example, the pattern: 4549 4550 foo\Kbar 4551 4552 matches "foobar", but reports that it has matched "bar". This feature 4553 is similar to a lookbehind assertion (described below). However, in 4554 this case, the part of the subject before the real match does not have 4555 to be of fixed length, as lookbehind assertions do. The use of \K does 4556 not interfere with the setting of captured substrings. For example, 4557 when the pattern 4558 4559 (foo)\Kbar 4560 4561 matches "foobar", the first substring is still set to "foo". 4562 4563 Perl documents that the use of \K within assertions is "not well 4564 defined". In PCRE, \K is acted upon when it occurs inside positive 4565 assertions, but is ignored in negative assertions. 4566 4567 Simple assertions 4568 4569 The final use of backslash is for certain simple assertions. An asser- 4570 tion specifies a condition that has to be met at a particular point in 4571 a match, without consuming any characters from the subject string. The 4572 use of subpatterns for more complicated assertions is described below. 4573 The backslashed assertions are: 4574 4575 \b matches at a word boundary 4576 \B matches when not at a word boundary 4577 \A matches at the start of the subject 4578 \Z matches at the end of the subject 4579 also matches before a newline at the end of the subject 4580 \z matches only at the end of the subject 4581 \G matches at the first matching position in the subject 4582 4583 Inside a character class, \b has a different meaning; it matches the 4584 backspace character. If any other of these assertions appears in a 4585 character class, by default it matches the corresponding literal char- 4586 acter (for example, \B matches the letter B). However, if the 4587 PCRE_EXTRA option is set, an "invalid escape sequence" error is gener- 4588 ated instead. 4589 4590 A word boundary is a position in the subject string where the current 4591 character and the previous character do not both match \w or \W (i.e. 4592 one matches \w and the other matches \W), or the start or end of the 4593 string if the first or last character matches \w, respectively. In a 4594 UTF mode, the meanings of \w and \W can be changed by setting the 4595 PCRE_UCP option. When this is done, it also affects \b and \B. Neither 4596 PCRE nor Perl has a separate "start of word" or "end of word" metase- 4597 quence. However, whatever follows \b normally determines which it is. 4598 For example, the fragment \ba matches "a" at the start of a word. 4599 4600 The \A, \Z, and \z assertions differ from the traditional circumflex 4601 and dollar (described in the next section) in that they only ever match 4602 at the very start and end of the subject string, whatever options are 4603 set. Thus, they are independent of multiline mode. These three asser- 4604 tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which 4605 affect only the behaviour of the circumflex and dollar metacharacters. 4606 However, if the startoffset argument of pcre_exec() is non-zero, indi- 4607 cating that matching is to start at a point other than the beginning of 4608 the subject, \A can never match. The difference between \Z and \z is 4609 that \Z matches before a newline at the end of the string as well as at 4610 the very end, whereas \z matches only at the end. 4611 4612 The \G assertion is true only when the current matching position is at 4613 the start point of the match, as specified by the startoffset argument 4614 of pcre_exec(). It differs from \A when the value of startoffset is 4615 non-zero. By calling pcre_exec() multiple times with appropriate argu- 4616 ments, you can mimic Perl's /g option, and it is in this kind of imple- 4617 mentation where \G can be useful. 4618 4619 Note, however, that PCRE's interpretation of \G, as the start of the 4620 current match, is subtly different from Perl's, which defines it as the 4621 end of the previous match. In Perl, these can be different when the 4622 previously matched string was empty. Because PCRE does just one match 4623 at a time, it cannot reproduce this behaviour. 4624 4625 If all the alternatives of a pattern begin with \G, the expression is 4626 anchored to the starting match position, and the "anchored" flag is set 4627 in the compiled regular expression. 4628 4629 4630CIRCUMFLEX AND DOLLAR 4631 4632 Outside a character class, in the default matching mode, the circumflex 4633 character is an assertion that is true only if the current matching 4634 point is at the start of the subject string. If the startoffset argu- 4635 ment of pcre_exec() is non-zero, circumflex can never match if the 4636 PCRE_MULTILINE option is unset. Inside a character class, circumflex 4637 has an entirely different meaning (see below). 4638 4639 Circumflex need not be the first character of the pattern if a number 4640 of alternatives are involved, but it should be the first thing in each 4641 alternative in which it appears if the pattern is ever to match that 4642 branch. If all possible alternatives start with a circumflex, that is, 4643 if the pattern is constrained to match only at the start of the sub- 4644 ject, it is said to be an "anchored" pattern. (There are also other 4645 constructs that can cause a pattern to be anchored.) 4646 4647 A dollar character is an assertion that is true only if the current 4648 matching point is at the end of the subject string, or immediately 4649 before a newline at the end of the string (by default). Dollar need not 4650 be the last character of the pattern if a number of alternatives are 4651 involved, but it should be the last item in any branch in which it 4652 appears. Dollar has no special meaning in a character class. 4653 4654 The meaning of dollar can be changed so that it matches only at the 4655 very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at 4656 compile time. This does not affect the \Z assertion. 4657 4658 The meanings of the circumflex and dollar characters are changed if the 4659 PCRE_MULTILINE option is set. When this is the case, a circumflex 4660 matches immediately after internal newlines as well as at the start of 4661 the subject string. It does not match after a newline that ends the 4662 string. A dollar matches before any newlines in the string, as well as 4663 at the very end, when PCRE_MULTILINE is set. When newline is specified 4664 as the two-character sequence CRLF, isolated CR and LF characters do 4665 not indicate newlines. 4666 4667 For example, the pattern /^abc$/ matches the subject string "def\nabc" 4668 (where \n represents a newline) in multiline mode, but not otherwise. 4669 Consequently, patterns that are anchored in single line mode because 4670 all branches start with ^ are not anchored in multiline mode, and a 4671 match for circumflex is possible when the startoffset argument of 4672 pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if 4673 PCRE_MULTILINE is set. 4674 4675 Note that the sequences \A, \Z, and \z can be used to match the start 4676 and end of the subject in both modes, and if all branches of a pattern 4677 start with \A it is always anchored, whether or not PCRE_MULTILINE is 4678 set. 4679 4680 4681FULL STOP (PERIOD, DOT) AND \N 4682 4683 Outside a character class, a dot in the pattern matches any one charac- 4684 ter in the subject string except (by default) a character that signi- 4685 fies the end of a line. 4686 4687 When a line ending is defined as a single character, dot never matches 4688 that character; when the two-character sequence CRLF is used, dot does 4689 not match CR if it is immediately followed by LF, but otherwise it 4690 matches all characters (including isolated CRs and LFs). When any Uni- 4691 code line endings are being recognized, dot does not match CR or LF or 4692 any of the other line ending characters. 4693 4694 The behaviour of dot with regard to newlines can be changed. If the 4695 PCRE_DOTALL option is set, a dot matches any one character, without 4696 exception. If the two-character sequence CRLF is present in the subject 4697 string, it takes two dots to match it. 4698 4699 The handling of dot is entirely independent of the handling of circum- 4700 flex and dollar, the only relationship being that they both involve 4701 newlines. Dot has no special meaning in a character class. 4702 4703 The escape sequence \N behaves like a dot, except that it is not 4704 affected by the PCRE_DOTALL option. In other words, it matches any 4705 character except one that signifies the end of a line. Perl also uses 4706 \N to match characters by name; PCRE does not support this. 4707 4708 4709MATCHING A SINGLE DATA UNIT 4710 4711 Outside a character class, the escape sequence \C matches any one data 4712 unit, whether or not a UTF mode is set. In the 8-bit library, one data 4713 unit is one byte; in the 16-bit library it is a 16-bit unit. Unlike a 4714 dot, \C always matches line-ending characters. The feature is provided 4715 in Perl in order to match individual bytes in UTF-8 mode, but it is 4716 unclear how it can usefully be used. Because \C breaks up characters 4717 into individual data units, matching one unit with \C in a UTF mode 4718 means that the rest of the string may start with a malformed UTF char- 4719 acter. This has undefined results, because PCRE assumes that it is 4720 dealing with valid UTF strings (and by default it checks this at the 4721 start of processing unless the PCRE_NO_UTF8_CHECK or 4722 PCRE_NO_UTF16_CHECK option is used). 4723 4724 PCRE does not allow \C to appear in lookbehind assertions (described 4725 below) in a UTF mode, because this would make it impossible to calcu- 4726 late the length of the lookbehind. 4727 4728 In general, the \C escape sequence is best avoided. However, one way of 4729 using it that avoids the problem of malformed UTF characters is to use 4730 a lookahead to check the length of the next character, as in this pat- 4731 tern, which could be used with a UTF-8 string (ignore white space and 4732 line breaks): 4733 4734 (?| (?=[\x00-\x7f])(\C) | 4735 (?=[\x80-\x{7ff}])(\C)(\C) | 4736 (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) | 4737 (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C)) 4738 4739 A group that starts with (?| resets the capturing parentheses numbers 4740 in each alternative (see "Duplicate Subpattern Numbers" below). The 4741 assertions at the start of each branch check the next UTF-8 character 4742 for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The 4743 character's individual bytes are then captured by the appropriate num- 4744 ber of groups. 4745 4746 4747SQUARE BRACKETS AND CHARACTER CLASSES 4748 4749 An opening square bracket introduces a character class, terminated by a 4750 closing square bracket. A closing square bracket on its own is not spe- 4751 cial by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set, 4752 a lone closing square bracket causes a compile-time error. If a closing 4753 square bracket is required as a member of the class, it should be the 4754 first data character in the class (after an initial circumflex, if 4755 present) or escaped with a backslash. 4756 4757 A character class matches a single character in the subject. In a UTF 4758 mode, the character may be more than one data unit long. A matched 4759 character must be in the set of characters defined by the class, unless 4760 the first character in the class definition is a circumflex, in which 4761 case the subject character must not be in the set defined by the class. 4762 If a circumflex is actually required as a member of the class, ensure 4763 it is not the first character, or escape it with a backslash. 4764 4765 For example, the character class [aeiou] matches any lower case vowel, 4766 while [^aeiou] matches any character that is not a lower case vowel. 4767 Note that a circumflex is just a convenient notation for specifying the 4768 characters that are in the class by enumerating those that are not. A 4769 class that starts with a circumflex is not an assertion; it still con- 4770 sumes a character from the subject string, and therefore it fails if 4771 the current pointer is at the end of the string. 4772 4773 In UTF-8 (UTF-16) mode, characters with values greater than 255 4774 (0xffff) can be included in a class as a literal string of data units, 4775 or by using the \x{ escaping mechanism. 4776 4777 When caseless matching is set, any letters in a class represent both 4778 their upper case and lower case versions, so for example, a caseless 4779 [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not 4780 match "A", whereas a caseful version would. In a UTF mode, PCRE always 4781 understands the concept of case for characters whose values are less 4782 than 128, so caseless matching is always possible. For characters with 4783 higher values, the concept of case is supported if PCRE is compiled 4784 with Unicode property support, but not otherwise. If you want to use 4785 caseless matching in a UTF mode for characters 128 and above, you must 4786 ensure that PCRE is compiled with Unicode property support as well as 4787 with UTF support. 4788 4789 Characters that might indicate line breaks are never treated in any 4790 special way when matching character classes, whatever line-ending 4791 sequence is in use, and whatever setting of the PCRE_DOTALL and 4792 PCRE_MULTILINE options is used. A class such as [^a] always matches one 4793 of these characters. 4794 4795 The minus (hyphen) character can be used to specify a range of charac- 4796 ters in a character class. For example, [d-m] matches any letter 4797 between d and m, inclusive. If a minus character is required in a 4798 class, it must be escaped with a backslash or appear in a position 4799 where it cannot be interpreted as indicating a range, typically as the 4800 first or last character in the class. 4801 4802 It is not possible to have the literal character "]" as the end charac- 4803 ter of a range. A pattern such as [W-]46] is interpreted as a class of 4804 two characters ("W" and "-") followed by a literal string "46]", so it 4805 would match "W46]" or "-46]". However, if the "]" is escaped with a 4806 backslash it is interpreted as the end of range, so [W-\]46] is inter- 4807 preted as a class containing a range followed by two other characters. 4808 The octal or hexadecimal representation of "]" can also be used to end 4809 a range. 4810 4811 Ranges operate in the collating sequence of character values. They can 4812 also be used for characters specified numerically, for example 4813 [\000-\037]. Ranges can include any characters that are valid for the 4814 current mode. 4815 4816 If a range that includes letters is used when caseless matching is set, 4817 it matches the letters in either case. For example, [W-c] is equivalent 4818 to [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if 4819 character tables for a French locale are in use, [\xc8-\xcb] matches 4820 accented E characters in both cases. In UTF modes, PCRE supports the 4821 concept of case for characters with values greater than 128 only when 4822 it is compiled with Unicode property support. 4823 4824 The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V, 4825 \w, and \W may appear in a character class, and add the characters that 4826 they match to the class. For example, [\dABCDEF] matches any hexadeci- 4827 mal digit. In UTF modes, the PCRE_UCP option affects the meanings of 4828 \d, \s, \w and their upper case partners, just as it does when they 4829 appear outside a character class, as described in the section entitled 4830 "Generic character types" above. The escape sequence \b has a different 4831 meaning inside a character class; it matches the backspace character. 4832 The sequences \B, \N, \R, and \X are not special inside a character 4833 class. Like any other unrecognized escape sequences, they are treated 4834 as the literal characters "B", "N", "R", and "X" by default, but cause 4835 an error if the PCRE_EXTRA option is set. 4836 4837 A circumflex can conveniently be used with the upper case character 4838 types to specify a more restricted set of characters than the matching 4839 lower case type. For example, the class [^\W_] matches any letter or 4840 digit, but not underscore, whereas [\w] includes underscore. A positive 4841 character class should be read as "something OR something OR ..." and a 4842 negative class as "NOT something AND NOT something AND NOT ...". 4843 4844 The only metacharacters that are recognized in character classes are 4845 backslash, hyphen (only where it can be interpreted as specifying a 4846 range), circumflex (only at the start), opening square bracket (only 4847 when it can be interpreted as introducing a POSIX class name - see the 4848 next section), and the terminating closing square bracket. However, 4849 escaping other non-alphanumeric characters does no harm. 4850 4851 4852POSIX CHARACTER CLASSES 4853 4854 Perl supports the POSIX notation for character classes. This uses names 4855 enclosed by [: and :] within the enclosing square brackets. PCRE also 4856 supports this notation. For example, 4857 4858 [01[:alpha:]%] 4859 4860 matches "0", "1", any alphabetic character, or "%". The supported class 4861 names are: 4862 4863 alnum letters and digits 4864 alpha letters 4865 ascii character codes 0 - 127 4866 blank space or tab only 4867 cntrl control characters 4868 digit decimal digits (same as \d) 4869 graph printing characters, excluding space 4870 lower lower case letters 4871 print printing characters, including space 4872 punct printing characters, excluding letters and digits and space 4873 space white space (not quite the same as \s) 4874 upper upper case letters 4875 word "word" characters (same as \w) 4876 xdigit hexadecimal digits 4877 4878 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), 4879 and space (32). Notice that this list includes the VT character (code 4880 11). This makes "space" different to \s, which does not include VT (for 4881 Perl compatibility). 4882 4883 The name "word" is a Perl extension, and "blank" is a GNU extension 4884 from Perl 5.8. Another Perl extension is negation, which is indicated 4885 by a ^ character after the colon. For example, 4886 4887 [12[:^digit:]] 4888 4889 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the 4890 POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but 4891 these are not supported, and an error is given if they are encountered. 4892 4893 By default, in UTF modes, characters with values greater than 128 do 4894 not match any of the POSIX character classes. However, if the PCRE_UCP 4895 option is passed to pcre_compile(), some of the classes are changed so 4896 that Unicode character properties are used. This is achieved by replac- 4897 ing the POSIX classes by other sequences, as follows: 4898 4899 [:alnum:] becomes \p{Xan} 4900 [:alpha:] becomes \p{L} 4901 [:blank:] becomes \h 4902 [:digit:] becomes \p{Nd} 4903 [:lower:] becomes \p{Ll} 4904 [:space:] becomes \p{Xps} 4905 [:upper:] becomes \p{Lu} 4906 [:word:] becomes \p{Xwd} 4907 4908 Negated versions, such as [:^alpha:] use \P instead of \p. The other 4909 POSIX classes are unchanged, and match only characters with code points 4910 less than 128. 4911 4912 4913VERTICAL BAR 4914 4915 Vertical bar characters are used to separate alternative patterns. For 4916 example, the pattern 4917 4918 gilbert|sullivan 4919 4920 matches either "gilbert" or "sullivan". Any number of alternatives may 4921 appear, and an empty alternative is permitted (matching the empty 4922 string). The matching process tries each alternative in turn, from left 4923 to right, and the first one that succeeds is used. If the alternatives 4924 are within a subpattern (defined below), "succeeds" means matching the 4925 rest of the main pattern as well as the alternative in the subpattern. 4926 4927 4928INTERNAL OPTION SETTING 4929 4930 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and 4931 PCRE_EXTENDED options (which are Perl-compatible) can be changed from 4932 within the pattern by a sequence of Perl option letters enclosed 4933 between "(?" and ")". The option letters are 4934 4935 i for PCRE_CASELESS 4936 m for PCRE_MULTILINE 4937 s for PCRE_DOTALL 4938 x for PCRE_EXTENDED 4939 4940 For example, (?im) sets caseless, multiline matching. It is also possi- 4941 ble to unset these options by preceding the letter with a hyphen, and a 4942 combined setting and unsetting such as (?im-sx), which sets PCRE_CASE- 4943 LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, 4944 is also permitted. If a letter appears both before and after the 4945 hyphen, the option is unset. 4946 4947 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA 4948 can be changed in the same way as the Perl-compatible options by using 4949 the characters J, U and X respectively. 4950 4951 When one of these option changes occurs at top level (that is, not 4952 inside subpattern parentheses), the change applies to the remainder of 4953 the pattern that follows. If the change is placed right at the start of 4954 a pattern, PCRE extracts it into the global options (and it will there- 4955 fore show up in data extracted by the pcre_fullinfo() function). 4956 4957 An option change within a subpattern (see below for a description of 4958 subpatterns) affects only that part of the subpattern that follows it, 4959 so 4960 4961 (a(?i)b)c 4962 4963 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not 4964 used). By this means, options can be made to have different settings 4965 in different parts of the pattern. Any changes made in one alternative 4966 do carry on into subsequent branches within the same subpattern. For 4967 example, 4968 4969 (a(?i)b|c) 4970 4971 matches "ab", "aB", "c", and "C", even though when matching "C" the 4972 first branch is abandoned before the option setting. This is because 4973 the effects of option settings happen at compile time. There would be 4974 some very weird behaviour otherwise. 4975 4976 Note: There are other PCRE-specific options that can be set by the 4977 application when the compiling or matching functions are called. In 4978 some cases the pattern can contain special leading sequences such as 4979 (*CRLF) to override what the application has set or what has been 4980 defaulted. Details are given in the section entitled "Newline 4981 sequences" above. There are also the (*UTF8), (*UTF16), and (*UCP) 4982 leading sequences that can be used to set UTF and Unicode property 4983 modes; they are equivalent to setting the PCRE_UTF8, PCRE_UTF16, and 4984 the PCRE_UCP options, respectively. 4985 4986 4987SUBPATTERNS 4988 4989 Subpatterns are delimited by parentheses (round brackets), which can be 4990 nested. Turning part of a pattern into a subpattern does two things: 4991 4992 1. It localizes a set of alternatives. For example, the pattern 4993 4994 cat(aract|erpillar|) 4995 4996 matches "cataract", "caterpillar", or "cat". Without the parentheses, 4997 it would match "cataract", "erpillar" or an empty string. 4998 4999 2. It sets up the subpattern as a capturing subpattern. This means 5000 that, when the whole pattern matches, that portion of the subject 5001 string that matched the subpattern is passed back to the caller via the 5002 ovector argument of the matching function. (This applies only to the 5003 traditional matching functions; the DFA matching functions do not sup- 5004 port capturing.) 5005 5006 Opening parentheses are counted from left to right (starting from 1) to 5007 obtain numbers for the capturing subpatterns. For example, if the 5008 string "the red king" is matched against the pattern 5009 5010 the ((red|white) (king|queen)) 5011 5012 the captured substrings are "red king", "red", and "king", and are num- 5013 bered 1, 2, and 3, respectively. 5014 5015 The fact that plain parentheses fulfil two functions is not always 5016 helpful. There are often times when a grouping subpattern is required 5017 without a capturing requirement. If an opening parenthesis is followed 5018 by a question mark and a colon, the subpattern does not do any captur- 5019 ing, and is not counted when computing the number of any subsequent 5020 capturing subpatterns. For example, if the string "the white queen" is 5021 matched against the pattern 5022 5023 the ((?:red|white) (king|queen)) 5024 5025 the captured substrings are "white queen" and "queen", and are numbered 5026 1 and 2. The maximum number of capturing subpatterns is 65535. 5027 5028 As a convenient shorthand, if any option settings are required at the 5029 start of a non-capturing subpattern, the option letters may appear 5030 between the "?" and the ":". Thus the two patterns 5031 5032 (?i:saturday|sunday) 5033 (?:(?i)saturday|sunday) 5034 5035 match exactly the same set of strings. Because alternative branches are 5036 tried from left to right, and options are not reset until the end of 5037 the subpattern is reached, an option setting in one branch does affect 5038 subsequent branches, so the above patterns match "SUNDAY" as well as 5039 "Saturday". 5040 5041 5042DUPLICATE SUBPATTERN NUMBERS 5043 5044 Perl 5.10 introduced a feature whereby each alternative in a subpattern 5045 uses the same numbers for its capturing parentheses. Such a subpattern 5046 starts with (?| and is itself a non-capturing subpattern. For example, 5047 consider this pattern: 5048 5049 (?|(Sat)ur|(Sun))day 5050 5051 Because the two alternatives are inside a (?| group, both sets of cap- 5052 turing parentheses are numbered one. Thus, when the pattern matches, 5053 you can look at captured substring number one, whichever alternative 5054 matched. This construct is useful when you want to capture part, but 5055 not all, of one of a number of alternatives. Inside a (?| group, paren- 5056 theses are numbered as usual, but the number is reset at the start of 5057 each branch. The numbers of any capturing parentheses that follow the 5058 subpattern start after the highest number used in any branch. The fol- 5059 lowing example is taken from the Perl documentation. The numbers under- 5060 neath show in which buffer the captured content will be stored. 5061 5062 # before ---------------branch-reset----------- after 5063 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x 5064 # 1 2 2 3 2 3 4 5065 5066 A back reference to a numbered subpattern uses the most recent value 5067 that is set for that number by any subpattern. The following pattern 5068 matches "abcabc" or "defdef": 5069 5070 /(?|(abc)|(def))\1/ 5071 5072 In contrast, a subroutine call to a numbered subpattern always refers 5073 to the first one in the pattern with the given number. The following 5074 pattern matches "abcabc" or "defabc": 5075 5076 /(?|(abc)|(def))(?1)/ 5077 5078 If a condition test for a subpattern's having matched refers to a non- 5079 unique number, the test is true if any of the subpatterns of that num- 5080 ber have matched. 5081 5082 An alternative approach to using this "branch reset" feature is to use 5083 duplicate named subpatterns, as described in the next section. 5084 5085 5086NAMED SUBPATTERNS 5087 5088 Identifying capturing parentheses by number is simple, but it can be 5089 very hard to keep track of the numbers in complicated regular expres- 5090 sions. Furthermore, if an expression is modified, the numbers may 5091 change. To help with this difficulty, PCRE supports the naming of sub- 5092 patterns. This feature was not added to Perl until release 5.10. Python 5093 had the feature earlier, and PCRE introduced it at release 4.0, using 5094 the Python syntax. PCRE now supports both the Perl and the Python syn- 5095 tax. Perl allows identically numbered subpatterns to have different 5096 names, but PCRE does not. 5097 5098 In PCRE, a subpattern can be named in one of three ways: (?<name>...) 5099 or (?'name'...) as in Perl, or (?P<name>...) as in Python. References 5100 to capturing parentheses from other parts of the pattern, such as back 5101 references, recursion, and conditions, can be made by name as well as 5102 by number. 5103 5104 Names consist of up to 32 alphanumeric characters and underscores. 5105 Named capturing parentheses are still allocated numbers as well as 5106 names, exactly as if the names were not present. The PCRE API provides 5107 function calls for extracting the name-to-number translation table from 5108 a compiled pattern. There is also a convenience function for extracting 5109 a captured substring by name. 5110 5111 By default, a name must be unique within a pattern, but it is possible 5112 to relax this constraint by setting the PCRE_DUPNAMES option at compile 5113 time. (Duplicate names are also always permitted for subpatterns with 5114 the same number, set up as described in the previous section.) Dupli- 5115 cate names can be useful for patterns where only one instance of the 5116 named parentheses can match. Suppose you want to match the name of a 5117 weekday, either as a 3-letter abbreviation or as the full name, and in 5118 both cases you want to extract the abbreviation. This pattern (ignoring 5119 the line breaks) does the job: 5120 5121 (?<DN>Mon|Fri|Sun)(?:day)?| 5122 (?<DN>Tue)(?:sday)?| 5123 (?<DN>Wed)(?:nesday)?| 5124 (?<DN>Thu)(?:rsday)?| 5125 (?<DN>Sat)(?:urday)? 5126 5127 There are five capturing substrings, but only one is ever set after a 5128 match. (An alternative way of solving this problem is to use a "branch 5129 reset" subpattern, as described in the previous section.) 5130 5131 The convenience function for extracting the data by name returns the 5132 substring for the first (and in this example, the only) subpattern of 5133 that name that matched. This saves searching to find which numbered 5134 subpattern it was. 5135 5136 If you make a back reference to a non-unique named subpattern from 5137 elsewhere in the pattern, the one that corresponds to the first occur- 5138 rence of the name is used. In the absence of duplicate numbers (see the 5139 previous section) this is the one with the lowest number. If you use a 5140 named reference in a condition test (see the section about conditions 5141 below), either to check whether a subpattern has matched, or to check 5142 for recursion, all subpatterns with the same name are tested. If the 5143 condition is true for any one of them, the overall condition is true. 5144 This is the same behaviour as testing by number. For further details of 5145 the interfaces for handling named subpatterns, see the pcreapi documen- 5146 tation. 5147 5148 Warning: You cannot use different names to distinguish between two sub- 5149 patterns with the same number because PCRE uses only the numbers when 5150 matching. For this reason, an error is given at compile time if differ- 5151 ent names are given to subpatterns with the same number. However, you 5152 can give the same name to subpatterns with the same number, even when 5153 PCRE_DUPNAMES is not set. 5154 5155 5156REPETITION 5157 5158 Repetition is specified by quantifiers, which can follow any of the 5159 following items: 5160 5161 a literal data character 5162 the dot metacharacter 5163 the \C escape sequence 5164 the \X escape sequence 5165 the \R escape sequence 5166 an escape such as \d or \pL that matches a single character 5167 a character class 5168 a back reference (see next section) 5169 a parenthesized subpattern (including assertions) 5170 a subroutine call to a subpattern (recursive or otherwise) 5171 5172 The general repetition quantifier specifies a minimum and maximum num- 5173 ber of permitted matches, by giving the two numbers in curly brackets 5174 (braces), separated by a comma. The numbers must be less than 65536, 5175 and the first must be less than or equal to the second. For example: 5176 5177 z{2,4} 5178 5179 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a 5180 special character. If the second number is omitted, but the comma is 5181 present, there is no upper limit; if the second number and the comma 5182 are both omitted, the quantifier specifies an exact number of required 5183 matches. Thus 5184 5185 [aeiou]{3,} 5186 5187 matches at least 3 successive vowels, but may match many more, while 5188 5189 \d{8} 5190 5191 matches exactly 8 digits. An opening curly bracket that appears in a 5192 position where a quantifier is not allowed, or one that does not match 5193 the syntax of a quantifier, is taken as a literal character. For exam- 5194 ple, {,6} is not a quantifier, but a literal string of four characters. 5195 5196 In UTF modes, quantifiers apply to characters rather than to individual 5197 data units. Thus, for example, \x{100}{2} matches two characters, each 5198 of which is represented by a two-byte sequence in a UTF-8 string. Simi- 5199 larly, \X{3} matches three Unicode extended sequences, each of which 5200 may be several data units long (and they may be of different lengths). 5201 5202 The quantifier {0} is permitted, causing the expression to behave as if 5203 the previous item and the quantifier were not present. This may be use- 5204 ful for subpatterns that are referenced as subroutines from elsewhere 5205 in the pattern (but see also the section entitled "Defining subpatterns 5206 for use by reference only" below). Items other than subpatterns that 5207 have a {0} quantifier are omitted from the compiled pattern. 5208 5209 For convenience, the three most common quantifiers have single-charac- 5210 ter abbreviations: 5211 5212 * is equivalent to {0,} 5213 + is equivalent to {1,} 5214 ? is equivalent to {0,1} 5215 5216 It is possible to construct infinite loops by following a subpattern 5217 that can match no characters with a quantifier that has no upper limit, 5218 for example: 5219 5220 (a?)* 5221 5222 Earlier versions of Perl and PCRE used to give an error at compile time 5223 for such patterns. However, because there are cases where this can be 5224 useful, such patterns are now accepted, but if any repetition of the 5225 subpattern does in fact match no characters, the loop is forcibly bro- 5226 ken. 5227 5228 By default, the quantifiers are "greedy", that is, they match as much 5229 as possible (up to the maximum number of permitted times), without 5230 causing the rest of the pattern to fail. The classic example of where 5231 this gives problems is in trying to match comments in C programs. These 5232 appear between /* and */ and within the comment, individual * and / 5233 characters may appear. An attempt to match C comments by applying the 5234 pattern 5235 5236 /\*.*\*/ 5237 5238 to the string 5239 5240 /* first comment */ not comment /* second comment */ 5241 5242 fails, because it matches the entire string owing to the greediness of 5243 the .* item. 5244 5245 However, if a quantifier is followed by a question mark, it ceases to 5246 be greedy, and instead matches the minimum number of times possible, so 5247 the pattern 5248 5249 /\*.*?\*/ 5250 5251 does the right thing with the C comments. The meaning of the various 5252 quantifiers is not otherwise changed, just the preferred number of 5253 matches. Do not confuse this use of question mark with its use as a 5254 quantifier in its own right. Because it has two uses, it can sometimes 5255 appear doubled, as in 5256 5257 \d??\d 5258 5259 which matches one digit by preference, but can match two if that is the 5260 only way the rest of the pattern matches. 5261 5262 If the PCRE_UNGREEDY option is set (an option that is not available in 5263 Perl), the quantifiers are not greedy by default, but individual ones 5264 can be made greedy by following them with a question mark. In other 5265 words, it inverts the default behaviour. 5266 5267 When a parenthesized subpattern is quantified with a minimum repeat 5268 count that is greater than 1 or with a limited maximum, more memory is 5269 required for the compiled pattern, in proportion to the size of the 5270 minimum or maximum. 5271 5272 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv- 5273 alent to Perl's /s) is set, thus allowing the dot to match newlines, 5274 the pattern is implicitly anchored, because whatever follows will be 5275 tried against every character position in the subject string, so there 5276 is no point in retrying the overall match at any position after the 5277 first. PCRE normally treats such a pattern as though it were preceded 5278 by \A. 5279 5280 In cases where it is known that the subject string contains no new- 5281 lines, it is worth setting PCRE_DOTALL in order to obtain this opti- 5282 mization, or alternatively using ^ to indicate anchoring explicitly. 5283 5284 However, there is one situation where the optimization cannot be used. 5285 When .* is inside capturing parentheses that are the subject of a back 5286 reference elsewhere in the pattern, a match at the start may fail where 5287 a later one succeeds. Consider, for example: 5288 5289 (.*)abc\1 5290 5291 If the subject is "xyz123abc123" the match point is the fourth charac- 5292 ter. For this reason, such a pattern is not implicitly anchored. 5293 5294 When a capturing subpattern is repeated, the value captured is the sub- 5295 string that matched the final iteration. For example, after 5296 5297 (tweedle[dume]{3}\s*)+ 5298 5299 has matched "tweedledum tweedledee" the value of the captured substring 5300 is "tweedledee". However, if there are nested capturing subpatterns, 5301 the corresponding captured values may have been set in previous itera- 5302 tions. For example, after 5303 5304 /(a|(b))+/ 5305 5306 matches "aba" the value of the second captured substring is "b". 5307 5308 5309ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS 5310 5311 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") 5312 repetition, failure of what follows normally causes the repeated item 5313 to be re-evaluated to see if a different number of repeats allows the 5314 rest of the pattern to match. Sometimes it is useful to prevent this, 5315 either to change the nature of the match, or to cause it fail earlier 5316 than it otherwise might, when the author of the pattern knows there is 5317 no point in carrying on. 5318 5319 Consider, for example, the pattern \d+foo when applied to the subject 5320 line 5321 5322 123456bar 5323 5324 After matching all 6 digits and then failing to match "foo", the normal 5325 action of the matcher is to try again with only 5 digits matching the 5326 \d+ item, and then with 4, and so on, before ultimately failing. 5327 "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides 5328 the means for specifying that once a subpattern has matched, it is not 5329 to be re-evaluated in this way. 5330 5331 If we use atomic grouping for the previous example, the matcher gives 5332 up immediately on failing to match "foo" the first time. The notation 5333 is a kind of special parenthesis, starting with (?> as in this example: 5334 5335 (?>\d+)foo 5336 5337 This kind of parenthesis "locks up" the part of the pattern it con- 5338 tains once it has matched, and a failure further into the pattern is 5339 prevented from backtracking into it. Backtracking past it to previous 5340 items, however, works as normal. 5341 5342 An alternative description is that a subpattern of this type matches 5343 the string of characters that an identical standalone pattern would 5344 match, if anchored at the current point in the subject string. 5345 5346 Atomic grouping subpatterns are not capturing subpatterns. Simple cases 5347 such as the above example can be thought of as a maximizing repeat that 5348 must swallow everything it can. So, while both \d+ and \d+? are pre- 5349 pared to adjust the number of digits they match in order to make the 5350 rest of the pattern match, (?>\d+) can only match an entire sequence of 5351 digits. 5352 5353 Atomic groups in general can of course contain arbitrarily complicated 5354 subpatterns, and can be nested. However, when the subpattern for an 5355 atomic group is just a single repeated item, as in the example above, a 5356 simpler notation, called a "possessive quantifier" can be used. This 5357 consists of an additional + character following a quantifier. Using 5358 this notation, the previous example can be rewritten as 5359 5360 \d++foo 5361 5362 Note that a possessive quantifier can be used with an entire group, for 5363 example: 5364 5365 (abc|xyz){2,3}+ 5366 5367 Possessive quantifiers are always greedy; the setting of the 5368 PCRE_UNGREEDY option is ignored. They are a convenient notation for the 5369 simpler forms of atomic group. However, there is no difference in the 5370 meaning of a possessive quantifier and the equivalent atomic group, 5371 though there may be a performance difference; possessive quantifiers 5372 should be slightly faster. 5373 5374 The possessive quantifier syntax is an extension to the Perl 5.8 syn- 5375 tax. Jeffrey Friedl originated the idea (and the name) in the first 5376 edition of his book. Mike McCloskey liked it, so implemented it when he 5377 built Sun's Java package, and PCRE copied it from there. It ultimately 5378 found its way into Perl at release 5.10. 5379 5380 PCRE has an optimization that automatically "possessifies" certain sim- 5381 ple pattern constructs. For example, the sequence A+B is treated as 5382 A++B because there is no point in backtracking into a sequence of A's 5383 when B must follow. 5384 5385 When a pattern contains an unlimited repeat inside a subpattern that 5386 can itself be repeated an unlimited number of times, the use of an 5387 atomic group is the only way to avoid some failing matches taking a 5388 very long time indeed. The pattern 5389 5390 (\D+|<\d+>)*[!?] 5391 5392 matches an unlimited number of substrings that either consist of non- 5393 digits, or digits enclosed in <>, followed by either ! or ?. When it 5394 matches, it runs quickly. However, if it is applied to 5395 5396 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa 5397 5398 it takes a long time before reporting failure. This is because the 5399 string can be divided between the internal \D+ repeat and the external 5400 * repeat in a large number of ways, and all have to be tried. (The 5401 example uses [!?] rather than a single character at the end, because 5402 both PCRE and Perl have an optimization that allows for fast failure 5403 when a single character is used. They remember the last single charac- 5404 ter that is required for a match, and fail early if it is not present 5405 in the string.) If the pattern is changed so that it uses an atomic 5406 group, like this: 5407 5408 ((?>\D+)|<\d+>)*[!?] 5409 5410 sequences of non-digits cannot be broken, and failure happens quickly. 5411 5412 5413BACK REFERENCES 5414 5415 Outside a character class, a backslash followed by a digit greater than 5416 0 (and possibly further digits) is a back reference to a capturing sub- 5417 pattern earlier (that is, to its left) in the pattern, provided there 5418 have been that many previous capturing left parentheses. 5419 5420 However, if the decimal number following the backslash is less than 10, 5421 it is always taken as a back reference, and causes an error only if 5422 there are not that many capturing left parentheses in the entire pat- 5423 tern. In other words, the parentheses that are referenced need not be 5424 to the left of the reference for numbers less than 10. A "forward back 5425 reference" of this type can make sense when a repetition is involved 5426 and the subpattern to the right has participated in an earlier itera- 5427 tion. 5428 5429 It is not possible to have a numerical "forward back reference" to a 5430 subpattern whose number is 10 or more using this syntax because a 5431 sequence such as \50 is interpreted as a character defined in octal. 5432 See the subsection entitled "Non-printing characters" above for further 5433 details of the handling of digits following a backslash. There is no 5434 such problem when named parentheses are used. A back reference to any 5435 subpattern is possible using named parentheses (see below). 5436 5437 Another way of avoiding the ambiguity inherent in the use of digits 5438 following a backslash is to use the \g escape sequence. This escape 5439 must be followed by an unsigned number or a negative number, optionally 5440 enclosed in braces. These examples are all identical: 5441 5442 (ring), \1 5443 (ring), \g1 5444 (ring), \g{1} 5445 5446 An unsigned number specifies an absolute reference without the ambigu- 5447 ity that is present in the older syntax. It is also useful when literal 5448 digits follow the reference. A negative number is a relative reference. 5449 Consider this example: 5450 5451 (abc(def)ghi)\g{-1} 5452 5453 The sequence \g{-1} is a reference to the most recently started captur- 5454 ing subpattern before \g, that is, is it equivalent to \2 in this exam- 5455 ple. Similarly, \g{-2} would be equivalent to \1. The use of relative 5456 references can be helpful in long patterns, and also in patterns that 5457 are created by joining together fragments that contain references 5458 within themselves. 5459 5460 A back reference matches whatever actually matched the capturing sub- 5461 pattern in the current subject string, rather than anything matching 5462 the subpattern itself (see "Subpatterns as subroutines" below for a way 5463 of doing that). So the pattern 5464 5465 (sens|respons)e and \1ibility 5466 5467 matches "sense and sensibility" and "response and responsibility", but 5468 not "sense and responsibility". If caseful matching is in force at the 5469 time of the back reference, the case of letters is relevant. For exam- 5470 ple, 5471 5472 ((?i)rah)\s+\1 5473 5474 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the 5475 original capturing subpattern is matched caselessly. 5476 5477 There are several different ways of writing back references to named 5478 subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or 5479 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's 5480 unified back reference syntax, in which \g can be used for both numeric 5481 and named references, is also supported. We could rewrite the above 5482 example in any of the following ways: 5483 5484 (?<p1>(?i)rah)\s+\k<p1> 5485 (?'p1'(?i)rah)\s+\k{p1} 5486 (?P<p1>(?i)rah)\s+(?P=p1) 5487 (?<p1>(?i)rah)\s+\g{p1} 5488 5489 A subpattern that is referenced by name may appear in the pattern 5490 before or after the reference. 5491 5492 There may be more than one back reference to the same subpattern. If a 5493 subpattern has not actually been used in a particular match, any back 5494 references to it always fail by default. For example, the pattern 5495 5496 (a|(bc))\2 5497 5498 always fails if it starts to match "a" rather than "bc". However, if 5499 the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer- 5500 ence to an unset value matches an empty string. 5501 5502 Because there may be many capturing parentheses in a pattern, all dig- 5503 its following a backslash are taken as part of a potential back refer- 5504 ence number. If the pattern continues with a digit character, some 5505 delimiter must be used to terminate the back reference. If the 5506 PCRE_EXTENDED option is set, this can be white space. Otherwise, the 5507 \g{ syntax or an empty comment (see "Comments" below) can be used. 5508 5509 Recursive back references 5510 5511 A back reference that occurs inside the parentheses to which it refers 5512 fails when the subpattern is first used, so, for example, (a\1) never 5513 matches. However, such references can be useful inside repeated sub- 5514 patterns. For example, the pattern 5515 5516 (a|b\1)+ 5517 5518 matches any number of "a"s and also "aba", "ababbaa" etc. At each iter- 5519 ation of the subpattern, the back reference matches the character 5520 string corresponding to the previous iteration. In order for this to 5521 work, the pattern must be such that the first iteration does not need 5522 to match the back reference. This can be done using alternation, as in 5523 the example above, or by a quantifier with a minimum of zero. 5524 5525 Back references of this type cause the group that they reference to be 5526 treated as an atomic group. Once the whole group has been matched, a 5527 subsequent matching failure cannot cause backtracking into the middle 5528 of the group. 5529 5530 5531ASSERTIONS 5532 5533 An assertion is a test on the characters following or preceding the 5534 current matching point that does not actually consume any characters. 5535 The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are 5536 described above. 5537 5538 More complicated assertions are coded as subpatterns. There are two 5539 kinds: those that look ahead of the current position in the subject 5540 string, and those that look behind it. An assertion subpattern is 5541 matched in the normal way, except that it does not cause the current 5542 matching position to be changed. 5543 5544 Assertion subpatterns are not capturing subpatterns. If such an asser- 5545 tion contains capturing subpatterns within it, these are counted for 5546 the purposes of numbering the capturing subpatterns in the whole pat- 5547 tern. However, substring capturing is carried out only for positive 5548 assertions, because it does not make sense for negative assertions. 5549 5550 For compatibility with Perl, assertion subpatterns may be repeated; 5551 though it makes no sense to assert the same thing several times, the 5552 side effect of capturing parentheses may occasionally be useful. In 5553 practice, there only three cases: 5554 5555 (1) If the quantifier is {0}, the assertion is never obeyed during 5556 matching. However, it may contain internal capturing parenthesized 5557 groups that are called from elsewhere via the subroutine mechanism. 5558 5559 (2) If quantifier is {0,n} where n is greater than zero, it is treated 5560 as if it were {0,1}. At run time, the rest of the pattern match is 5561 tried with and without the assertion, the order depending on the greed- 5562 iness of the quantifier. 5563 5564 (3) If the minimum repetition is greater than zero, the quantifier is 5565 ignored. The assertion is obeyed just once when encountered during 5566 matching. 5567 5568 Lookahead assertions 5569 5570 Lookahead assertions start with (?= for positive assertions and (?! for 5571 negative assertions. For example, 5572 5573 \w+(?=;) 5574 5575 matches a word followed by a semicolon, but does not include the semi- 5576 colon in the match, and 5577 5578 foo(?!bar) 5579 5580 matches any occurrence of "foo" that is not followed by "bar". Note 5581 that the apparently similar pattern 5582 5583 (?!foo)bar 5584 5585 does not find an occurrence of "bar" that is preceded by something 5586 other than "foo"; it finds any occurrence of "bar" whatsoever, because 5587 the assertion (?!foo) is always true when the next three characters are 5588 "bar". A lookbehind assertion is needed to achieve the other effect. 5589 5590 If you want to force a matching failure at some point in a pattern, the 5591 most convenient way to do it is with (?!) because an empty string 5592 always matches, so an assertion that requires there not to be an empty 5593 string must always fail. The backtracking control verb (*FAIL) or (*F) 5594 is a synonym for (?!). 5595 5596 Lookbehind assertions 5597 5598 Lookbehind assertions start with (?<= for positive assertions and (?<! 5599 for negative assertions. For example, 5600 5601 (?<!foo)bar 5602 5603 does find an occurrence of "bar" that is not preceded by "foo". The 5604 contents of a lookbehind assertion are restricted such that all the 5605 strings it matches must have a fixed length. However, if there are sev- 5606 eral top-level alternatives, they do not all have to have the same 5607 fixed length. Thus 5608 5609 (?<=bullock|donkey) 5610 5611 is permitted, but 5612 5613 (?<!dogs?|cats?) 5614 5615 causes an error at compile time. Branches that match different length 5616 strings are permitted only at the top level of a lookbehind assertion. 5617 This is an extension compared with Perl, which requires all branches to 5618 match the same length of string. An assertion such as 5619 5620 (?<=ab(c|de)) 5621 5622 is not permitted, because its single top-level branch can match two 5623 different lengths, but it is acceptable to PCRE if rewritten to use two 5624 top-level branches: 5625 5626 (?<=abc|abde) 5627 5628 In some cases, the escape sequence \K (see above) can be used instead 5629 of a lookbehind assertion to get round the fixed-length restriction. 5630 5631 The implementation of lookbehind assertions is, for each alternative, 5632 to temporarily move the current position back by the fixed length and 5633 then try to match. If there are insufficient characters before the cur- 5634 rent position, the assertion fails. 5635 5636 In a UTF mode, PCRE does not allow the \C escape (which matches a sin- 5637 gle data unit even in a UTF mode) to appear in lookbehind assertions, 5638 because it makes it impossible to calculate the length of the lookbe- 5639 hind. The \X and \R escapes, which can match different numbers of data 5640 units, are also not permitted. 5641 5642 "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in 5643 lookbehinds, as long as the subpattern matches a fixed-length string. 5644 Recursion, however, is not supported. 5645 5646 Possessive quantifiers can be used in conjunction with lookbehind 5647 assertions to specify efficient matching of fixed-length strings at the 5648 end of subject strings. Consider a simple pattern such as 5649 5650 abcd$ 5651 5652 when applied to a long string that does not match. Because matching 5653 proceeds from left to right, PCRE will look for each "a" in the subject 5654 and then see if what follows matches the rest of the pattern. If the 5655 pattern is specified as 5656 5657 ^.*abcd$ 5658 5659 the initial .* matches the entire string at first, but when this fails 5660 (because there is no following "a"), it backtracks to match all but the 5661 last character, then all but the last two characters, and so on. Once 5662 again the search for "a" covers the entire string, from right to left, 5663 so we are no better off. However, if the pattern is written as 5664 5665 ^.*+(?<=abcd) 5666 5667 there can be no backtracking for the .*+ item; it can match only the 5668 entire string. The subsequent lookbehind assertion does a single test 5669 on the last four characters. If it fails, the match fails immediately. 5670 For long strings, this approach makes a significant difference to the 5671 processing time. 5672 5673 Using multiple assertions 5674 5675 Several assertions (of any sort) may occur in succession. For example, 5676 5677 (?<=\d{3})(?<!999)foo 5678 5679 matches "foo" preceded by three digits that are not "999". Notice that 5680 each of the assertions is applied independently at the same point in 5681 the subject string. First there is a check that the previous three 5682 characters are all digits, and then there is a check that the same 5683 three characters are not "999". This pattern does not match "foo" pre- 5684 ceded by six characters, the first of which are digits and the last 5685 three of which are not "999". For example, it doesn't match "123abc- 5686 foo". A pattern to do that is 5687 5688 (?<=\d{3}...)(?<!999)foo 5689 5690 This time the first assertion looks at the preceding six characters, 5691 checking that the first three are digits, and then the second assertion 5692 checks that the preceding three characters are not "999". 5693 5694 Assertions can be nested in any combination. For example, 5695 5696 (?<=(?<!foo)bar)baz 5697 5698 matches an occurrence of "baz" that is preceded by "bar" which in turn 5699 is not preceded by "foo", while 5700 5701 (?<=\d{3}(?!999)...)foo 5702 5703 is another pattern that matches "foo" preceded by three digits and any 5704 three characters that are not "999". 5705 5706 5707CONDITIONAL SUBPATTERNS 5708 5709 It is possible to cause the matching process to obey a subpattern con- 5710 ditionally or to choose between two alternative subpatterns, depending 5711 on the result of an assertion, or whether a specific capturing subpat- 5712 tern has already been matched. The two possible forms of conditional 5713 subpattern are: 5714 5715 (?(condition)yes-pattern) 5716 (?(condition)yes-pattern|no-pattern) 5717 5718 If the condition is satisfied, the yes-pattern is used; otherwise the 5719 no-pattern (if present) is used. If there are more than two alterna- 5720 tives in the subpattern, a compile-time error occurs. Each of the two 5721 alternatives may itself contain nested subpatterns of any form, includ- 5722 ing conditional subpatterns; the restriction to two alternatives 5723 applies only at the level of the condition. This pattern fragment is an 5724 example where the alternatives are complex: 5725 5726 (?(1) (A|B|C) | (D | (?(2)E|F) | E) ) 5727 5728 5729 There are four kinds of condition: references to subpatterns, refer- 5730 ences to recursion, a pseudo-condition called DEFINE, and assertions. 5731 5732 Checking for a used subpattern by number 5733 5734 If the text between the parentheses consists of a sequence of digits, 5735 the condition is true if a capturing subpattern of that number has pre- 5736 viously matched. If there is more than one capturing subpattern with 5737 the same number (see the earlier section about duplicate subpattern 5738 numbers), the condition is true if any of them have matched. An alter- 5739 native notation is to precede the digits with a plus or minus sign. In 5740 this case, the subpattern number is relative rather than absolute. The 5741 most recently opened parentheses can be referenced by (?(-1), the next 5742 most recent by (?(-2), and so on. Inside loops it can also make sense 5743 to refer to subsequent groups. The next parentheses to be opened can be 5744 referenced as (?(+1), and so on. (The value zero in any of these forms 5745 is not used; it provokes a compile-time error.) 5746 5747 Consider the following pattern, which contains non-significant white 5748 space to make it more readable (assume the PCRE_EXTENDED option) and to 5749 divide it into three parts for ease of discussion: 5750 5751 ( \( )? [^()]+ (?(1) \) ) 5752 5753 The first part matches an optional opening parenthesis, and if that 5754 character is present, sets it as the first captured substring. The sec- 5755 ond part matches one or more characters that are not parentheses. The 5756 third part is a conditional subpattern that tests whether or not the 5757 first set of parentheses matched. If they did, that is, if subject 5758 started with an opening parenthesis, the condition is true, and so the 5759 yes-pattern is executed and a closing parenthesis is required. Other- 5760 wise, since no-pattern is not present, the subpattern matches nothing. 5761 In other words, this pattern matches a sequence of non-parentheses, 5762 optionally enclosed in parentheses. 5763 5764 If you were embedding this pattern in a larger one, you could use a 5765 relative reference: 5766 5767 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ... 5768 5769 This makes the fragment independent of the parentheses in the larger 5770 pattern. 5771 5772 Checking for a used subpattern by name 5773 5774 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a 5775 used subpattern by name. For compatibility with earlier versions of 5776 PCRE, which had this facility before Perl, the syntax (?(name)...) is 5777 also recognized. However, there is a possible ambiguity with this syn- 5778 tax, because subpattern names may consist entirely of digits. PCRE 5779 looks first for a named subpattern; if it cannot find one and the name 5780 consists entirely of digits, PCRE looks for a subpattern of that num- 5781 ber, which must be greater than zero. Using subpattern names that con- 5782 sist entirely of digits is not recommended. 5783 5784 Rewriting the above example to use a named subpattern gives this: 5785 5786 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) ) 5787 5788 If the name used in a condition of this kind is a duplicate, the test 5789 is applied to all subpatterns of the same name, and is true if any one 5790 of them has matched. 5791 5792 Checking for pattern recursion 5793 5794 If the condition is the string (R), and there is no subpattern with the 5795 name R, the condition is true if a recursive call to the whole pattern 5796 or any subpattern has been made. If digits or a name preceded by amper- 5797 sand follow the letter R, for example: 5798 5799 (?(R3)...) or (?(R&name)...) 5800 5801 the condition is true if the most recent recursion is into a subpattern 5802 whose number or name is given. This condition does not check the entire 5803 recursion stack. If the name used in a condition of this kind is a 5804 duplicate, the test is applied to all subpatterns of the same name, and 5805 is true if any one of them is the most recent recursion. 5806 5807 At "top level", all these recursion test conditions are false. The 5808 syntax for recursive patterns is described below. 5809 5810 Defining subpatterns for use by reference only 5811 5812 If the condition is the string (DEFINE), and there is no subpattern 5813 with the name DEFINE, the condition is always false. In this case, 5814 there may be only one alternative in the subpattern. It is always 5815 skipped if control reaches this point in the pattern; the idea of 5816 DEFINE is that it can be used to define subroutines that can be refer- 5817 enced from elsewhere. (The use of subroutines is described below.) For 5818 example, a pattern to match an IPv4 address such as "192.168.23.245" 5819 could be written like this (ignore white space and line breaks): 5820 5821 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) 5822 \b (?&byte) (\.(?&byte)){3} \b 5823 5824 The first part of the pattern is a DEFINE group inside which a another 5825 group named "byte" is defined. This matches an individual component of 5826 an IPv4 address (a number less than 256). When matching takes place, 5827 this part of the pattern is skipped because DEFINE acts like a false 5828 condition. The rest of the pattern uses references to the named group 5829 to match the four dot-separated components of an IPv4 address, insist- 5830 ing on a word boundary at each end. 5831 5832 Assertion conditions 5833 5834 If the condition is not in any of the above formats, it must be an 5835 assertion. This may be a positive or negative lookahead or lookbehind 5836 assertion. Consider this pattern, again containing non-significant 5837 white space, and with the two alternatives on the second line: 5838 5839 (?(?=[^a-z]*[a-z]) 5840 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) 5841 5842 The condition is a positive lookahead assertion that matches an 5843 optional sequence of non-letters followed by a letter. In other words, 5844 it tests for the presence of at least one letter in the subject. If a 5845 letter is found, the subject is matched against the first alternative; 5846 otherwise it is matched against the second. This pattern matches 5847 strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are 5848 letters and dd are digits. 5849 5850 5851COMMENTS 5852 5853 There are two ways of including comments in patterns that are processed 5854 by PCRE. In both cases, the start of the comment must not be in a char- 5855 acter class, nor in the middle of any other sequence of related charac- 5856 ters such as (?: or a subpattern name or number. The characters that 5857 make up a comment play no part in the pattern matching. 5858 5859 The sequence (?# marks the start of a comment that continues up to the 5860 next closing parenthesis. Nested parentheses are not permitted. If the 5861 PCRE_EXTENDED option is set, an unescaped # character also introduces a 5862 comment, which in this case continues to immediately after the next 5863 newline character or character sequence in the pattern. Which charac- 5864 ters are interpreted as newlines is controlled by the options passed to 5865 a compiling function or by a special sequence at the start of the pat- 5866 tern, as described in the section entitled "Newline conventions" above. 5867 Note that the end of this type of comment is a literal newline sequence 5868 in the pattern; escape sequences that happen to represent a newline do 5869 not count. For example, consider this pattern when PCRE_EXTENDED is 5870 set, and the default newline convention is in force: 5871 5872 abc #comment \n still comment 5873 5874 On encountering the # character, pcre_compile() skips along, looking 5875 for a newline in the pattern. The sequence \n is still literal at this 5876 stage, so it does not terminate the comment. Only an actual character 5877 with the code value 0x0a (the default newline) does so. 5878 5879 5880RECURSIVE PATTERNS 5881 5882 Consider the problem of matching a string in parentheses, allowing for 5883 unlimited nested parentheses. Without the use of recursion, the best 5884 that can be done is to use a pattern that matches up to some fixed 5885 depth of nesting. It is not possible to handle an arbitrary nesting 5886 depth. 5887 5888 For some time, Perl has provided a facility that allows regular expres- 5889 sions to recurse (amongst other things). It does this by interpolating 5890 Perl code in the expression at run time, and the code can refer to the 5891 expression itself. A Perl pattern using code interpolation to solve the 5892 parentheses problem can be created like this: 5893 5894 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; 5895 5896 The (?p{...}) item interpolates Perl code at run time, and in this case 5897 refers recursively to the pattern in which it appears. 5898 5899 Obviously, PCRE cannot support the interpolation of Perl code. Instead, 5900 it supports special syntax for recursion of the entire pattern, and 5901 also for individual subpattern recursion. After its introduction in 5902 PCRE and Python, this kind of recursion was subsequently introduced 5903 into Perl at release 5.10. 5904 5905 A special item that consists of (? followed by a number greater than 5906 zero and a closing parenthesis is a recursive subroutine call of the 5907 subpattern of the given number, provided that it occurs inside that 5908 subpattern. (If not, it is a non-recursive subroutine call, which is 5909 described in the next section.) The special item (?R) or (?0) is a 5910 recursive call of the entire regular expression. 5911 5912 This PCRE pattern solves the nested parentheses problem (assume the 5913 PCRE_EXTENDED option is set so that white space is ignored): 5914 5915 \( ( [^()]++ | (?R) )* \) 5916 5917 First it matches an opening parenthesis. Then it matches any number of 5918 substrings which can either be a sequence of non-parentheses, or a 5919 recursive match of the pattern itself (that is, a correctly parenthe- 5920 sized substring). Finally there is a closing parenthesis. Note the use 5921 of a possessive quantifier to avoid backtracking into sequences of non- 5922 parentheses. 5923 5924 If this were part of a larger pattern, you would not want to recurse 5925 the entire pattern, so instead you could use this: 5926 5927 ( \( ( [^()]++ | (?1) )* \) ) 5928 5929 We have put the pattern into parentheses, and caused the recursion to 5930 refer to them instead of the whole pattern. 5931 5932 In a larger pattern, keeping track of parenthesis numbers can be 5933 tricky. This is made easier by the use of relative references. Instead 5934 of (?1) in the pattern above you can write (?-2) to refer to the second 5935 most recently opened parentheses preceding the recursion. In other 5936 words, a negative number counts capturing parentheses leftwards from 5937 the point at which it is encountered. 5938 5939 It is also possible to refer to subsequently opened parentheses, by 5940 writing references such as (?+2). However, these cannot be recursive 5941 because the reference is not inside the parentheses that are refer- 5942 enced. They are always non-recursive subroutine calls, as described in 5943 the next section. 5944 5945 An alternative approach is to use named parentheses instead. The Perl 5946 syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also 5947 supported. We could rewrite the above example as follows: 5948 5949 (?<pn> \( ( [^()]++ | (?&pn) )* \) ) 5950 5951 If there is more than one subpattern with the same name, the earliest 5952 one is used. 5953 5954 This particular example pattern that we have been looking at contains 5955 nested unlimited repeats, and so the use of a possessive quantifier for 5956 matching strings of non-parentheses is important when applying the pat- 5957 tern to strings that do not match. For example, when this pattern is 5958 applied to 5959 5960 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() 5961 5962 it yields "no match" quickly. However, if a possessive quantifier is 5963 not used, the match runs for a very long time indeed because there are 5964 so many different ways the + and * repeats can carve up the subject, 5965 and all have to be tested before failure can be reported. 5966 5967 At the end of a match, the values of capturing parentheses are those 5968 from the outermost level. If you want to obtain intermediate values, a 5969 callout function can be used (see below and the pcrecallout documenta- 5970 tion). If the pattern above is matched against 5971 5972 (ab(cd)ef) 5973 5974 the value for the inner capturing parentheses (numbered 2) is "ef", 5975 which is the last value taken on at the top level. If a capturing sub- 5976 pattern is not matched at the top level, its final captured value is 5977 unset, even if it was (temporarily) set at a deeper level during the 5978 matching process. 5979 5980 If there are more than 15 capturing parentheses in a pattern, PCRE has 5981 to obtain extra memory to store data during a recursion, which it does 5982 by using pcre_malloc, freeing it via pcre_free afterwards. If no memory 5983 can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. 5984 5985 Do not confuse the (?R) item with the condition (R), which tests for 5986 recursion. Consider this pattern, which matches text in angle brack- 5987 ets, allowing for arbitrary nesting. Only digits are allowed in nested 5988 brackets (that is, when recursing), whereas any characters are permit- 5989 ted at the outer level. 5990 5991 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > 5992 5993 In this pattern, (?(R) is the start of a conditional subpattern, with 5994 two different alternatives for the recursive and non-recursive cases. 5995 The (?R) item is the actual recursive call. 5996 5997 Differences in recursion processing between PCRE and Perl 5998 5999 Recursion processing in PCRE differs from Perl in two important ways. 6000 In PCRE (like Python, but unlike Perl), a recursive subpattern call is 6001 always treated as an atomic group. That is, once it has matched some of 6002 the subject string, it is never re-entered, even if it contains untried 6003 alternatives and there is a subsequent matching failure. This can be 6004 illustrated by the following pattern, which purports to match a palin- 6005 dromic string that contains an odd number of characters (for example, 6006 "a", "aba", "abcba", "abcdcba"): 6007 6008 ^(.|(.)(?1)\2)$ 6009 6010 The idea is that it either matches a single character, or two identical 6011 characters surrounding a sub-palindrome. In Perl, this pattern works; 6012 in PCRE it does not if the pattern is longer than three characters. 6013 Consider the subject string "abcba": 6014 6015 At the top level, the first character is matched, but as it is not at 6016 the end of the string, the first alternative fails; the second alterna- 6017 tive is taken and the recursion kicks in. The recursive call to subpat- 6018 tern 1 successfully matches the next character ("b"). (Note that the 6019 beginning and end of line tests are not part of the recursion). 6020 6021 Back at the top level, the next character ("c") is compared with what 6022 subpattern 2 matched, which was "a". This fails. Because the recursion 6023 is treated as an atomic group, there are now no backtracking points, 6024 and so the entire match fails. (Perl is able, at this point, to re- 6025 enter the recursion and try the second alternative.) However, if the 6026 pattern is written with the alternatives in the other order, things are 6027 different: 6028 6029 ^((.)(?1)\2|.)$ 6030 6031 This time, the recursing alternative is tried first, and continues to 6032 recurse until it runs out of characters, at which point the recursion 6033 fails. But this time we do have another alternative to try at the 6034 higher level. That is the big difference: in the previous case the 6035 remaining alternative is at a deeper recursion level, which PCRE cannot 6036 use. 6037 6038 To change the pattern so that it matches all palindromic strings, not 6039 just those with an odd number of characters, it is tempting to change 6040 the pattern to this: 6041 6042 ^((.)(?1)\2|.?)$ 6043 6044 Again, this works in Perl, but not in PCRE, and for the same reason. 6045 When a deeper recursion has matched a single character, it cannot be 6046 entered again in order to match an empty string. The solution is to 6047 separate the two cases, and write out the odd and even cases as alter- 6048 natives at the higher level: 6049 6050 ^(?:((.)(?1)\2|)|((.)(?3)\4|.)) 6051 6052 If you want to match typical palindromic phrases, the pattern has to 6053 ignore all non-word characters, which can be done like this: 6054 6055 ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$ 6056 6057 If run with the PCRE_CASELESS option, this pattern matches phrases such 6058 as "A man, a plan, a canal: Panama!" and it works well in both PCRE and 6059 Perl. Note the use of the possessive quantifier *+ to avoid backtrack- 6060 ing into sequences of non-word characters. Without this, PCRE takes a 6061 great deal longer (ten times or more) to match typical phrases, and 6062 Perl takes so long that you think it has gone into a loop. 6063 6064 WARNING: The palindrome-matching patterns above work only if the sub- 6065 ject string does not start with a palindrome that is shorter than the 6066 entire string. For example, although "abcba" is correctly matched, if 6067 the subject is "ababa", PCRE finds the palindrome "aba" at the start, 6068 then fails at top level because the end of the string does not follow. 6069 Once again, it cannot jump back into the recursion to try other alter- 6070 natives, so the entire match fails. 6071 6072 The second way in which PCRE and Perl differ in their recursion pro- 6073 cessing is in the handling of captured values. In Perl, when a subpat- 6074 tern is called recursively or as a subpattern (see the next section), 6075 it has no access to any values that were captured outside the recur- 6076 sion, whereas in PCRE these values can be referenced. Consider this 6077 pattern: 6078 6079 ^(.)(\1|a(?2)) 6080 6081 In PCRE, this pattern matches "bab". The first capturing parentheses 6082 match "b", then in the second group, when the back reference \1 fails 6083 to match "b", the second alternative matches "a" and then recurses. In 6084 the recursion, \1 does now match "b" and so the whole match succeeds. 6085 In Perl, the pattern fails to match because inside the recursive call 6086 \1 cannot access the externally set value. 6087 6088 6089SUBPATTERNS AS SUBROUTINES 6090 6091 If the syntax for a recursive subpattern call (either by number or by 6092 name) is used outside the parentheses to which it refers, it operates 6093 like a subroutine in a programming language. The called subpattern may 6094 be defined before or after the reference. A numbered reference can be 6095 absolute or relative, as in these examples: 6096 6097 (...(absolute)...)...(?2)... 6098 (...(relative)...)...(?-1)... 6099 (...(?+1)...(relative)... 6100 6101 An earlier example pointed out that the pattern 6102 6103 (sens|respons)e and \1ibility 6104 6105 matches "sense and sensibility" and "response and responsibility", but 6106 not "sense and responsibility". If instead the pattern 6107 6108 (sens|respons)e and (?1)ibility 6109 6110 is used, it does match "sense and responsibility" as well as the other 6111 two strings. Another example is given in the discussion of DEFINE 6112 above. 6113 6114 All subroutine calls, whether recursive or not, are always treated as 6115 atomic groups. That is, once a subroutine has matched some of the sub- 6116 ject string, it is never re-entered, even if it contains untried alter- 6117 natives and there is a subsequent matching failure. Any capturing 6118 parentheses that are set during the subroutine call revert to their 6119 previous values afterwards. 6120 6121 Processing options such as case-independence are fixed when a subpat- 6122 tern is defined, so if it is used as a subroutine, such options cannot 6123 be changed for different calls. For example, consider this pattern: 6124 6125 (abc)(?i:(?-1)) 6126 6127 It matches "abcabc". It does not match "abcABC" because the change of 6128 processing option does not affect the called subpattern. 6129 6130 6131ONIGURUMA SUBROUTINE SYNTAX 6132 6133 For compatibility with Oniguruma, the non-Perl syntax \g followed by a 6134 name or a number enclosed either in angle brackets or single quotes, is 6135 an alternative syntax for referencing a subpattern as a subroutine, 6136 possibly recursively. Here are two of the examples used above, rewrit- 6137 ten using this syntax: 6138 6139 (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) ) 6140 (sens|respons)e and \g'1'ibility 6141 6142 PCRE supports an extension to Oniguruma: if a number is preceded by a 6143 plus or a minus sign it is taken as a relative reference. For example: 6144 6145 (abc)(?i:\g<-1>) 6146 6147 Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not 6148 synonymous. The former is a back reference; the latter is a subroutine 6149 call. 6150 6151 6152CALLOUTS 6153 6154 Perl has a feature whereby using the sequence (?{...}) causes arbitrary 6155 Perl code to be obeyed in the middle of matching a regular expression. 6156 This makes it possible, amongst other things, to extract different sub- 6157 strings that match the same pair of parentheses when there is a repeti- 6158 tion. 6159 6160 PCRE provides a similar feature, but of course it cannot obey arbitrary 6161 Perl code. The feature is called "callout". The caller of PCRE provides 6162 an external function by putting its entry point in the global variable 6163 pcre_callout (8-bit library) or pcre16_callout (16-bit library). By 6164 default, this variable contains NULL, which disables all calling out. 6165 6166 Within a regular expression, (?C) indicates the points at which the 6167 external function is to be called. If you want to identify different 6168 callout points, you can put a number less than 256 after the letter C. 6169 The default value is zero. For example, this pattern has two callout 6170 points: 6171 6172 (?C1)abc(?C2)def 6173 6174 If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, call- 6175 outs are automatically installed before each item in the pattern. They 6176 are all numbered 255. 6177 6178 During matching, when PCRE reaches a callout point, the external func- 6179 tion is called. It is provided with the number of the callout, the 6180 position in the pattern, and, optionally, one item of data originally 6181 supplied by the caller of the matching function. The callout function 6182 may cause matching to proceed, to backtrack, or to fail altogether. A 6183 complete description of the interface to the callout function is given 6184 in the pcrecallout documentation. 6185 6186 6187BACKTRACKING CONTROL 6188 6189 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", 6190 which are described in the Perl documentation as "experimental and sub- 6191 ject to change or removal in a future version of Perl". It goes on to 6192 say: "Their usage in production code should be noted to avoid problems 6193 during upgrades." The same remarks apply to the PCRE features described 6194 in this section. 6195 6196 Since these verbs are specifically related to backtracking, most of 6197 them can be used only when the pattern is to be matched using one of 6198 the traditional matching functions, which use a backtracking algorithm. 6199 With the exception of (*FAIL), which behaves like a failing negative 6200 assertion, they cause an error if encountered by a DFA matching func- 6201 tion. 6202 6203 If any of these verbs are used in an assertion or in a subpattern that 6204 is called as a subroutine (whether or not recursively), their effect is 6205 confined to that subpattern; it does not extend to the surrounding pat- 6206 tern, with one exception: the name from a *(MARK), (*PRUNE), or (*THEN) 6207 that is encountered in a successful positive assertion is passed back 6208 when a match succeeds (compare capturing parentheses in assertions). 6209 Note that such subpatterns are processed as anchored at the point where 6210 they are tested. Note also that Perl's treatment of subroutines and 6211 assertions is different in some cases. 6212 6213 The new verbs make use of what was previously invalid syntax: an open- 6214 ing parenthesis followed by an asterisk. They are generally of the form 6215 (*VERB) or (*VERB:NAME). Some may take either form, with differing be- 6216 haviour, depending on whether or not an argument is present. A name is 6217 any sequence of characters that does not include a closing parenthesis. 6218 The maximum length of name is 255 in the 8-bit library and 65535 in the 6219 16-bit library. If the name is empty, that is, if the closing parenthe- 6220 sis immediately follows the colon, the effect is as if the colon were 6221 not there. Any number of these verbs may occur in a pattern. 6222 6223 Optimizations that affect backtracking verbs 6224 6225 PCRE contains some optimizations that are used to speed up matching by 6226 running some checks at the start of each match attempt. For example, it 6227 may know the minimum length of matching subject, or that a particular 6228 character must be present. When one of these optimizations suppresses 6229 the running of a match, any included backtracking verbs will not, of 6230 course, be processed. You can suppress the start-of-match optimizations 6231 by setting the PCRE_NO_START_OPTIMIZE option when calling pcre_com- 6232 pile() or pcre_exec(), or by starting the pattern with (*NO_START_OPT). 6233 There is more discussion of this option in the section entitled "Option 6234 bits for pcre_exec()" in the pcreapi documentation. 6235 6236 Experiments with Perl suggest that it too has similar optimizations, 6237 sometimes leading to anomalous results. 6238 6239 Verbs that act immediately 6240 6241 The following verbs act as soon as they are encountered. They may not 6242 be followed by a name. 6243 6244 (*ACCEPT) 6245 6246 This verb causes the match to end successfully, skipping the remainder 6247 of the pattern. However, when it is inside a subpattern that is called 6248 as a subroutine, only that subpattern is ended successfully. Matching 6249 then continues at the outer level. If (*ACCEPT) is inside capturing 6250 parentheses, the data so far is captured. For example: 6251 6252 A((?:A|B(*ACCEPT)|C)D) 6253 6254 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap- 6255 tured by the outer parentheses. 6256 6257 (*FAIL) or (*F) 6258 6259 This verb causes a matching failure, forcing backtracking to occur. It 6260 is equivalent to (?!) but easier to read. The Perl documentation notes 6261 that it is probably useful only when combined with (?{}) or (??{}). 6262 Those are, of course, Perl features that are not present in PCRE. The 6263 nearest equivalent is the callout feature, as for example in this pat- 6264 tern: 6265 6266 a+(?C)(*FAIL) 6267 6268 A match with the string "aaaa" always fails, but the callout is taken 6269 before each backtrack happens (in this example, 10 times). 6270 6271 Recording which path was taken 6272 6273 There is one verb whose main purpose is to track how a match was 6274 arrived at, though it also has a secondary use in conjunction with 6275 advancing the match starting point (see (*SKIP) below). 6276 6277 (*MARK:NAME) or (*:NAME) 6278 6279 A name is always required with this verb. There may be as many 6280 instances of (*MARK) as you like in a pattern, and their names do not 6281 have to be unique. 6282 6283 When a match succeeds, the name of the last-encountered (*MARK) on the 6284 matching path is passed back to the caller as described in the section 6285 entitled "Extra data for pcre_exec()" in the pcreapi documentation. 6286 Here is an example of pcretest output, where the /K modifier requests 6287 the retrieval and outputting of (*MARK) data: 6288 6289 re> /X(*MARK:A)Y|X(*MARK:B)Z/K 6290 data> XY 6291 0: XY 6292 MK: A 6293 XZ 6294 0: XZ 6295 MK: B 6296 6297 The (*MARK) name is tagged with "MK:" in this output, and in this exam- 6298 ple it indicates which of the two alternatives matched. This is a more 6299 efficient way of obtaining this information than putting each alterna- 6300 tive in its own capturing parentheses. 6301 6302 If (*MARK) is encountered in a positive assertion, its name is recorded 6303 and passed back if it is the last-encountered. This does not happen for 6304 negative assertions. 6305 6306 After a partial match or a failed match, the name of the last encoun- 6307 tered (*MARK) in the entire match process is returned. For example: 6308 6309 re> /X(*MARK:A)Y|X(*MARK:B)Z/K 6310 data> XP 6311 No match, mark = B 6312 6313 Note that in this unanchored example the mark is retained from the 6314 match attempt that started at the letter "X" in the subject. Subsequent 6315 match attempts starting at "P" and then with an empty string do not get 6316 as far as the (*MARK) item, but nevertheless do not reset it. 6317 6318 If you are interested in (*MARK) values after failed matches, you 6319 should probably set the PCRE_NO_START_OPTIMIZE option (see above) to 6320 ensure that the match is always attempted. 6321 6322 Verbs that act after backtracking 6323 6324 The following verbs do nothing when they are encountered. Matching con- 6325 tinues with what follows, but if there is no subsequent match, causing 6326 a backtrack to the verb, a failure is forced. That is, backtracking 6327 cannot pass to the left of the verb. However, when one of these verbs 6328 appears inside an atomic group, its effect is confined to that group, 6329 because once the group has been matched, there is never any backtrack- 6330 ing into it. In this situation, backtracking can "jump back" to the 6331 left of the entire atomic group. (Remember also, as stated above, that 6332 this localization also applies in subroutine calls and assertions.) 6333 6334 These verbs differ in exactly what kind of failure occurs when back- 6335 tracking reaches them. 6336 6337 (*COMMIT) 6338 6339 This verb, which may not be followed by a name, causes the whole match 6340 to fail outright if the rest of the pattern does not match. Even if the 6341 pattern is unanchored, no further attempts to find a match by advancing 6342 the starting point take place. Once (*COMMIT) has been passed, 6343 pcre_exec() is committed to finding a match at the current starting 6344 point, or not at all. For example: 6345 6346 a+(*COMMIT)b 6347 6348 This matches "xxaab" but not "aacaab". It can be thought of as a kind 6349 of dynamic anchor, or "I've started, so I must finish." The name of the 6350 most recently passed (*MARK) in the path is passed back when (*COMMIT) 6351 forces a match failure. 6352 6353 Note that (*COMMIT) at the start of a pattern is not the same as an 6354 anchor, unless PCRE's start-of-match optimizations are turned off, as 6355 shown in this pcretest example: 6356 6357 re> /(*COMMIT)abc/ 6358 data> xyzabc 6359 0: abc 6360 xyzabc\Y 6361 No match 6362 6363 PCRE knows that any match must start with "a", so the optimization 6364 skips along the subject to "a" before running the first match attempt, 6365 which succeeds. When the optimization is disabled by the \Y escape in 6366 the second subject, the match starts at "x" and so the (*COMMIT) causes 6367 it to fail without trying any other starting points. 6368 6369 (*PRUNE) or (*PRUNE:NAME) 6370 6371 This verb causes the match to fail at the current starting position in 6372 the subject if the rest of the pattern does not match. If the pattern 6373 is unanchored, the normal "bumpalong" advance to the next starting 6374 character then happens. Backtracking can occur as usual to the left of 6375 (*PRUNE), before it is reached, or when matching to the right of 6376 (*PRUNE), but if there is no match to the right, backtracking cannot 6377 cross (*PRUNE). In simple cases, the use of (*PRUNE) is just an alter- 6378 native to an atomic group or possessive quantifier, but there are some 6379 uses of (*PRUNE) that cannot be expressed in any other way. The behav- 6380 iour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an 6381 anchored pattern (*PRUNE) has the same effect as (*COMMIT). 6382 6383 (*SKIP) 6384 6385 This verb, when given without a name, is like (*PRUNE), except that if 6386 the pattern is unanchored, the "bumpalong" advance is not to the next 6387 character, but to the position in the subject where (*SKIP) was encoun- 6388 tered. (*SKIP) signifies that whatever text was matched leading up to 6389 it cannot be part of a successful match. Consider: 6390 6391 a+(*SKIP)b 6392 6393 If the subject is "aaaac...", after the first match attempt fails 6394 (starting at the first character in the string), the starting point 6395 skips on to start the next attempt at "c". Note that a possessive quan- 6396 tifer does not have the same effect as this example; although it would 6397 suppress backtracking during the first match attempt, the second 6398 attempt would start at the second character instead of skipping on to 6399 "c". 6400 6401 (*SKIP:NAME) 6402 6403 When (*SKIP) has an associated name, its behaviour is modified. If the 6404 following pattern fails to match, the previous path through the pattern 6405 is searched for the most recent (*MARK) that has the same name. If one 6406 is found, the "bumpalong" advance is to the subject position that cor- 6407 responds to that (*MARK) instead of to where (*SKIP) was encountered. 6408 If no (*MARK) with a matching name is found, the (*SKIP) is ignored. 6409 6410 (*THEN) or (*THEN:NAME) 6411 6412 This verb causes a skip to the next innermost alternative if the rest 6413 of the pattern does not match. That is, it cancels pending backtrack- 6414 ing, but only within the current alternative. Its name comes from the 6415 observation that it can be used for a pattern-based if-then-else block: 6416 6417 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... 6418 6419 If the COND1 pattern matches, FOO is tried (and possibly further items 6420 after the end of the group if FOO succeeds); on failure, the matcher 6421 skips to the second alternative and tries COND2, without backtracking 6422 into COND1. The behaviour of (*THEN:NAME) is exactly the same as 6423 (*MARK:NAME)(*THEN). If (*THEN) is not inside an alternation, it acts 6424 like (*PRUNE). 6425 6426 Note that a subpattern that does not contain a | character is just a 6427 part of the enclosing alternative; it is not a nested alternation with 6428 only one alternative. The effect of (*THEN) extends beyond such a sub- 6429 pattern to the enclosing alternative. Consider this pattern, where A, 6430 B, etc. are complex pattern fragments that do not contain any | charac- 6431 ters at this level: 6432 6433 A (B(*THEN)C) | D 6434 6435 If A and B are matched, but there is a failure in C, matching does not 6436 backtrack into A; instead it moves to the next alternative, that is, D. 6437 However, if the subpattern containing (*THEN) is given an alternative, 6438 it behaves differently: 6439 6440 A (B(*THEN)C | (*FAIL)) | D 6441 6442 The effect of (*THEN) is now confined to the inner subpattern. After a 6443 failure in C, matching moves to (*FAIL), which causes the whole subpat- 6444 tern to fail because there are no more alternatives to try. In this 6445 case, matching does now backtrack into A. 6446 6447 Note also that a conditional subpattern is not considered as having two 6448 alternatives, because only one is ever used. In other words, the | 6449 character in a conditional subpattern has a different meaning. Ignoring 6450 white space, consider: 6451 6452 ^.*? (?(?=a) a | b(*THEN)c ) 6453 6454 If the subject is "ba", this pattern does not match. Because .*? is 6455 ungreedy, it initially matches zero characters. The condition (?=a) 6456 then fails, the character "b" is matched, but "c" is not. At this 6457 point, matching does not backtrack to .*? as might perhaps be expected 6458 from the presence of the | character. The conditional subpattern is 6459 part of the single alternative that comprises the whole pattern, and so 6460 the match fails. (If there was a backtrack into .*?, allowing it to 6461 match "b", the match would succeed.) 6462 6463 The verbs just described provide four different "strengths" of control 6464 when subsequent matching fails. (*THEN) is the weakest, carrying on the 6465 match at the next alternative. (*PRUNE) comes next, failing the match 6466 at the current starting position, but allowing an advance to the next 6467 character (for an unanchored pattern). (*SKIP) is similar, except that 6468 the advance may be more than one character. (*COMMIT) is the strongest, 6469 causing the entire match to fail. 6470 6471 If more than one such verb is present in a pattern, the "strongest" one 6472 wins. For example, consider this pattern, where A, B, etc. are complex 6473 pattern fragments: 6474 6475 (A(*COMMIT)B(*THEN)C|D) 6476 6477 Once A has matched, PCRE is committed to this match, at the current 6478 starting position. If subsequently B matches, but C does not, the nor- 6479 mal (*THEN) action of trying the next alternative (that is, D) does not 6480 happen because (*COMMIT) overrides. 6481 6482 6483SEE ALSO 6484 6485 pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3), 6486 pcre16(3). 6487 6488 6489AUTHOR 6490 6491 Philip Hazel 6492 University Computing Service 6493 Cambridge CB2 3QH, England. 6494 6495 6496REVISION 6497 6498 Last updated: 17 June 2012 6499 Copyright (c) 1997-2012 University of Cambridge. 6500------------------------------------------------------------------------------ 6501 6502 6503PCRESYNTAX(3) PCRESYNTAX(3) 6504 6505 6506NAME 6507 PCRE - Perl-compatible regular expressions 6508 6509 6510PCRE REGULAR EXPRESSION SYNTAX SUMMARY 6511 6512 The full syntax and semantics of the regular expressions that are sup- 6513 ported by PCRE are described in the pcrepattern documentation. This 6514 document contains a quick-reference summary of the syntax. 6515 6516 6517QUOTING 6518 6519 \x where x is non-alphanumeric is a literal x 6520 \Q...\E treat enclosed characters as literal 6521 6522 6523CHARACTERS 6524 6525 \a alarm, that is, the BEL character (hex 07) 6526 \cx "control-x", where x is any ASCII character 6527 \e escape (hex 1B) 6528 \f form feed (hex 0C) 6529 \n newline (hex 0A) 6530 \r carriage return (hex 0D) 6531 \t tab (hex 09) 6532 \ddd character with octal code ddd, or backreference 6533 \xhh character with hex code hh 6534 \x{hhh..} character with hex code hhh.. 6535 6536 6537CHARACTER TYPES 6538 6539 . any character except newline; 6540 in dotall mode, any character whatsoever 6541 \C one data unit, even in UTF mode (best avoided) 6542 \d a decimal digit 6543 \D a character that is not a decimal digit 6544 \h a horizontal white space character 6545 \H a character that is not a horizontal white space character 6546 \N a character that is not a newline 6547 \p{xx} a character with the xx property 6548 \P{xx} a character without the xx property 6549 \R a newline sequence 6550 \s a white space character 6551 \S a character that is not a white space character 6552 \v a vertical white space character 6553 \V a character that is not a vertical white space character 6554 \w a "word" character 6555 \W a "non-word" character 6556 \X an extended Unicode sequence 6557 6558 In PCRE, by default, \d, \D, \s, \S, \w, and \W recognize only ASCII 6559 characters, even in a UTF mode. However, this can be changed by setting 6560 the PCRE_UCP option. 6561 6562 6563GENERAL CATEGORY PROPERTIES FOR \p and \P 6564 6565 C Other 6566 Cc Control 6567 Cf Format 6568 Cn Unassigned 6569 Co Private use 6570 Cs Surrogate 6571 6572 L Letter 6573 Ll Lower case letter 6574 Lm Modifier letter 6575 Lo Other letter 6576 Lt Title case letter 6577 Lu Upper case letter 6578 L& Ll, Lu, or Lt 6579 6580 M Mark 6581 Mc Spacing mark 6582 Me Enclosing mark 6583 Mn Non-spacing mark 6584 6585 N Number 6586 Nd Decimal number 6587 Nl Letter number 6588 No Other number 6589 6590 P Punctuation 6591 Pc Connector punctuation 6592 Pd Dash punctuation 6593 Pe Close punctuation 6594 Pf Final punctuation 6595 Pi Initial punctuation 6596 Po Other punctuation 6597 Ps Open punctuation 6598 6599 S Symbol 6600 Sc Currency symbol 6601 Sk Modifier symbol 6602 Sm Mathematical symbol 6603 So Other symbol 6604 6605 Z Separator 6606 Zl Line separator 6607 Zp Paragraph separator 6608 Zs Space separator 6609 6610 6611PCRE SPECIAL CATEGORY PROPERTIES FOR \p and \P 6612 6613 Xan Alphanumeric: union of properties L and N 6614 Xps POSIX space: property Z or tab, NL, VT, FF, CR 6615 Xsp Perl space: property Z or tab, NL, FF, CR 6616 Xwd Perl word: property Xan or underscore 6617 6618 6619SCRIPT NAMES FOR \p AND \P 6620 6621 Arabic, Armenian, Avestan, Balinese, Bamum, Batak, Bengali, Bopomofo, 6622 Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Carian, Chakma, 6623 Cham, Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, 6624 Devanagari, Egyptian_Hieroglyphs, Ethiopic, Georgian, Glagolitic, 6625 Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira- 6626 gana, Imperial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscrip- 6627 tional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li, 6628 Kharoshthi, Khmer, Lao, Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian, 6629 Lydian, Malayalam, Mandaic, Meetei_Mayek, Meroitic_Cursive, 6630 Meroitic_Hieroglyphs, Miao, Mongolian, Myanmar, New_Tai_Lue, Nko, 6631 Ogham, Old_Italic, Old_Persian, Old_South_Arabian, Old_Turkic, 6632 Ol_Chiki, Oriya, Osmanya, Phags_Pa, Phoenician, Rejang, Runic, Samari- 6633 tan, Saurashtra, Sharada, Shavian, Sinhala, Sora_Sompeng, Sundanese, 6634 Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, 6635 Takri, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Vai, 6636 Yi. 6637 6638 6639CHARACTER CLASSES 6640 6641 [...] positive character class 6642 [^...] negative character class 6643 [x-y] range (can be used for hex characters) 6644 [[:xxx:]] positive POSIX named set 6645 [[:^xxx:]] negative POSIX named set 6646 6647 alnum alphanumeric 6648 alpha alphabetic 6649 ascii 0-127 6650 blank space or tab 6651 cntrl control character 6652 digit decimal digit 6653 graph printing, excluding space 6654 lower lower case letter 6655 print printing, including space 6656 punct printing, excluding alphanumeric 6657 space white space 6658 upper upper case letter 6659 word same as \w 6660 xdigit hexadecimal digit 6661 6662 In PCRE, POSIX character set names recognize only ASCII characters by 6663 default, but some of them use Unicode properties if PCRE_UCP is set. 6664 You can use \Q...\E inside a character class. 6665 6666 6667QUANTIFIERS 6668 6669 ? 0 or 1, greedy 6670 ?+ 0 or 1, possessive 6671 ?? 0 or 1, lazy 6672 * 0 or more, greedy 6673 *+ 0 or more, possessive 6674 *? 0 or more, lazy 6675 + 1 or more, greedy 6676 ++ 1 or more, possessive 6677 +? 1 or more, lazy 6678 {n} exactly n 6679 {n,m} at least n, no more than m, greedy 6680 {n,m}+ at least n, no more than m, possessive 6681 {n,m}? at least n, no more than m, lazy 6682 {n,} n or more, greedy 6683 {n,}+ n or more, possessive 6684 {n,}? n or more, lazy 6685 6686 6687ANCHORS AND SIMPLE ASSERTIONS 6688 6689 \b word boundary 6690 \B not a word boundary 6691 ^ start of subject 6692 also after internal newline in multiline mode 6693 \A start of subject 6694 $ end of subject 6695 also before newline at end of subject 6696 also before internal newline in multiline mode 6697 \Z end of subject 6698 also before newline at end of subject 6699 \z end of subject 6700 \G first matching position in subject 6701 6702 6703MATCH POINT RESET 6704 6705 \K reset start of match 6706 6707 6708ALTERNATION 6709 6710 expr|expr|expr... 6711 6712 6713CAPTURING 6714 6715 (...) capturing group 6716 (?<name>...) named capturing group (Perl) 6717 (?'name'...) named capturing group (Perl) 6718 (?P<name>...) named capturing group (Python) 6719 (?:...) non-capturing group 6720 (?|...) non-capturing group; reset group numbers for 6721 capturing groups in each alternative 6722 6723 6724ATOMIC GROUPS 6725 6726 (?>...) atomic, non-capturing group 6727 6728 6729COMMENT 6730 6731 (?#....) comment (not nestable) 6732 6733 6734OPTION SETTING 6735 6736 (?i) caseless 6737 (?J) allow duplicate names 6738 (?m) multiline 6739 (?s) single line (dotall) 6740 (?U) default ungreedy (lazy) 6741 (?x) extended (ignore white space) 6742 (?-...) unset option(s) 6743 6744 The following are recognized only at the start of a pattern or after 6745 one of the newline-setting options with similar syntax: 6746 6747 (*NO_START_OPT) no start-match optimization (PCRE_NO_START_OPTIMIZE) 6748 (*UTF8) set UTF-8 mode: 8-bit library (PCRE_UTF8) 6749 (*UTF16) set UTF-16 mode: 16-bit library (PCRE_UTF16) 6750 (*UCP) set PCRE_UCP (use Unicode properties for \d etc) 6751 6752 6753LOOKAHEAD AND LOOKBEHIND ASSERTIONS 6754 6755 (?=...) positive look ahead 6756 (?!...) negative look ahead 6757 (?<=...) positive look behind 6758 (?<!...) negative look behind 6759 6760 Each top-level branch of a look behind must be of a fixed length. 6761 6762 6763BACKREFERENCES 6764 6765 \n reference by number (can be ambiguous) 6766 \gn reference by number 6767 \g{n} reference by number 6768 \g{-n} relative reference by number 6769 \k<name> reference by name (Perl) 6770 \k'name' reference by name (Perl) 6771 \g{name} reference by name (Perl) 6772 \k{name} reference by name (.NET) 6773 (?P=name) reference by name (Python) 6774 6775 6776SUBROUTINE REFERENCES (POSSIBLY RECURSIVE) 6777 6778 (?R) recurse whole pattern 6779 (?n) call subpattern by absolute number 6780 (?+n) call subpattern by relative number 6781 (?-n) call subpattern by relative number 6782 (?&name) call subpattern by name (Perl) 6783 (?P>name) call subpattern by name (Python) 6784 \g<name> call subpattern by name (Oniguruma) 6785 \g'name' call subpattern by name (Oniguruma) 6786 \g<n> call subpattern by absolute number (Oniguruma) 6787 \g'n' call subpattern by absolute number (Oniguruma) 6788 \g<+n> call subpattern by relative number (PCRE extension) 6789 \g'+n' call subpattern by relative number (PCRE extension) 6790 \g<-n> call subpattern by relative number (PCRE extension) 6791 \g'-n' call subpattern by relative number (PCRE extension) 6792 6793 6794CONDITIONAL PATTERNS 6795 6796 (?(condition)yes-pattern) 6797 (?(condition)yes-pattern|no-pattern) 6798 6799 (?(n)... absolute reference condition 6800 (?(+n)... relative reference condition 6801 (?(-n)... relative reference condition 6802 (?(<name>)... named reference condition (Perl) 6803 (?('name')... named reference condition (Perl) 6804 (?(name)... named reference condition (PCRE) 6805 (?(R)... overall recursion condition 6806 (?(Rn)... specific group recursion condition 6807 (?(R&name)... specific recursion condition 6808 (?(DEFINE)... define subpattern for reference 6809 (?(assert)... assertion condition 6810 6811 6812BACKTRACKING CONTROL 6813 6814 The following act immediately they are reached: 6815 6816 (*ACCEPT) force successful match 6817 (*FAIL) force backtrack; synonym (*F) 6818 (*MARK:NAME) set name to be passed back; synonym (*:NAME) 6819 6820 The following act only when a subsequent match failure causes a back- 6821 track to reach them. They all force a match failure, but they differ in 6822 what happens afterwards. Those that advance the start-of-match point do 6823 so only if the pattern is not anchored. 6824 6825 (*COMMIT) overall failure, no advance of starting point 6826 (*PRUNE) advance to next starting character 6827 (*PRUNE:NAME) equivalent to (*MARK:NAME)(*PRUNE) 6828 (*SKIP) advance to current matching position 6829 (*SKIP:NAME) advance to position corresponding to an earlier 6830 (*MARK:NAME); if not found, the (*SKIP) is ignored 6831 (*THEN) local failure, backtrack to next alternation 6832 (*THEN:NAME) equivalent to (*MARK:NAME)(*THEN) 6833 6834 6835NEWLINE CONVENTIONS 6836 6837 These are recognized only at the very start of the pattern or after a 6838 (*BSR_...), (*UTF8), (*UTF16) or (*UCP) option. 6839 6840 (*CR) carriage return only 6841 (*LF) linefeed only 6842 (*CRLF) carriage return followed by linefeed 6843 (*ANYCRLF) all three of the above 6844 (*ANY) any Unicode newline sequence 6845 6846 6847WHAT \R MATCHES 6848 6849 These are recognized only at the very start of the pattern or after a 6850 (*...) option that sets the newline convention or a UTF or UCP mode. 6851 6852 (*BSR_ANYCRLF) CR, LF, or CRLF 6853 (*BSR_UNICODE) any Unicode newline sequence 6854 6855 6856CALLOUTS 6857 6858 (?C) callout 6859 (?Cn) callout with data n 6860 6861 6862SEE ALSO 6863 6864 pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3). 6865 6866 6867AUTHOR 6868 6869 Philip Hazel 6870 University Computing Service 6871 Cambridge CB2 3QH, England. 6872 6873 6874REVISION 6875 6876 Last updated: 10 January 2012 6877 Copyright (c) 1997-2012 University of Cambridge. 6878------------------------------------------------------------------------------ 6879 6880 6881PCREUNICODE(3) PCREUNICODE(3) 6882 6883 6884NAME 6885 PCRE - Perl-compatible regular expressions 6886 6887 6888UTF-8, UTF-16, AND UNICODE PROPERTY SUPPORT 6889 6890 From Release 8.30, in addition to its previous UTF-8 support, PCRE also 6891 supports UTF-16 by means of a separate 16-bit library. This can be 6892 built as well as, or instead of, the 8-bit library. 6893 6894 6895UTF-8 SUPPORT 6896 6897 In order process UTF-8 strings, you must build PCRE's 8-bit library 6898 with UTF support, and, in addition, you must call pcre_compile() with 6899 the PCRE_UTF8 option flag, or the pattern must start with the sequence 6900 (*UTF8). When either of these is the case, both the pattern and any 6901 subject strings that are matched against it are treated as UTF-8 6902 strings instead of strings of 1-byte characters. 6903 6904 6905UTF-16 SUPPORT 6906 6907 In order process UTF-16 strings, you must build PCRE's 16-bit library 6908 with UTF support, and, in addition, you must call pcre16_compile() with 6909 the PCRE_UTF16 option flag, or the pattern must start with the sequence 6910 (*UTF16). When either of these is the case, both the pattern and any 6911 subject strings that are matched against it are treated as UTF-16 6912 strings instead of strings of 16-bit characters. 6913 6914 6915UTF SUPPORT OVERHEAD 6916 6917 If you compile PCRE with UTF support, but do not use it at run time, 6918 the library will be a bit bigger, but the additional run time overhead 6919 is limited to testing the PCRE_UTF8/16 flag occasionally, so should not 6920 be very big. 6921 6922 6923UNICODE PROPERTY SUPPORT 6924 6925 If PCRE is built with Unicode character property support (which implies 6926 UTF support), the escape sequences \p{..}, \P{..}, and \X can be used. 6927 The available properties that can be tested are limited to the general 6928 category properties such as Lu for an upper case letter or Nd for a 6929 decimal number, the Unicode script names such as Arabic or Han, and the 6930 derived properties Any and L&. A full list is given in the pcrepattern 6931 documentation. Only the short names for properties are supported. For 6932 example, \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not 6933 supported. Furthermore, in Perl, many properties may optionally be 6934 prefixed by "Is", for compatibility with Perl 5.6. PCRE does not sup- 6935 port this. 6936 6937 Validity of UTF-8 strings 6938 6939 When you set the PCRE_UTF8 flag, the byte strings passed as patterns 6940 and subjects are (by default) checked for validity on entry to the rel- 6941 evant functions. The entire string is checked before any other process- 6942 ing takes place. From release 7.3 of PCRE, the check is according the 6943 rules of RFC 3629, which are themselves derived from the Unicode speci- 6944 fication. Earlier releases of PCRE followed the rules of RFC 2279, 6945 which allows the full range of 31-bit values (0 to 0x7FFFFFFF). The 6946 current check allows only values in the range U+0 to U+10FFFF, exclud- 6947 ing U+D800 to U+DFFF. 6948 6949 The excluded code points are the "Surrogate Area" of Unicode. They are 6950 reserved for use by UTF-16, where they are used in pairs to encode 6951 codepoints with values greater than 0xFFFF. The code points that are 6952 encoded by UTF-16 pairs are available independently in the UTF-8 encod- 6953 ing. (In other words, the whole surrogate thing is a fudge for UTF-16 6954 which unfortunately messes up UTF-8.) 6955 6956 If an invalid UTF-8 string is passed to PCRE, an error return is given. 6957 At compile time, the only additional information is the offset to the 6958 first byte of the failing character. The run-time functions pcre_exec() 6959 and pcre_dfa_exec() also pass back this information, as well as a more 6960 detailed reason code if the caller has provided memory in which to do 6961 this. 6962 6963 In some situations, you may already know that your strings are valid, 6964 and therefore want to skip these checks in order to improve perfor- 6965 mance, for example in the case of a long subject string that is being 6966 scanned repeatedly with different patterns. If you set the 6967 PCRE_NO_UTF8_CHECK flag at compile time or at run time, PCRE assumes 6968 that the pattern or subject it is given (respectively) contains only 6969 valid UTF-8 codes. In this case, it does not diagnose an invalid UTF-8 6970 string. 6971 6972 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, 6973 what happens depends on why the string is invalid. If the string con- 6974 forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a 6975 string of characters in the range 0 to 0x7FFFFFFF by pcre_dfa_exec() 6976 and the interpreted version of pcre_exec(). In other words, apart from 6977 the initial validity test, these functions (when in UTF-8 mode) handle 6978 strings according to the more liberal rules of RFC 2279. However, the 6979 just-in-time (JIT) optimization for pcre_exec() supports only RFC 3629. 6980 If you are using JIT optimization, or if the string does not even con- 6981 form to RFC 2279, the result is undefined. Your program may crash. 6982 6983 If you want to process strings of values in the full range 0 to 6984 0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you can 6985 set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in 6986 this situation, you will have to apply your own validity check, and 6987 avoid the use of JIT optimization. 6988 6989 Validity of UTF-16 strings 6990 6991 When you set the PCRE_UTF16 flag, the strings of 16-bit data units that 6992 are passed as patterns and subjects are (by default) checked for valid- 6993 ity on entry to the relevant functions. Values other than those in the 6994 surrogate range U+D800 to U+DFFF are independent code points. Values in 6995 the surrogate range must be used in pairs in the correct manner. 6996 6997 If an invalid UTF-16 string is passed to PCRE, an error return is 6998 given. At compile time, the only additional information is the offset 6999 to the first data unit of the failing character. The run-time functions 7000 pcre16_exec() and pcre16_dfa_exec() also pass back this information, as 7001 well as a more detailed reason code if the caller has provided memory 7002 in which to do this. 7003 7004 In some situations, you may already know that your strings are valid, 7005 and therefore want to skip these checks in order to improve perfor- 7006 mance. If you set the PCRE_NO_UTF16_CHECK flag at compile time or at 7007 run time, PCRE assumes that the pattern or subject it is given (respec- 7008 tively) contains only valid UTF-16 sequences. In this case, it does not 7009 diagnose an invalid UTF-16 string. 7010 7011 General comments about UTF modes 7012 7013 1. Codepoints less than 256 can be specified by either braced or 7014 unbraced hexadecimal escape sequences (for example, \x{b3} or \xb3). 7015 Larger values have to use braced sequences. 7016 7017 2. Octal numbers up to \777 are recognized, and in UTF-8 mode, they 7018 match two-byte characters for values greater than \177. 7019 7020 3. Repeat quantifiers apply to complete UTF characters, not to individ- 7021 ual data units, for example: \x{100}{3}. 7022 7023 4. The dot metacharacter matches one UTF character instead of a single 7024 data unit. 7025 7026 5. The escape sequence \C can be used to match a single byte in UTF-8 7027 mode, or a single 16-bit data unit in UTF-16 mode, but its use can lead 7028 to some strange effects because it breaks up multi-unit characters (see 7029 the description of \C in the pcrepattern documentation). The use of \C 7030 is not supported in the alternative matching function 7031 pcre[16]_dfa_exec(), nor is it supported in UTF mode by the JIT opti- 7032 mization of pcre[16]_exec(). If JIT optimization is requested for a UTF 7033 pattern that contains \C, it will not succeed, and so the matching will 7034 be carried out by the normal interpretive function. 7035 7036 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly 7037 test characters of any code value, but, by default, the characters that 7038 PCRE recognizes as digits, spaces, or word characters remain the same 7039 set as in non-UTF mode, all with values less than 256. This remains 7040 true even when PCRE is built to include Unicode property support, 7041 because to do otherwise would slow down PCRE in many common cases. Note 7042 in particular that this applies to \b and \B, because they are defined 7043 in terms of \w and \W. If you really want to test for a wider sense of, 7044 say, "digit", you can use explicit Unicode property tests such as 7045 \p{Nd}. Alternatively, if you set the PCRE_UCP option, the way that the 7046 character escapes work is changed so that Unicode properties are used 7047 to determine which characters match. There are more details in the sec- 7048 tion on generic character types in the pcrepattern documentation. 7049 7050 7. Similarly, characters that match the POSIX named character classes 7051 are all low-valued characters, unless the PCRE_UCP option is set. 7052 7053 8. However, the horizontal and vertical white space matching escapes 7054 (\h, \H, \v, and \V) do match all the appropriate Unicode characters, 7055 whether or not PCRE_UCP is set. 7056 7057 9. Case-insensitive matching applies only to characters whose values 7058 are less than 128, unless PCRE is built with Unicode property support. 7059 Even when Unicode property support is available, PCRE still uses its 7060 own character tables when checking the case of low-valued characters, 7061 so as not to degrade performance. The Unicode property information is 7062 used only for characters with higher values. Furthermore, PCRE supports 7063 case-insensitive matching only when there is a one-to-one mapping 7064 between a letter's cases. There are a small number of many-to-one map- 7065 pings in Unicode; these are not supported by PCRE. 7066 7067 7068AUTHOR 7069 7070 Philip Hazel 7071 University Computing Service 7072 Cambridge CB2 3QH, England. 7073 7074 7075REVISION 7076 7077 Last updated: 14 April 2012 7078 Copyright (c) 1997-2012 University of Cambridge. 7079------------------------------------------------------------------------------ 7080 7081 7082PCREJIT(3) PCREJIT(3) 7083 7084 7085NAME 7086 PCRE - Perl-compatible regular expressions 7087 7088 7089PCRE JUST-IN-TIME COMPILER SUPPORT 7090 7091 Just-in-time compiling is a heavyweight optimization that can greatly 7092 speed up pattern matching. However, it comes at the cost of extra pro- 7093 cessing before the match is performed. Therefore, it is of most benefit 7094 when the same pattern is going to be matched many times. This does not 7095 necessarily mean many calls of a matching function; if the pattern is 7096 not anchored, matching attempts may take place many times at various 7097 positions in the subject, even for a single call. Therefore, if the 7098 subject string is very long, it may still pay to use JIT for one-off 7099 matches. 7100 7101 JIT support applies only to the traditional Perl-compatible matching 7102 function. It does not apply when the DFA matching function is being 7103 used. The code for this support was written by Zoltan Herczeg. 7104 7105 71068-BIT and 16-BIT SUPPORT 7107 7108 JIT support is available for both the 8-bit and 16-bit PCRE libraries. 7109 To keep this documentation simple, only the 8-bit interface is 7110 described in what follows. If you are using the 16-bit library, substi- 7111 tute the 16-bit functions and 16-bit structures (for example, 7112 pcre16_jit_stack instead of pcre_jit_stack). 7113 7114 7115AVAILABILITY OF JIT SUPPORT 7116 7117 JIT support is an optional feature of PCRE. The "configure" option 7118 --enable-jit (or equivalent CMake option) must be set when PCRE is 7119 built if you want to use JIT. The support is limited to the following 7120 hardware platforms: 7121 7122 ARM v5, v7, and Thumb2 7123 Intel x86 32-bit and 64-bit 7124 MIPS 32-bit 7125 Power PC 32-bit and 64-bit 7126 7127 If --enable-jit is set on an unsupported platform, compilation fails. 7128 7129 A program that is linked with PCRE 8.20 or later can tell if JIT sup- 7130 port is available by calling pcre_config() with the PCRE_CONFIG_JIT 7131 option. The result is 1 when JIT is available, and 0 otherwise. How- 7132 ever, a simple program does not need to check this in order to use JIT. 7133 The API is implemented in a way that falls back to the interpretive 7134 code if JIT is not available. 7135 7136 If your program may sometimes be linked with versions of PCRE that are 7137 older than 8.20, but you want to use JIT when it is available, you can 7138 test the values of PCRE_MAJOR and PCRE_MINOR, or the existence of a JIT 7139 macro such as PCRE_CONFIG_JIT, for compile-time control of your code. 7140 7141 7142SIMPLE USE OF JIT 7143 7144 You have to do two things to make use of the JIT support in the sim- 7145 plest way: 7146 7147 (1) Call pcre_study() with the PCRE_STUDY_JIT_COMPILE option for 7148 each compiled pattern, and pass the resulting pcre_extra block to 7149 pcre_exec(). 7150 7151 (2) Use pcre_free_study() to free the pcre_extra block when it is 7152 no longer needed, instead of just freeing it yourself. This 7153 ensures that any JIT data is also freed. 7154 7155 For a program that may be linked with pre-8.20 versions of PCRE, you 7156 can insert 7157 7158 #ifndef PCRE_STUDY_JIT_COMPILE 7159 #define PCRE_STUDY_JIT_COMPILE 0 7160 #endif 7161 7162 so that no option is passed to pcre_study(), and then use something 7163 like this to free the study data: 7164 7165 #ifdef PCRE_CONFIG_JIT 7166 pcre_free_study(study_ptr); 7167 #else 7168 pcre_free(study_ptr); 7169 #endif 7170 7171 PCRE_STUDY_JIT_COMPILE requests the JIT compiler to generate code for 7172 complete matches. If you want to run partial matches using the 7173 PCRE_PARTIAL_HARD or PCRE_PARTIAL_SOFT options of pcre_exec(), you 7174 should set one or both of the following options in addition to, or 7175 instead of, PCRE_STUDY_JIT_COMPILE when you call pcre_study(): 7176 7177 PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE 7178 PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE 7179 7180 The JIT compiler generates different optimized code for each of the 7181 three modes (normal, soft partial, hard partial). When pcre_exec() is 7182 called, the appropriate code is run if it is available. Otherwise, the 7183 pattern is matched using interpretive code. 7184 7185 In some circumstances you may need to call additional functions. These 7186 are described in the section entitled "Controlling the JIT stack" 7187 below. 7188 7189 If JIT support is not available, PCRE_STUDY_JIT_COMPILE etc. are 7190 ignored, and no JIT data is created. Otherwise, the compiled pattern is 7191 passed to the JIT compiler, which turns it into machine code that exe- 7192 cutes much faster than the normal interpretive code. When pcre_exec() 7193 is passed a pcre_extra block containing a pointer to JIT code of the 7194 appropriate mode (normal or hard/soft partial), it obeys that code 7195 instead of running the interpreter. The result is identical, but the 7196 compiled JIT code runs much faster. 7197 7198 There are some pcre_exec() options that are not supported for JIT exe- 7199 cution. There are also some pattern items that JIT cannot handle. 7200 Details are given below. In both cases, execution automatically falls 7201 back to the interpretive code. If you want to know whether JIT was 7202 actually used for a particular match, you should arrange for a JIT 7203 callback function to be set up as described in the section entitled 7204 "Controlling the JIT stack" below, even if you do not need to supply a 7205 non-default JIT stack. Such a callback function is called whenever JIT 7206 code is about to be obeyed. If the execution options are not right for 7207 JIT execution, the callback function is not obeyed. 7208 7209 If the JIT compiler finds an unsupported item, no JIT data is gener- 7210 ated. You can find out if JIT execution is available after studying a 7211 pattern by calling pcre_fullinfo() with the PCRE_INFO_JIT option. A 7212 result of 1 means that JIT compilation was successful. A result of 0 7213 means that JIT support is not available, or the pattern was not studied 7214 with PCRE_STUDY_JIT_COMPILE etc., or the JIT compiler was not able to 7215 handle the pattern. 7216 7217 Once a pattern has been studied, with or without JIT, it can be used as 7218 many times as you like for matching different subject strings. 7219 7220 7221UNSUPPORTED OPTIONS AND PATTERN ITEMS 7222 7223 The only pcre_exec() options that are supported for JIT execution are 7224 PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, 7225 PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and PCRE_PAR- 7226 TIAL_SOFT. 7227 7228 The unsupported pattern items are: 7229 7230 \C match a single byte; not supported in UTF-8 mode 7231 (?Cn) callouts 7232 (*PRUNE) ) 7233 (*SKIP) ) backtracking control verbs 7234 (*THEN) ) 7235 7236 Support for some of these may be added in future. 7237 7238 7239RETURN VALUES FROM JIT EXECUTION 7240 7241 When a pattern is matched using JIT execution, the return values are 7242 the same as those given by the interpretive pcre_exec() code, with the 7243 addition of one new error code: PCRE_ERROR_JIT_STACKLIMIT. This means 7244 that the memory used for the JIT stack was insufficient. See "Control- 7245 ling the JIT stack" below for a discussion of JIT stack usage. For com- 7246 patibility with the interpretive pcre_exec() code, no more than two- 7247 thirds of the ovector argument is used for passing back captured sub- 7248 strings. 7249 7250 The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if 7251 searching a very large pattern tree goes on for too long, as it is in 7252 the same circumstance when JIT is not used, but the details of exactly 7253 what is counted are not the same. The PCRE_ERROR_RECURSIONLIMIT error 7254 code is never returned by JIT execution. 7255 7256 7257SAVING AND RESTORING COMPILED PATTERNS 7258 7259 The code that is generated by the JIT compiler is architecture-spe- 7260 cific, and is also position dependent. For those reasons it cannot be 7261 saved (in a file or database) and restored later like the bytecode and 7262 other data of a compiled pattern. Saving and restoring compiled pat- 7263 terns is not something many people do. More detail about this facility 7264 is given in the pcreprecompile documentation. It should be possible to 7265 run pcre_study() on a saved and restored pattern, and thereby recreate 7266 the JIT data, but because JIT compilation uses significant resources, 7267 it is probably not worth doing this; you might as well recompile the 7268 original pattern. 7269 7270 7271CONTROLLING THE JIT STACK 7272 7273 When the compiled JIT code runs, it needs a block of memory to use as a 7274 stack. By default, it uses 32K on the machine stack. However, some 7275 large or complicated patterns need more than this. The error 7276 PCRE_ERROR_JIT_STACKLIMIT is given when there is not enough stack. 7277 Three functions are provided for managing blocks of memory for use as 7278 JIT stacks. There is further discussion about the use of JIT stacks in 7279 the section entitled "JIT stack FAQ" below. 7280 7281 The pcre_jit_stack_alloc() function creates a JIT stack. Its arguments 7282 are a starting size and a maximum size, and it returns a pointer to an 7283 opaque structure of type pcre_jit_stack, or NULL if there is an error. 7284 The pcre_jit_stack_free() function can be used to free a stack that is 7285 no longer needed. (For the technically minded: the address space is 7286 allocated by mmap or VirtualAlloc.) 7287 7288 JIT uses far less memory for recursion than the interpretive code, and 7289 a maximum stack size of 512K to 1M should be more than enough for any 7290 pattern. 7291 7292 The pcre_assign_jit_stack() function specifies which stack JIT code 7293 should use. Its arguments are as follows: 7294 7295 pcre_extra *extra 7296 pcre_jit_callback callback 7297 void *data 7298 7299 The extra argument must be the result of studying a pattern with 7300 PCRE_STUDY_JIT_COMPILE etc. There are three cases for the values of the 7301 other two options: 7302 7303 (1) If callback is NULL and data is NULL, an internal 32K block 7304 on the machine stack is used. 7305 7306 (2) If callback is NULL and data is not NULL, data must be 7307 a valid JIT stack, the result of calling pcre_jit_stack_alloc(). 7308 7309 (3) If callback is not NULL, it must point to a function that is 7310 called with data as an argument at the start of matching, in 7311 order to set up a JIT stack. If the return from the callback 7312 function is NULL, the internal 32K stack is used; otherwise the 7313 return value must be a valid JIT stack, the result of calling 7314 pcre_jit_stack_alloc(). 7315 7316 A callback function is obeyed whenever JIT code is about to be run; it 7317 is not obeyed when pcre_exec() is called with options that are incom- 7318 patible for JIT execution. A callback function can therefore be used to 7319 determine whether a match operation was executed by JIT or by the 7320 interpreter. 7321 7322 You may safely use the same JIT stack for more than one pattern (either 7323 by assigning directly or by callback), as long as the patterns are all 7324 matched sequentially in the same thread. In a multithread application, 7325 if you do not specify a JIT stack, or if you assign or pass back NULL 7326 from a callback, that is thread-safe, because each thread has its own 7327 machine stack. However, if you assign or pass back a non-NULL JIT 7328 stack, this must be a different stack for each thread so that the 7329 application is thread-safe. 7330 7331 Strictly speaking, even more is allowed. You can assign the same non- 7332 NULL stack to any number of patterns as long as they are not used for 7333 matching by multiple threads at the same time. For example, you can 7334 assign the same stack to all compiled patterns, and use a global mutex 7335 in the callback to wait until the stack is available for use. However, 7336 this is an inefficient solution, and not recommended. 7337 7338 This is a suggestion for how a multithreaded program that needs to set 7339 up non-default JIT stacks might operate: 7340 7341 During thread initalization 7342 thread_local_var = pcre_jit_stack_alloc(...) 7343 7344 During thread exit 7345 pcre_jit_stack_free(thread_local_var) 7346 7347 Use a one-line callback function 7348 return thread_local_var 7349 7350 All the functions described in this section do nothing if JIT is not 7351 available, and pcre_assign_jit_stack() does nothing unless the extra 7352 argument is non-NULL and points to a pcre_extra block that is the 7353 result of a successful study with PCRE_STUDY_JIT_COMPILE etc. 7354 7355 7356JIT STACK FAQ 7357 7358 (1) Why do we need JIT stacks? 7359 7360 PCRE (and JIT) is a recursive, depth-first engine, so it needs a stack 7361 where the local data of the current node is pushed before checking its 7362 child nodes. Allocating real machine stack on some platforms is diffi- 7363 cult. For example, the stack chain needs to be updated every time if we 7364 extend the stack on PowerPC. Although it is possible, its updating 7365 time overhead decreases performance. So we do the recursion in memory. 7366 7367 (2) Why don't we simply allocate blocks of memory with malloc()? 7368 7369 Modern operating systems have a nice feature: they can reserve an 7370 address space instead of allocating memory. We can safely allocate mem- 7371 ory pages inside this address space, so the stack could grow without 7372 moving memory data (this is important because of pointers). Thus we can 7373 allocate 1M address space, and use only a single memory page (usually 7374 4K) if that is enough. However, we can still grow up to 1M anytime if 7375 needed. 7376 7377 (3) Who "owns" a JIT stack? 7378 7379 The owner of the stack is the user program, not the JIT studied pattern 7380 or anything else. The user program must ensure that if a stack is used 7381 by pcre_exec(), (that is, it is assigned to the pattern currently run- 7382 ning), that stack must not be used by any other threads (to avoid over- 7383 writing the same memory area). The best practice for multithreaded pro- 7384 grams is to allocate a stack for each thread, and return this stack 7385 through the JIT callback function. 7386 7387 (4) When should a JIT stack be freed? 7388 7389 You can free a JIT stack at any time, as long as it will not be used by 7390 pcre_exec() again. When you assign the stack to a pattern, only a 7391 pointer is set. There is no reference counting or any other magic. You 7392 can free the patterns and stacks in any order, anytime. Just do not 7393 call pcre_exec() with a pattern pointing to an already freed stack, as 7394 that will cause SEGFAULT. (Also, do not free a stack currently used by 7395 pcre_exec() in another thread). You can also replace the stack for a 7396 pattern at any time. You can even free the previous stack before 7397 assigning a replacement. 7398 7399 (5) Should I allocate/free a stack every time before/after calling 7400 pcre_exec()? 7401 7402 No, because this is too costly in terms of resources. However, you 7403 could implement some clever idea which release the stack if it is not 7404 used in let's say two minutes. The JIT callback can help to achive this 7405 without keeping a list of the currently JIT studied patterns. 7406 7407 (6) OK, the stack is for long term memory allocation. But what happens 7408 if a pattern causes stack overflow with a stack of 1M? Is that 1M kept 7409 until the stack is freed? 7410 7411 Especially on embedded sytems, it might be a good idea to release mem- 7412 ory sometimes without freeing the stack. There is no API for this at 7413 the moment. Probably a function call which returns with the currently 7414 allocated memory for any stack and another which allows releasing mem- 7415 ory (shrinking the stack) would be a good idea if someone needs this. 7416 7417 (7) This is too much of a headache. Isn't there any better solution for 7418 JIT stack handling? 7419 7420 No, thanks to Windows. If POSIX threads were used everywhere, we could 7421 throw out this complicated API. 7422 7423 7424EXAMPLE CODE 7425 7426 This is a single-threaded example that specifies a JIT stack without 7427 using a callback. 7428 7429 int rc; 7430 int ovector[30]; 7431 pcre *re; 7432 pcre_extra *extra; 7433 pcre_jit_stack *jit_stack; 7434 7435 re = pcre_compile(pattern, 0, &error, &erroffset, NULL); 7436 /* Check for errors */ 7437 extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error); 7438 jit_stack = pcre_jit_stack_alloc(32*1024, 512*1024); 7439 /* Check for error (NULL) */ 7440 pcre_assign_jit_stack(extra, NULL, jit_stack); 7441 rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30); 7442 /* Check results */ 7443 pcre_free(re); 7444 pcre_free_study(extra); 7445 pcre_jit_stack_free(jit_stack); 7446 7447 7448SEE ALSO 7449 7450 pcreapi(3) 7451 7452 7453AUTHOR 7454 7455 Philip Hazel (FAQ by Zoltan Herczeg) 7456 University Computing Service 7457 Cambridge CB2 3QH, England. 7458 7459 7460REVISION 7461 7462 Last updated: 04 May 2012 7463 Copyright (c) 1997-2012 University of Cambridge. 7464------------------------------------------------------------------------------ 7465 7466 7467PCREPARTIAL(3) PCREPARTIAL(3) 7468 7469 7470NAME 7471 PCRE - Perl-compatible regular expressions 7472 7473 7474PARTIAL MATCHING IN PCRE 7475 7476 In normal use of PCRE, if the subject string that is passed to a match- 7477 ing function matches as far as it goes, but is too short to match the 7478 entire pattern, PCRE_ERROR_NOMATCH is returned. There are circumstances 7479 where it might be helpful to distinguish this case from other cases in 7480 which there is no match. 7481 7482 Consider, for example, an application where a human is required to type 7483 in data for a field with specific formatting requirements. An example 7484 might be a date in the form ddmmmyy, defined by this pattern: 7485 7486 ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$ 7487 7488 If the application sees the user's keystrokes one by one, and can check 7489 that what has been typed so far is potentially valid, it is able to 7490 raise an error as soon as a mistake is made, by beeping and not 7491 reflecting the character that has been typed, for example. This immedi- 7492 ate feedback is likely to be a better user interface than a check that 7493 is delayed until the entire string has been entered. Partial matching 7494 can also be useful when the subject string is very long and is not all 7495 available at once. 7496 7497 PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and 7498 PCRE_PARTIAL_HARD options, which can be set when calling any of the 7499 matching functions. For backwards compatibility, PCRE_PARTIAL is a syn- 7500 onym for PCRE_PARTIAL_SOFT. The essential difference between the two 7501 options is whether or not a partial match is preferred to an alterna- 7502 tive complete match, though the details differ between the two types of 7503 matching function. If both options are set, PCRE_PARTIAL_HARD takes 7504 precedence. 7505 7506 If you want to use partial matching with just-in-time optimized code, 7507 you must call pcre_study() or pcre16_study() with one or both of these 7508 options: 7509 7510 PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE 7511 PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE 7512 7513 PCRE_STUDY_JIT_COMPILE should also be set if you are going to run non- 7514 partial matches on the same pattern. If the appropriate JIT study mode 7515 has not been set for a match, the interpretive matching code is used. 7516 7517 Setting a partial matching option disables two of PCRE's standard opti- 7518 mizations. PCRE remembers the last literal data unit in a pattern, and 7519 abandons matching immediately if it is not present in the subject 7520 string. This optimization cannot be used for a subject string that 7521 might match only partially. If the pattern was studied, PCRE knows the 7522 minimum length of a matching string, and does not bother to run the 7523 matching function on shorter strings. This optimization is also dis- 7524 abled for partial matching. 7525 7526 7527PARTIAL MATCHING USING pcre_exec() OR pcre16_exec() 7528 7529 A partial match occurs during a call to pcre_exec() or pcre16_exec() 7530 when the end of the subject string is reached successfully, but match- 7531 ing cannot continue because more characters are needed. However, at 7532 least one character in the subject must have been inspected. This char- 7533 acter need not form part of the final matched string; lookbehind asser- 7534 tions and the \K escape sequence provide ways of inspecting characters 7535 before the start of a matched substring. The requirement for inspecting 7536 at least one character exists because an empty string can always be 7537 matched; without such a restriction there would always be a partial 7538 match of an empty string at the end of the subject. 7539 7540 If there are at least two slots in the offsets vector when a partial 7541 match is returned, the first slot is set to the offset of the earliest 7542 character that was inspected. For convenience, the second offset points 7543 to the end of the subject so that a substring can easily be identified. 7544 7545 For the majority of patterns, the first offset identifies the start of 7546 the partially matched string. However, for patterns that contain look- 7547 behind assertions, or \K, or begin with \b or \B, earlier characters 7548 have been inspected while carrying out the match. For example: 7549 7550 /(?<=abc)123/ 7551 7552 This pattern matches "123", but only if it is preceded by "abc". If the 7553 subject string is "xyzabc12", the offsets after a partial match are for 7554 the substring "abc12", because all these characters are needed if 7555 another match is tried with extra characters added to the subject. 7556 7557 What happens when a partial match is identified depends on which of the 7558 two partial matching options are set. 7559 7560 PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre16_exec() 7561 7562 If PCRE_PARTIAL_SOFT is set when pcre_exec() or pcre16_exec() identi- 7563 fies a partial match, the partial match is remembered, but matching 7564 continues as normal, and other alternatives in the pattern are tried. 7565 If no complete match can be found, PCRE_ERROR_PARTIAL is returned 7566 instead of PCRE_ERROR_NOMATCH. 7567 7568 This option is "soft" because it prefers a complete match over a par- 7569 tial match. All the various matching items in a pattern behave as if 7570 the subject string is potentially complete. For example, \z, \Z, and $ 7571 match at the end of the subject, as normal, and for \b and \B the end 7572 of the subject is treated as a non-alphanumeric. 7573 7574 If there is more than one partial match, the first one that was found 7575 provides the data that is returned. Consider this pattern: 7576 7577 /123\w+X|dogY/ 7578 7579 If this is matched against the subject string "abc123dog", both alter- 7580 natives fail to match, but the end of the subject is reached during 7581 matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set to 3 7582 and 9, identifying "123dog" as the first partial match that was found. 7583 (In this example, there are two partial matches, because "dog" on its 7584 own partially matches the second alternative.) 7585 7586 PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre16_exec() 7587 7588 If PCRE_PARTIAL_HARD is set for pcre_exec() or pcre16_exec(), 7589 PCRE_ERROR_PARTIAL is returned as soon as a partial match is found, 7590 without continuing to search for possible complete matches. This option 7591 is "hard" because it prefers an earlier partial match over a later com- 7592 plete match. For this reason, the assumption is made that the end of 7593 the supplied subject string may not be the true end of the available 7594 data, and so, if \z, \Z, \b, \B, or $ are encountered at the end of the 7595 subject, the result is PCRE_ERROR_PARTIAL, provided that at least one 7596 character in the subject has been inspected. 7597 7598 Setting PCRE_PARTIAL_HARD also affects the way UTF-8 and UTF-16 subject 7599 strings are checked for validity. Normally, an invalid sequence causes 7600 the error PCRE_ERROR_BADUTF8 or PCRE_ERROR_BADUTF16. However, in the 7601 special case of a truncated character at the end of the subject, 7602 PCRE_ERROR_SHORTUTF8 or PCRE_ERROR_SHORTUTF16 is returned when 7603 PCRE_PARTIAL_HARD is set. 7604 7605 Comparing hard and soft partial matching 7606 7607 The difference between the two partial matching options can be illus- 7608 trated by a pattern such as: 7609 7610 /dog(sbody)?/ 7611 7612 This matches either "dog" or "dogsbody", greedily (that is, it prefers 7613 the longer string if possible). If it is matched against the string 7614 "dog" with PCRE_PARTIAL_SOFT, it yields a complete match for "dog". 7615 However, if PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. 7616 On the other hand, if the pattern is made ungreedy the result is dif- 7617 ferent: 7618 7619 /dog(sbody)??/ 7620 7621 In this case the result is always a complete match because that is 7622 found first, and matching never continues after finding a complete 7623 match. It might be easier to follow this explanation by thinking of the 7624 two patterns like this: 7625 7626 /dog(sbody)?/ is the same as /dogsbody|dog/ 7627 /dog(sbody)??/ is the same as /dog|dogsbody/ 7628 7629 The second pattern will never match "dogsbody", because it will always 7630 find the shorter match first. 7631 7632 7633PARTIAL MATCHING USING pcre_dfa_exec() OR pcre16_dfa_exec() 7634 7635 The DFA functions move along the subject string character by character, 7636 without backtracking, searching for all possible matches simultane- 7637 ously. If the end of the subject is reached before the end of the pat- 7638 tern, there is the possibility of a partial match, again provided that 7639 at least one character has been inspected. 7640 7641 When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if 7642 there have been no complete matches. Otherwise, the complete matches 7643 are returned. However, if PCRE_PARTIAL_HARD is set, a partial match 7644 takes precedence over any complete matches. The portion of the string 7645 that was inspected when the longest partial match was found is set as 7646 the first matching string, provided there are at least two slots in the 7647 offsets vector. 7648 7649 Because the DFA functions always search for all possible matches, and 7650 there is no difference between greedy and ungreedy repetition, their 7651 behaviour is different from the standard functions when PCRE_PAR- 7652 TIAL_HARD is set. Consider the string "dog" matched against the 7653 ungreedy pattern shown above: 7654 7655 /dog(sbody)??/ 7656 7657 Whereas the standard functions stop as soon as they find the complete 7658 match for "dog", the DFA functions also find the partial match for 7659 "dogsbody", and so return that when PCRE_PARTIAL_HARD is set. 7660 7661 7662PARTIAL MATCHING AND WORD BOUNDARIES 7663 7664 If a pattern ends with one of sequences \b or \B, which test for word 7665 boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter- 7666 intuitive results. Consider this pattern: 7667 7668 /\bcat\b/ 7669 7670 This matches "cat", provided there is a word boundary at either end. If 7671 the subject string is "the cat", the comparison of the final "t" with a 7672 following character cannot take place, so a partial match is found. 7673 However, normal matching carries on, and \b matches at the end of the 7674 subject when the last character is a letter, so a complete match is 7675 found. The result, therefore, is not PCRE_ERROR_PARTIAL. Using 7676 PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because 7677 then the partial match takes precedence. 7678 7679 7680FORMERLY RESTRICTED PATTERNS 7681 7682 For releases of PCRE prior to 8.00, because of the way certain internal 7683 optimizations were implemented in the pcre_exec() function, the 7684 PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be 7685 used with all patterns. From release 8.00 onwards, the restrictions no 7686 longer apply, and partial matching with can be requested for any pat- 7687 tern. 7688 7689 Items that were formerly restricted were repeated single characters and 7690 repeated metasequences. If PCRE_PARTIAL was set for a pattern that did 7691 not conform to the restrictions, pcre_exec() returned the error code 7692 PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The 7693 PCRE_INFO_OKPARTIAL call to pcre_fullinfo() to find out if a compiled 7694 pattern can be used for partial matching now always returns 1. 7695 7696 7697EXAMPLE OF PARTIAL MATCHING USING PCRETEST 7698 7699 If the escape sequence \P is present in a pcretest data line, the 7700 PCRE_PARTIAL_SOFT option is used for the match. Here is a run of 7701 pcretest that uses the date example quoted above: 7702 7703 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ 7704 data> 25jun04\P 7705 0: 25jun04 7706 1: jun 7707 data> 25dec3\P 7708 Partial match: 23dec3 7709 data> 3ju\P 7710 Partial match: 3ju 7711 data> 3juj\P 7712 No match 7713 data> j\P 7714 No match 7715 7716 The first data string is matched completely, so pcretest shows the 7717 matched substrings. The remaining four strings do not match the com- 7718 plete pattern, but the first two are partial matches. Similar output is 7719 obtained if DFA matching is used. 7720 7721 If the escape sequence \P is present more than once in a pcretest data 7722 line, the PCRE_PARTIAL_HARD option is set for the match. 7723 7724 7725MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre16_dfa_exec() 7726 7727 When a partial match has been found using a DFA matching function, it 7728 is possible to continue the match by providing additional subject data 7729 and calling the function again with the same compiled regular expres- 7730 sion, this time setting the PCRE_DFA_RESTART option. You must pass the 7731 same working space as before, because this is where details of the pre- 7732 vious partial match are stored. Here is an example using pcretest, 7733 using the \R escape sequence to set the PCRE_DFA_RESTART option (\D 7734 specifies the use of the DFA matching function): 7735 7736 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ 7737 data> 23ja\P\D 7738 Partial match: 23ja 7739 data> n05\R\D 7740 0: n05 7741 7742 The first call has "23ja" as the subject, and requests partial match- 7743 ing; the second call has "n05" as the subject for the continued 7744 (restarted) match. Notice that when the match is complete, only the 7745 last part is shown; PCRE does not retain the previously partially- 7746 matched string. It is up to the calling program to do that if it needs 7747 to. 7748 7749 You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with 7750 PCRE_DFA_RESTART to continue partial matching over multiple segments. 7751 This facility can be used to pass very long subject strings to the DFA 7752 matching functions. 7753 7754 7755MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre16_exec() 7756 7757 From release 8.00, the standard matching functions can also be used to 7758 do multi-segment matching. Unlike the DFA functions, it is not possible 7759 to restart the previous match with a new segment of data. Instead, new 7760 data must be added to the previous subject string, and the entire match 7761 re-run, starting from the point where the partial match occurred. Ear- 7762 lier data can be discarded. 7763 7764 It is best to use PCRE_PARTIAL_HARD in this situation, because it does 7765 not treat the end of a segment as the end of the subject when matching 7766 \z, \Z, \b, \B, and $. Consider an unanchored pattern that matches 7767 dates: 7768 7769 re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/ 7770 data> The date is 23ja\P\P 7771 Partial match: 23ja 7772 7773 At this stage, an application could discard the text preceding "23ja", 7774 add on text from the next segment, and call the matching function 7775 again. Unlike the DFA matching functions, the entire matching string 7776 must always be available, and the complete matching process occurs for 7777 each call, so more memory and more processing time is needed. 7778 7779 Note: If the pattern contains lookbehind assertions, or \K, or starts 7780 with \b or \B, the string that is returned for a partial match includes 7781 characters that precede the partially matched string itself, because 7782 these must be retained when adding on more characters for a subsequent 7783 matching attempt. However, in some cases you may need to retain even 7784 earlier characters, as discussed in the next section. 7785 7786 7787ISSUES WITH MULTI-SEGMENT MATCHING 7788 7789 Certain types of pattern may give problems with multi-segment matching, 7790 whichever matching function is used. 7791 7792 1. If the pattern contains a test for the beginning of a line, you need 7793 to pass the PCRE_NOTBOL option when the subject string for any call 7794 does start at the beginning of a line. There is also a PCRE_NOTEOL 7795 option, but in practice when doing multi-segment matching you should be 7796 using PCRE_PARTIAL_HARD, which includes the effect of PCRE_NOTEOL. 7797 7798 2. Lookbehind assertions that have already been obeyed are catered for 7799 in the offsets that are returned for a partial match. However a lookbe- 7800 hind assertion later in the pattern could require even earlier charac- 7801 ters to be inspected. You can handle this case by using the 7802 PCRE_INFO_MAXLOOKBEHIND option of the pcre_fullinfo() or 7803 pcre16_fullinfo() functions to obtain the length of the largest lookbe- 7804 hind in the pattern. This length is given in characters, not bytes. If 7805 you always retain at least that many characters before the partially 7806 matched string, all should be well. (Of course, near the start of the 7807 subject, fewer characters may be present; in that case all characters 7808 should be retained.) 7809 7810 3. Because a partial match must always contain at least one character, 7811 what might be considered a partial match of an empty string actually 7812 gives a "no match" result. For example: 7813 7814 re> /c(?<=abc)x/ 7815 data> ab\P 7816 No match 7817 7818 If the next segment begins "cx", a match should be found, but this will 7819 only happen if characters from the previous segment are retained. For 7820 this reason, a "no match" result should be interpreted as "partial 7821 match of an empty string" when the pattern contains lookbehinds. 7822 7823 4. Matching a subject string that is split into multiple segments may 7824 not always produce exactly the same result as matching over one single 7825 long string, especially when PCRE_PARTIAL_SOFT is used. The section 7826 "Partial Matching and Word Boundaries" above describes an issue that 7827 arises if the pattern ends with \b or \B. Another kind of difference 7828 may occur when there are multiple matching possibilities, because (for 7829 PCRE_PARTIAL_SOFT) a partial match result is given only when there are 7830 no completed matches. This means that as soon as the shortest match has 7831 been found, continuation to a new subject segment is no longer possi- 7832 ble. Consider again this pcretest example: 7833 7834 re> /dog(sbody)?/ 7835 data> dogsb\P 7836 0: dog 7837 data> do\P\D 7838 Partial match: do 7839 data> gsb\R\P\D 7840 0: g 7841 data> dogsbody\D 7842 0: dogsbody 7843 1: dog 7844 7845 The first data line passes the string "dogsb" to a standard matching 7846 function, setting the PCRE_PARTIAL_SOFT option. Although the string is 7847 a partial match for "dogsbody", the result is not PCRE_ERROR_PARTIAL, 7848 because the shorter string "dog" is a complete match. Similarly, when 7849 the subject is presented to a DFA matching function in several parts 7850 ("do" and "gsb" being the first two) the match stops when "dog" has 7851 been found, and it is not possible to continue. On the other hand, if 7852 "dogsbody" is presented as a single string, a DFA matching function 7853 finds both matches. 7854 7855 Because of these problems, it is best to use PCRE_PARTIAL_HARD when 7856 matching multi-segment data. The example above then behaves differ- 7857 ently: 7858 7859 re> /dog(sbody)?/ 7860 data> dogsb\P\P 7861 Partial match: dogsb 7862 data> do\P\D 7863 Partial match: do 7864 data> gsb\R\P\P\D 7865 Partial match: gsb 7866 7867 5. Patterns that contain alternatives at the top level which do not all 7868 start with the same pattern item may not work as expected when 7869 PCRE_DFA_RESTART is used. For example, consider this pattern: 7870 7871 1234|3789 7872 7873 If the first part of the subject is "ABC123", a partial match of the 7874 first alternative is found at offset 3. There is no partial match for 7875 the second alternative, because such a match does not start at the same 7876 point in the subject string. Attempting to continue with the string 7877 "7890" does not yield a match because only those alternatives that 7878 match at one point in the subject are remembered. The problem arises 7879 because the start of the second alternative matches within the first 7880 alternative. There is no problem with anchored patterns or patterns 7881 such as: 7882 7883 1234|ABCD 7884 7885 where no string can be a partial match for both alternatives. This is 7886 not a problem if a standard matching function is used, because the 7887 entire match has to be rerun each time: 7888 7889 re> /1234|3789/ 7890 data> ABC123\P\P 7891 Partial match: 123 7892 data> 1237890 7893 0: 3789 7894 7895 Of course, instead of using PCRE_DFA_RESTART, the same technique of re- 7896 running the entire match can also be used with the DFA matching func- 7897 tions. Another possibility is to work with two buffers. If a partial 7898 match at offset n in the first buffer is followed by "no match" when 7899 PCRE_DFA_RESTART is used on the second buffer, you can then try a new 7900 match starting at offset n+1 in the first buffer. 7901 7902 7903AUTHOR 7904 7905 Philip Hazel 7906 University Computing Service 7907 Cambridge CB2 3QH, England. 7908 7909 7910REVISION 7911 7912 Last updated: 24 February 2012 7913 Copyright (c) 1997-2012 University of Cambridge. 7914------------------------------------------------------------------------------ 7915 7916 7917PCREPRECOMPILE(3) PCREPRECOMPILE(3) 7918 7919 7920NAME 7921 PCRE - Perl-compatible regular expressions 7922 7923 7924SAVING AND RE-USING PRECOMPILED PCRE PATTERNS 7925 7926 If you are running an application that uses a large number of regular 7927 expression patterns, it may be useful to store them in a precompiled 7928 form instead of having to compile them every time the application is 7929 run. If you are not using any private character tables (see the 7930 pcre_maketables() documentation), this is relatively straightforward. 7931 If you are using private tables, it is a little bit more complicated. 7932 However, if you are using the just-in-time optimization feature, it is 7933 not possible to save and reload the JIT data. 7934 7935 If you save compiled patterns to a file, you can copy them to a differ- 7936 ent host and run them there. If the two hosts have different endianness 7937 (byte order), you should run the pcre[16]_pattern_to_host_byte_order() 7938 function on the new host before trying to match the pattern. The match- 7939 ing functions return PCRE_ERROR_BADENDIANNESS if they detect a pattern 7940 with the wrong endianness. 7941 7942 Compiling regular expressions with one version of PCRE for use with a 7943 different version is not guaranteed to work and may cause crashes, and 7944 saving and restoring a compiled pattern loses any JIT optimization 7945 data. 7946 7947 7948SAVING A COMPILED PATTERN 7949 7950 The value returned by pcre[16]_compile() points to a single block of 7951 memory that holds the compiled pattern and associated data. You can 7952 find the length of this block in bytes by calling pcre[16]_fullinfo() 7953 with an argument of PCRE_INFO_SIZE. You can then save the data in any 7954 appropriate manner. Here is sample code for the 8-bit library that com- 7955 piles a pattern and writes it to a file. It assumes that the variable 7956 fd refers to a file that is open for output: 7957 7958 int erroroffset, rc, size; 7959 char *error; 7960 pcre *re; 7961 7962 re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL); 7963 if (re == NULL) { ... handle errors ... } 7964 rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size); 7965 if (rc < 0) { ... handle errors ... } 7966 rc = fwrite(re, 1, size, fd); 7967 if (rc != size) { ... handle errors ... } 7968 7969 In this example, the bytes that comprise the compiled pattern are 7970 copied exactly. Note that this is binary data that may contain any of 7971 the 256 possible byte values. On systems that make a distinction 7972 between binary and non-binary data, be sure that the file is opened for 7973 binary output. 7974 7975 If you want to write more than one pattern to a file, you will have to 7976 devise a way of separating them. For binary data, preceding each pat- 7977 tern with its length is probably the most straightforward approach. 7978 Another possibility is to write out the data in hexadecimal instead of 7979 binary, one pattern to a line. 7980 7981 Saving compiled patterns in a file is only one possible way of storing 7982 them for later use. They could equally well be saved in a database, or 7983 in the memory of some daemon process that passes them via sockets to 7984 the processes that want them. 7985 7986 If the pattern has been studied, it is also possible to save the normal 7987 study data in a similar way to the compiled pattern itself. However, if 7988 the PCRE_STUDY_JIT_COMPILE was used, the just-in-time data that is cre- 7989 ated cannot be saved because it is too dependent on the current envi- 7990 ronment. When studying generates additional information, 7991 pcre[16]_study() returns a pointer to a pcre[16]_extra data block. Its 7992 format is defined in the section on matching a pattern in the pcreapi 7993 documentation. The study_data field points to the binary study data, 7994 and this is what you must save (not the pcre[16]_extra block itself). 7995 The length of the study data can be obtained by calling 7996 pcre[16]_fullinfo() with an argument of PCRE_INFO_STUDYSIZE. Remember 7997 to check that pcre[16]_study() did return a non-NULL value before try- 7998 ing to save the study data. 7999 8000 8001RE-USING A PRECOMPILED PATTERN 8002 8003 Re-using a precompiled pattern is straightforward. Having reloaded it 8004 into main memory, called pcre[16]_pattern_to_host_byte_order() if nec- 8005 essary, you pass its pointer to pcre[16]_exec() or pcre[16]_dfa_exec() 8006 in the usual way. 8007 8008 However, if you passed a pointer to custom character tables when the 8009 pattern was compiled (the tableptr argument of pcre[16]_compile()), you 8010 must now pass a similar pointer to pcre[16]_exec() or 8011 pcre[16]_dfa_exec(), because the value saved with the compiled pattern 8012 will obviously be nonsense. A field in a pcre[16]_extra() block is used 8013 to pass this data, as described in the section on matching a pattern in 8014 the pcreapi documentation. 8015 8016 If you did not provide custom character tables when the pattern was 8017 compiled, the pointer in the compiled pattern is NULL, which causes the 8018 matching functions to use PCRE's internal tables. Thus, you do not need 8019 to take any special action at run time in this case. 8020 8021 If you saved study data with the compiled pattern, you need to create 8022 your own pcre[16]_extra data block and set the study_data field to 8023 point to the reloaded study data. You must also set the 8024 PCRE_EXTRA_STUDY_DATA bit in the flags field to indicate that study 8025 data is present. Then pass the pcre[16]_extra block to the matching 8026 function in the usual way. If the pattern was studied for just-in-time 8027 optimization, that data cannot be saved, and so is lost by a 8028 save/restore cycle. 8029 8030 8031COMPATIBILITY WITH DIFFERENT PCRE RELEASES 8032 8033 In general, it is safest to recompile all saved patterns when you 8034 update to a new PCRE release, though not all updates actually require 8035 this. 8036 8037 8038AUTHOR 8039 8040 Philip Hazel 8041 University Computing Service 8042 Cambridge CB2 3QH, England. 8043 8044 8045REVISION 8046 8047 Last updated: 10 January 2012 8048 Copyright (c) 1997-2012 University of Cambridge. 8049------------------------------------------------------------------------------ 8050 8051 8052PCREPERFORM(3) PCREPERFORM(3) 8053 8054 8055NAME 8056 PCRE - Perl-compatible regular expressions 8057 8058 8059PCRE PERFORMANCE 8060 8061 Two aspects of performance are discussed below: memory usage and pro- 8062 cessing time. The way you express your pattern as a regular expression 8063 can affect both of them. 8064 8065 8066COMPILED PATTERN MEMORY USAGE 8067 8068 Patterns are compiled by PCRE into a reasonably efficient interpretive 8069 code, so that most simple patterns do not use much memory. However, 8070 there is one case where the memory usage of a compiled pattern can be 8071 unexpectedly large. If a parenthesized subpattern has a quantifier with 8072 a minimum greater than 1 and/or a limited maximum, the whole subpattern 8073 is repeated in the compiled code. For example, the pattern 8074 8075 (abc|def){2,4} 8076 8077 is compiled as if it were 8078 8079 (abc|def)(abc|def)((abc|def)(abc|def)?)? 8080 8081 (Technical aside: It is done this way so that backtrack points within 8082 each of the repetitions can be independently maintained.) 8083 8084 For regular expressions whose quantifiers use only small numbers, this 8085 is not usually a problem. However, if the numbers are large, and par- 8086 ticularly if such repetitions are nested, the memory usage can become 8087 an embarrassment. For example, the very simple pattern 8088 8089 ((ab){1,1000}c){1,3} 8090 8091 uses 51K bytes when compiled using the 8-bit library. When PCRE is com- 8092 piled with its default internal pointer size of two bytes, the size 8093 limit on a compiled pattern is 64K data units, and this is reached with 8094 the above pattern if the outer repetition is increased from 3 to 4. 8095 PCRE can be compiled to use larger internal pointers and thus handle 8096 larger compiled patterns, but it is better to try to rewrite your pat- 8097 tern to use less memory if you can. 8098 8099 One way of reducing the memory usage for such patterns is to make use 8100 of PCRE's "subroutine" facility. Re-writing the above pattern as 8101 8102 ((ab)(?2){0,999}c)(?1){0,2} 8103 8104 reduces the memory requirements to 18K, and indeed it remains under 20K 8105 even with the outer repetition increased to 100. However, this pattern 8106 is not exactly equivalent, because the "subroutine" calls are treated 8107 as atomic groups into which there can be no backtracking if there is a 8108 subsequent matching failure. Therefore, PCRE cannot do this kind of 8109 rewriting automatically. Furthermore, there is a noticeable loss of 8110 speed when executing the modified pattern. Nevertheless, if the atomic 8111 grouping is not a problem and the loss of speed is acceptable, this 8112 kind of rewriting will allow you to process patterns that PCRE cannot 8113 otherwise handle. 8114 8115 8116STACK USAGE AT RUN TIME 8117 8118 When pcre_exec() or pcre16_exec() is used for matching, certain kinds 8119 of pattern can cause it to use large amounts of the process stack. In 8120 some environments the default process stack is quite small, and if it 8121 runs out the result is often SIGSEGV. This issue is probably the most 8122 frequently raised problem with PCRE. Rewriting your pattern can often 8123 help. The pcrestack documentation discusses this issue in detail. 8124 8125 8126PROCESSING TIME 8127 8128 Certain items in regular expression patterns are processed more effi- 8129 ciently than others. It is more efficient to use a character class like 8130 [aeiou] than a set of single-character alternatives such as 8131 (a|e|i|o|u). In general, the simplest construction that provides the 8132 required behaviour is usually the most efficient. Jeffrey Friedl's book 8133 contains a lot of useful general discussion about optimizing regular 8134 expressions for efficient performance. This document contains a few 8135 observations about PCRE. 8136 8137 Using Unicode character properties (the \p, \P, and \X escapes) is 8138 slow, because PCRE has to scan a structure that contains data for over 8139 fifteen thousand characters whenever it needs a character's property. 8140 If you can find an alternative pattern that does not use character 8141 properties, it will probably be faster. 8142 8143 By default, the escape sequences \b, \d, \s, and \w, and the POSIX 8144 character classes such as [:alpha:] do not use Unicode properties, 8145 partly for backwards compatibility, and partly for performance reasons. 8146 However, you can set PCRE_UCP if you want Unicode character properties 8147 to be used. This can double the matching time for items such as \d, 8148 when matched with a traditional matching function; the performance loss 8149 is less with a DFA matching function, and in both cases there is not 8150 much difference for \b. 8151 8152 When a pattern begins with .* not in parentheses, or in parentheses 8153 that are not the subject of a backreference, and the PCRE_DOTALL option 8154 is set, the pattern is implicitly anchored by PCRE, since it can match 8155 only at the start of a subject string. However, if PCRE_DOTALL is not 8156 set, PCRE cannot make this optimization, because the . metacharacter 8157 does not then match a newline, and if the subject string contains new- 8158 lines, the pattern may match from the character immediately following 8159 one of them instead of from the very start. For example, the pattern 8160 8161 .*second 8162 8163 matches the subject "first\nand second" (where \n stands for a newline 8164 character), with the match starting at the seventh character. In order 8165 to do this, PCRE has to retry the match starting after every newline in 8166 the subject. 8167 8168 If you are using such a pattern with subject strings that do not con- 8169 tain newlines, the best performance is obtained by setting PCRE_DOTALL, 8170 or starting the pattern with ^.* or ^.*? to indicate explicit anchor- 8171 ing. That saves PCRE from having to scan along the subject looking for 8172 a newline to restart at. 8173 8174 Beware of patterns that contain nested indefinite repeats. These can 8175 take a long time to run when applied to a string that does not match. 8176 Consider the pattern fragment 8177 8178 ^(a+)* 8179 8180 This can match "aaaa" in 16 different ways, and this number increases 8181 very rapidly as the string gets longer. (The * repeat can match 0, 1, 8182 2, 3, or 4 times, and for each of those cases other than 0 or 4, the + 8183 repeats can match different numbers of times.) When the remainder of 8184 the pattern is such that the entire match is going to fail, PCRE has in 8185 principle to try every possible variation, and this can take an 8186 extremely long time, even for relatively short strings. 8187 8188 An optimization catches some of the more simple cases such as 8189 8190 (a+)*b 8191 8192 where a literal character follows. Before embarking on the standard 8193 matching procedure, PCRE checks that there is a "b" later in the sub- 8194 ject string, and if there is not, it fails the match immediately. How- 8195 ever, when there is no following literal this optimization cannot be 8196 used. You can see the difference by comparing the behaviour of 8197 8198 (a+)*\d 8199 8200 with the pattern above. The former gives a failure almost instantly 8201 when applied to a whole line of "a" characters, whereas the latter 8202 takes an appreciable time with strings longer than about 20 characters. 8203 8204 In many cases, the solution to this kind of performance issue is to use 8205 an atomic group or a possessive quantifier. 8206 8207 8208AUTHOR 8209 8210 Philip Hazel 8211 University Computing Service 8212 Cambridge CB2 3QH, England. 8213 8214 8215REVISION 8216 8217 Last updated: 09 January 2012 8218 Copyright (c) 1997-2012 University of Cambridge. 8219------------------------------------------------------------------------------ 8220 8221 8222PCREPOSIX(3) PCREPOSIX(3) 8223 8224 8225NAME 8226 PCRE - Perl-compatible regular expressions. 8227 8228 8229SYNOPSIS OF POSIX API 8230 8231 #include <pcreposix.h> 8232 8233 int regcomp(regex_t *preg, const char *pattern, 8234 int cflags); 8235 8236 int regexec(regex_t *preg, const char *string, 8237 size_t nmatch, regmatch_t pmatch[], int eflags); 8238 8239 size_t regerror(int errcode, const regex_t *preg, 8240 char *errbuf, size_t errbuf_size); 8241 8242 void regfree(regex_t *preg); 8243 8244 8245DESCRIPTION 8246 8247 This set of functions provides a POSIX-style API for the PCRE regular 8248 expression 8-bit library. See the pcreapi documentation for a descrip- 8249 tion of PCRE's native API, which contains much additional functional- 8250 ity. There is no POSIX-style wrapper for PCRE's 16-bit library. 8251 8252 The functions described here are just wrapper functions that ultimately 8253 call the PCRE native API. Their prototypes are defined in the 8254 pcreposix.h header file, and on Unix systems the library itself is 8255 called pcreposix.a, so can be accessed by adding -lpcreposix to the 8256 command for linking an application that uses them. Because the POSIX 8257 functions call the native ones, it is also necessary to add -lpcre. 8258 8259 I have implemented only those POSIX option bits that can be reasonably 8260 mapped to PCRE native options. In addition, the option REG_EXTENDED is 8261 defined with the value zero. This has no effect, but since programs 8262 that are written to the POSIX interface often use it, this makes it 8263 easier to slot in PCRE as a replacement library. Other POSIX options 8264 are not even defined. 8265 8266 There are also some other options that are not defined by POSIX. These 8267 have been added at the request of users who want to make use of certain 8268 PCRE-specific features via the POSIX calling interface. 8269 8270 When PCRE is called via these functions, it is only the API that is 8271 POSIX-like in style. The syntax and semantics of the regular expres- 8272 sions themselves are still those of Perl, subject to the setting of 8273 various PCRE options, as described below. "POSIX-like in style" means 8274 that the API approximates to the POSIX definition; it is not fully 8275 POSIX-compatible, and in multi-byte encoding domains it is probably 8276 even less compatible. 8277 8278 The header for these functions is supplied as pcreposix.h to avoid any 8279 potential clash with other POSIX libraries. It can, of course, be 8280 renamed or aliased as regex.h, which is the "correct" name. It provides 8281 two structure types, regex_t for compiled internal forms, and reg- 8282 match_t for returning captured substrings. It also defines some con- 8283 stants whose names start with "REG_"; these are used for setting 8284 options and identifying error codes. 8285 8286 8287COMPILING A PATTERN 8288 8289 The function regcomp() is called to compile a pattern into an internal 8290 form. The pattern is a C string terminated by a binary zero, and is 8291 passed in the argument pattern. The preg argument is a pointer to a 8292 regex_t structure that is used as a base for storing information about 8293 the compiled regular expression. 8294 8295 The argument cflags is either zero, or contains one or more of the bits 8296 defined by the following macros: 8297 8298 REG_DOTALL 8299 8300 The PCRE_DOTALL option is set when the regular expression is passed for 8301 compilation to the native function. Note that REG_DOTALL is not part of 8302 the POSIX standard. 8303 8304 REG_ICASE 8305 8306 The PCRE_CASELESS option is set when the regular expression is passed 8307 for compilation to the native function. 8308 8309 REG_NEWLINE 8310 8311 The PCRE_MULTILINE option is set when the regular expression is passed 8312 for compilation to the native function. Note that this does not mimic 8313 the defined POSIX behaviour for REG_NEWLINE (see the following sec- 8314 tion). 8315 8316 REG_NOSUB 8317 8318 The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is 8319 passed for compilation to the native function. In addition, when a pat- 8320 tern that is compiled with this flag is passed to regexec() for match- 8321 ing, the nmatch and pmatch arguments are ignored, and no captured 8322 strings are returned. 8323 8324 REG_UCP 8325 8326 The PCRE_UCP option is set when the regular expression is passed for 8327 compilation to the native function. This causes PCRE to use Unicode 8328 properties when matchine \d, \w, etc., instead of just recognizing 8329 ASCII values. Note that REG_UTF8 is not part of the POSIX standard. 8330 8331 REG_UNGREEDY 8332 8333 The PCRE_UNGREEDY option is set when the regular expression is passed 8334 for compilation to the native function. Note that REG_UNGREEDY is not 8335 part of the POSIX standard. 8336 8337 REG_UTF8 8338 8339 The PCRE_UTF8 option is set when the regular expression is passed for 8340 compilation to the native function. This causes the pattern itself and 8341 all data strings used for matching it to be treated as UTF-8 strings. 8342 Note that REG_UTF8 is not part of the POSIX standard. 8343 8344 In the absence of these flags, no options are passed to the native 8345 function. This means the the regex is compiled with PCRE default 8346 semantics. In particular, the way it handles newline characters in the 8347 subject string is the Perl way, not the POSIX way. Note that setting 8348 PCRE_MULTILINE has only some of the effects specified for REG_NEWLINE. 8349 It does not affect the way newlines are matched by . (they are not) or 8350 by a negative class such as [^a] (they are). 8351 8352 The yield of regcomp() is zero on success, and non-zero otherwise. The 8353 preg structure is filled in on success, and one member of the structure 8354 is public: re_nsub contains the number of capturing subpatterns in the 8355 regular expression. Various error codes are defined in the header file. 8356 8357 NOTE: If the yield of regcomp() is non-zero, you must not attempt to 8358 use the contents of the preg structure. If, for example, you pass it to 8359 regexec(), the result is undefined and your program is likely to crash. 8360 8361 8362MATCHING NEWLINE CHARACTERS 8363 8364 This area is not simple, because POSIX and Perl take different views of 8365 things. It is not possible to get PCRE to obey POSIX semantics, but 8366 then PCRE was never intended to be a POSIX engine. The following table 8367 lists the different possibilities for matching newline characters in 8368 PCRE: 8369 8370 Default Change with 8371 8372 . matches newline no PCRE_DOTALL 8373 newline matches [^a] yes not changeable 8374 $ matches \n at end yes PCRE_DOLLARENDONLY 8375 $ matches \n in middle no PCRE_MULTILINE 8376 ^ matches \n in middle no PCRE_MULTILINE 8377 8378 This is the equivalent table for POSIX: 8379 8380 Default Change with 8381 8382 . matches newline yes REG_NEWLINE 8383 newline matches [^a] yes REG_NEWLINE 8384 $ matches \n at end no REG_NEWLINE 8385 $ matches \n in middle no REG_NEWLINE 8386 ^ matches \n in middle no REG_NEWLINE 8387 8388 PCRE's behaviour is the same as Perl's, except that there is no equiva- 8389 lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is 8390 no way to stop newline from matching [^a]. 8391 8392 The default POSIX newline handling can be obtained by setting 8393 PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE 8394 behave exactly as for the REG_NEWLINE action. 8395 8396 8397MATCHING A PATTERN 8398 8399 The function regexec() is called to match a compiled pattern preg 8400 against a given string, which is by default terminated by a zero byte 8401 (but see REG_STARTEND below), subject to the options in eflags. These 8402 can be: 8403 8404 REG_NOTBOL 8405 8406 The PCRE_NOTBOL option is set when calling the underlying PCRE matching 8407 function. 8408 8409 REG_NOTEMPTY 8410 8411 The PCRE_NOTEMPTY option is set when calling the underlying PCRE match- 8412 ing function. Note that REG_NOTEMPTY is not part of the POSIX standard. 8413 However, setting this option can give more POSIX-like behaviour in some 8414 situations. 8415 8416 REG_NOTEOL 8417 8418 The PCRE_NOTEOL option is set when calling the underlying PCRE matching 8419 function. 8420 8421 REG_STARTEND 8422 8423 The string is considered to start at string + pmatch[0].rm_so and to 8424 have a terminating NUL located at string + pmatch[0].rm_eo (there need 8425 not actually be a NUL at that location), regardless of the value of 8426 nmatch. This is a BSD extension, compatible with but not specified by 8427 IEEE Standard 1003.2 (POSIX.2), and should be used with caution in 8428 software intended to be portable to other systems. Note that a non-zero 8429 rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location 8430 of the string, not how it is matched. 8431 8432 If the pattern was compiled with the REG_NOSUB flag, no data about any 8433 matched strings is returned. The nmatch and pmatch arguments of 8434 regexec() are ignored. 8435 8436 If the value of nmatch is zero, or if the value pmatch is NULL, no data 8437 about any matched strings is returned. 8438 8439 Otherwise,the portion of the string that was matched, and also any cap- 8440 tured substrings, are returned via the pmatch argument, which points to 8441 an array of nmatch structures of type regmatch_t, containing the mem- 8442 bers rm_so and rm_eo. These contain the offset to the first character 8443 of each substring and the offset to the first character after the end 8444 of each substring, respectively. The 0th element of the vector relates 8445 to the entire portion of string that was matched; subsequent elements 8446 relate to the capturing subpatterns of the regular expression. Unused 8447 entries in the array have both structure members set to -1. 8448 8449 A successful match yields a zero return; various error codes are 8450 defined in the header file, of which REG_NOMATCH is the "expected" 8451 failure code. 8452 8453 8454ERROR MESSAGES 8455 8456 The regerror() function maps a non-zero errorcode from either regcomp() 8457 or regexec() to a printable message. If preg is not NULL, the error 8458 should have arisen from the use of that structure. A message terminated 8459 by a binary zero is placed in errbuf. The length of the message, 8460 including the zero, is limited to errbuf_size. The yield of the func- 8461 tion is the size of buffer needed to hold the whole message. 8462 8463 8464MEMORY USAGE 8465 8466 Compiling a regular expression causes memory to be allocated and asso- 8467 ciated with the preg structure. The function regfree() frees all such 8468 memory, after which preg may no longer be used as a compiled expres- 8469 sion. 8470 8471 8472AUTHOR 8473 8474 Philip Hazel 8475 University Computing Service 8476 Cambridge CB2 3QH, England. 8477 8478 8479REVISION 8480 8481 Last updated: 09 January 2012 8482 Copyright (c) 1997-2012 University of Cambridge. 8483------------------------------------------------------------------------------ 8484 8485 8486PCRECPP(3) PCRECPP(3) 8487 8488 8489NAME 8490 PCRE - Perl-compatible regular expressions. 8491 8492 8493SYNOPSIS OF C++ WRAPPER 8494 8495 #include <pcrecpp.h> 8496 8497 8498DESCRIPTION 8499 8500 The C++ wrapper for PCRE was provided by Google Inc. Some additional 8501 functionality was added by Giuseppe Maxia. This brief man page was con- 8502 structed from the notes in the pcrecpp.h file, which should be con- 8503 sulted for further details. Note that the C++ wrapper supports only the 8504 original 8-bit PCRE library. There is no 16-bit support at present. 8505 8506 8507MATCHING INTERFACE 8508 8509 The "FullMatch" operation checks that supplied text matches a supplied 8510 pattern exactly. If pointer arguments are supplied, it copies matched 8511 sub-strings that match sub-patterns into them. 8512 8513 Example: successful match 8514 pcrecpp::RE re("h.*o"); 8515 re.FullMatch("hello"); 8516 8517 Example: unsuccessful match (requires full match): 8518 pcrecpp::RE re("e"); 8519 !re.FullMatch("hello"); 8520 8521 Example: creating a temporary RE object: 8522 pcrecpp::RE("h.*o").FullMatch("hello"); 8523 8524 You can pass in a "const char*" or a "string" for "text". The examples 8525 below tend to use a const char*. You can, as in the different examples 8526 above, store the RE object explicitly in a variable or use a temporary 8527 RE object. The examples below use one mode or the other arbitrarily. 8528 Either could correctly be used for any of these examples. 8529 8530 You must supply extra pointer arguments to extract matched subpieces. 8531 8532 Example: extracts "ruby" into "s" and 1234 into "i" 8533 int i; 8534 string s; 8535 pcrecpp::RE re("(\\w+):(\\d+)"); 8536 re.FullMatch("ruby:1234", &s, &i); 8537 8538 Example: does not try to extract any extra sub-patterns 8539 re.FullMatch("ruby:1234", &s); 8540 8541 Example: does not try to extract into NULL 8542 re.FullMatch("ruby:1234", NULL, &i); 8543 8544 Example: integer overflow causes failure 8545 !re.FullMatch("ruby:1234567891234", NULL, &i); 8546 8547 Example: fails because there aren't enough sub-patterns: 8548 !pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s); 8549 8550 Example: fails because string cannot be stored in integer 8551 !pcrecpp::RE("(.*)").FullMatch("ruby", &i); 8552 8553 The provided pointer arguments can be pointers to any scalar numeric 8554 type, or one of: 8555 8556 string (matched piece is copied to string) 8557 StringPiece (StringPiece is mutated to point to matched piece) 8558 T (where "bool T::ParseFrom(const char*, int)" exists) 8559 NULL (the corresponding matched sub-pattern is not copied) 8560 8561 The function returns true iff all of the following conditions are sat- 8562 isfied: 8563 8564 a. "text" matches "pattern" exactly; 8565 8566 b. The number of matched sub-patterns is >= number of supplied 8567 pointers; 8568 8569 c. The "i"th argument has a suitable type for holding the 8570 string captured as the "i"th sub-pattern. If you pass in 8571 void * NULL for the "i"th argument, or a non-void * NULL 8572 of the correct type, or pass fewer arguments than the 8573 number of sub-patterns, "i"th captured sub-pattern is 8574 ignored. 8575 8576 CAVEAT: An optional sub-pattern that does not exist in the matched 8577 string is assigned the empty string. Therefore, the following will 8578 return false (because the empty string is not a valid number): 8579 8580 int number; 8581 pcrecpp::RE::FullMatch("abc", "[a-z]+(\\d+)?", &number); 8582 8583 The matching interface supports at most 16 arguments per call. If you 8584 need more, consider using the more general interface 8585 pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch. 8586 8587 NOTE: Do not use no_arg, which is used internally to mark the end of a 8588 list of optional arguments, as a placeholder for missing arguments, as 8589 this can lead to segfaults. 8590 8591 8592QUOTING METACHARACTERS 8593 8594 You can use the "QuoteMeta" operation to insert backslashes before all 8595 potentially meaningful characters in a string. The returned string, 8596 used as a regular expression, will exactly match the original string. 8597 8598 Example: 8599 string quoted = RE::QuoteMeta(unquoted); 8600 8601 Note that it's legal to escape a character even if it has no special 8602 meaning in a regular expression -- so this function does that. (This 8603 also makes it identical to the perl function of the same name; see 8604 "perldoc -f quotemeta".) For example, "1.5-2.0?" becomes 8605 "1\.5\-2\.0\?". 8606 8607 8608PARTIAL MATCHES 8609 8610 You can use the "PartialMatch" operation when you want the pattern to 8611 match any substring of the text. 8612 8613 Example: simple search for a string: 8614 pcrecpp::RE("ell").PartialMatch("hello"); 8615 8616 Example: find first number in a string: 8617 int number; 8618 pcrecpp::RE re("(\\d+)"); 8619 re.PartialMatch("x*100 + 20", &number); 8620 assert(number == 100); 8621 8622 8623UTF-8 AND THE MATCHING INTERFACE 8624 8625 By default, pattern and text are plain text, one byte per character. 8626 The UTF8 flag, passed to the constructor, causes both pattern and 8627 string to be treated as UTF-8 text, still a byte stream but potentially 8628 multiple bytes per character. In practice, the text is likelier to be 8629 UTF-8 than the pattern, but the match returned may depend on the UTF8 8630 flag, so always use it when matching UTF8 text. For example, "." will 8631 match one byte normally but with UTF8 set may match up to three bytes 8632 of a multi-byte character. 8633 8634 Example: 8635 pcrecpp::RE_Options options; 8636 options.set_utf8(); 8637 pcrecpp::RE re(utf8_pattern, options); 8638 re.FullMatch(utf8_string); 8639 8640 Example: using the convenience function UTF8(): 8641 pcrecpp::RE re(utf8_pattern, pcrecpp::UTF8()); 8642 re.FullMatch(utf8_string); 8643 8644 NOTE: The UTF8 flag is ignored if pcre was not configured with the 8645 --enable-utf8 flag. 8646 8647 8648PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE 8649 8650 PCRE defines some modifiers to change the behavior of the regular 8651 expression engine. The C++ wrapper defines an auxiliary class, 8652 RE_Options, as a vehicle to pass such modifiers to a RE class. Cur- 8653 rently, the following modifiers are supported: 8654 8655 modifier description Perl corresponding 8656 8657 PCRE_CASELESS case insensitive match /i 8658 PCRE_MULTILINE multiple lines match /m 8659 PCRE_DOTALL dot matches newlines /s 8660 PCRE_DOLLAR_ENDONLY $ matches only at end N/A 8661 PCRE_EXTRA strict escape parsing N/A 8662 PCRE_EXTENDED ignore white spaces /x 8663 PCRE_UTF8 handles UTF8 chars built-in 8664 PCRE_UNGREEDY reverses * and *? N/A 8665 PCRE_NO_AUTO_CAPTURE disables capturing parens N/A (*) 8666 8667 (*) Both Perl and PCRE allow non capturing parentheses by means of the 8668 "?:" modifier within the pattern itself. e.g. (?:ab|cd) does not cap- 8669 ture, while (ab|cd) does. 8670 8671 For a full account on how each modifier works, please check the PCRE 8672 API reference page. 8673 8674 For each modifier, there are two member functions whose name is made 8675 out of the modifier in lowercase, without the "PCRE_" prefix. For 8676 instance, PCRE_CASELESS is handled by 8677 8678 bool caseless() 8679 8680 which returns true if the modifier is set, and 8681 8682 RE_Options & set_caseless(bool) 8683 8684 which sets or unsets the modifier. Moreover, PCRE_EXTRA_MATCH_LIMIT can 8685 be accessed through the set_match_limit() and match_limit() member 8686 functions. Setting match_limit to a non-zero value will limit the exe- 8687 cution of pcre to keep it from doing bad things like blowing the stack 8688 or taking an eternity to return a result. A value of 5000 is good 8689 enough to stop stack blowup in a 2MB thread stack. Setting match_limit 8690 to zero disables match limiting. Alternatively, you can call 8691 match_limit_recursion() which uses PCRE_EXTRA_MATCH_LIMIT_RECURSION to 8692 limit how much PCRE recurses. match_limit() limits the number of 8693 matches PCRE does; match_limit_recursion() limits the depth of internal 8694 recursion, and therefore the amount of stack that is used. 8695 8696 Normally, to pass one or more modifiers to a RE class, you declare a 8697 RE_Options object, set the appropriate options, and pass this object to 8698 a RE constructor. Example: 8699 8700 RE_Options opt; 8701 opt.set_caseless(true); 8702 if (RE("HELLO", opt).PartialMatch("hello world")) ... 8703 8704 RE_options has two constructors. The default constructor takes no argu- 8705 ments and creates a set of flags that are off by default. The optional 8706 parameter option_flags is to facilitate transfer of legacy code from C 8707 programs. This lets you do 8708 8709 RE(pattern, 8710 RE_Options(PCRE_CASELESS|PCRE_MULTILINE)).PartialMatch(str); 8711 8712 However, new code is better off doing 8713 8714 RE(pattern, 8715 RE_Options().set_caseless(true).set_multiline(true)) 8716 .PartialMatch(str); 8717 8718 If you are going to pass one of the most used modifiers, there are some 8719 convenience functions that return a RE_Options class with the appropri- 8720 ate modifier already set: CASELESS(), UTF8(), MULTILINE(), DOTALL(), 8721 and EXTENDED(). 8722 8723 If you need to set several options at once, and you don't want to go 8724 through the pains of declaring a RE_Options object and setting several 8725 options, there is a parallel method that give you such ability on the 8726 fly. You can concatenate several set_xxxxx() member functions, since 8727 each of them returns a reference to its class object. For example, to 8728 pass PCRE_CASELESS, PCRE_EXTENDED, and PCRE_MULTILINE to a RE with one 8729 statement, you may write: 8730 8731 RE(" ^ xyz \\s+ .* blah$", 8732 RE_Options() 8733 .set_caseless(true) 8734 .set_extended(true) 8735 .set_multiline(true)).PartialMatch(sometext); 8736 8737 8738SCANNING TEXT INCREMENTALLY 8739 8740 The "Consume" operation may be useful if you want to repeatedly match 8741 regular expressions at the front of a string and skip over them as they 8742 match. This requires use of the "StringPiece" type, which represents a 8743 sub-range of a real string. Like RE, StringPiece is defined in the 8744 pcrecpp namespace. 8745 8746 Example: read lines of the form "var = value" from a string. 8747 string contents = ...; // Fill string somehow 8748 pcrecpp::StringPiece input(contents); // Wrap in a StringPiece 8749 8750 string var; 8751 int value; 8752 pcrecpp::RE re("(\\w+) = (\\d+)\n"); 8753 while (re.Consume(&input, &var, &value)) { 8754 ...; 8755 } 8756 8757 Each successful call to "Consume" will set "var/value", and also 8758 advance "input" so it points past the matched text. 8759 8760 The "FindAndConsume" operation is similar to "Consume" but does not 8761 anchor your match at the beginning of the string. For example, you 8762 could extract all words from a string by repeatedly calling 8763 8764 pcrecpp::RE("(\\w+)").FindAndConsume(&input, &word) 8765 8766 8767PARSING HEX/OCTAL/C-RADIX NUMBERS 8768 8769 By default, if you pass a pointer to a numeric value, the corresponding 8770 text is interpreted as a base-10 number. You can instead wrap the 8771 pointer with a call to one of the operators Hex(), Octal(), or CRadix() 8772 to interpret the text in another base. The CRadix operator interprets 8773 C-style "0" (base-8) and "0x" (base-16) prefixes, but defaults to 8774 base-10. 8775 8776 Example: 8777 int a, b, c, d; 8778 pcrecpp::RE re("(.*) (.*) (.*) (.*)"); 8779 re.FullMatch("100 40 0100 0x40", 8780 pcrecpp::Octal(&a), pcrecpp::Hex(&b), 8781 pcrecpp::CRadix(&c), pcrecpp::CRadix(&d)); 8782 8783 will leave 64 in a, b, c, and d. 8784 8785 8786REPLACING PARTS OF STRINGS 8787 8788 You can replace the first match of "pattern" in "str" with "rewrite". 8789 Within "rewrite", backslash-escaped digits (\1 to \9) can be used to 8790 insert text matching corresponding parenthesized group from the pat- 8791 tern. \0 in "rewrite" refers to the entire matching text. For example: 8792 8793 string s = "yabba dabba doo"; 8794 pcrecpp::RE("b+").Replace("d", &s); 8795 8796 will leave "s" containing "yada dabba doo". The result is true if the 8797 pattern matches and a replacement occurs, false otherwise. 8798 8799 GlobalReplace is like Replace except that it replaces all occurrences 8800 of the pattern in the string with the rewrite. Replacements are not 8801 subject to re-matching. For example: 8802 8803 string s = "yabba dabba doo"; 8804 pcrecpp::RE("b+").GlobalReplace("d", &s); 8805 8806 will leave "s" containing "yada dada doo". It returns the number of 8807 replacements made. 8808 8809 Extract is like Replace, except that if the pattern matches, "rewrite" 8810 is copied into "out" (an additional argument) with substitutions. The 8811 non-matching portions of "text" are ignored. Returns true iff a match 8812 occurred and the extraction happened successfully; if no match occurs, 8813 the string is left unaffected. 8814 8815 8816AUTHOR 8817 8818 The C++ wrapper was contributed by Google Inc. 8819 Copyright (c) 2007 Google Inc. 8820 8821 8822REVISION 8823 8824 Last updated: 08 January 2012 8825------------------------------------------------------------------------------ 8826 8827 8828PCRESAMPLE(3) PCRESAMPLE(3) 8829 8830 8831NAME 8832 PCRE - Perl-compatible regular expressions 8833 8834 8835PCRE SAMPLE PROGRAM 8836 8837 A simple, complete demonstration program, to get you started with using 8838 PCRE, is supplied in the file pcredemo.c in the PCRE distribution. A 8839 listing of this program is given in the pcredemo documentation. If you 8840 do not have a copy of the PCRE distribution, you can save this listing 8841 to re-create pcredemo.c. 8842 8843 The demonstration program, which uses the original PCRE 8-bit library, 8844 compiles the regular expression that is its first argument, and matches 8845 it against the subject string in its second argument. No PCRE options 8846 are set, and default character tables are used. If matching succeeds, 8847 the program outputs the portion of the subject that matched, together 8848 with the contents of any captured substrings. 8849 8850 If the -g option is given on the command line, the program then goes on 8851 to check for further matches of the same regular expression in the same 8852 subject string. The logic is a little bit tricky because of the possi- 8853 bility of matching an empty string. Comments in the code explain what 8854 is going on. 8855 8856 If PCRE is installed in the standard include and library directories 8857 for your operating system, you should be able to compile the demonstra- 8858 tion program using this command: 8859 8860 gcc -o pcredemo pcredemo.c -lpcre 8861 8862 If PCRE is installed elsewhere, you may need to add additional options 8863 to the command line. For example, on a Unix-like system that has PCRE 8864 installed in /usr/local, you can compile the demonstration program 8865 using a command like this: 8866 8867 gcc -o pcredemo -I/usr/local/include pcredemo.c \ 8868 -L/usr/local/lib -lpcre 8869 8870 In a Windows environment, if you want to statically link the program 8871 against a non-dll pcre.a file, you must uncomment the line that defines 8872 PCRE_STATIC before including pcre.h, because otherwise the pcre_mal- 8873 loc() and pcre_free() exported functions will be declared 8874 __declspec(dllimport), with unwanted results. 8875 8876 Once you have compiled and linked the demonstration program, you can 8877 run simple tests like this: 8878 8879 ./pcredemo 'cat|dog' 'the cat sat on the mat' 8880 ./pcredemo -g 'cat|dog' 'the dog sat on the cat' 8881 8882 Note that there is a much more comprehensive test program, called 8883 pcretest, which supports many more facilities for testing regular 8884 expressions and both PCRE libraries. The pcredemo program is provided 8885 as a simple coding example. 8886 8887 If you try to run pcredemo when PCRE is not installed in the standard 8888 library directory, you may get an error like this on some operating 8889 systems (e.g. Solaris): 8890 8891 ld.so.1: a.out: fatal: libpcre.so.0: open failed: No such file or 8892 directory 8893 8894 This is caused by the way shared library support works on those sys- 8895 tems. You need to add 8896 8897 -R/usr/local/lib 8898 8899 (for example) to the compile command to get round this problem. 8900 8901 8902AUTHOR 8903 8904 Philip Hazel 8905 University Computing Service 8906 Cambridge CB2 3QH, England. 8907 8908 8909REVISION 8910 8911 Last updated: 10 January 2012 8912 Copyright (c) 1997-2012 University of Cambridge. 8913------------------------------------------------------------------------------ 8914PCRELIMITS(3) PCRELIMITS(3) 8915 8916 8917NAME 8918 PCRE - Perl-compatible regular expressions 8919 8920 8921SIZE AND OTHER LIMITATIONS 8922 8923 There are some size limitations in PCRE but it is hoped that they will 8924 never in practice be relevant. 8925 8926 The maximum length of a compiled pattern is approximately 64K data 8927 units (bytes for the 8-bit library, 16-bit units for the 16-bit 8928 library) if PCRE is compiled with the default internal linkage size of 8929 2 bytes. If you want to process regular expressions that are truly 8930 enormous, you can compile PCRE with an internal linkage size of 3 or 4 8931 (when building the 16-bit library, 3 is rounded up to 4). See the 8932 README file in the source distribution and the pcrebuild documentation 8933 for details. In these cases the limit is substantially larger. How- 8934 ever, the speed of execution is slower. 8935 8936 All values in repeating quantifiers must be less than 65536. 8937 8938 There is no limit to the number of parenthesized subpatterns, but there 8939 can be no more than 65535 capturing subpatterns. 8940 8941 There is a limit to the number of forward references to subsequent sub- 8942 patterns of around 200,000. Repeated forward references with fixed 8943 upper limits, for example, (?2){0,100} when subpattern number 2 is to 8944 the right, are included in the count. There is no limit to the number 8945 of backward references. 8946 8947 The maximum length of name for a named subpattern is 32 characters, and 8948 the maximum number of named subpatterns is 10000. 8949 8950 The maximum length of a name in a (*MARK), (*PRUNE), (*SKIP), or 8951 (*THEN) verb is 255 for the 8-bit library and 65535 for the 16-bit 8952 library. 8953 8954 The maximum length of a subject string is the largest positive number 8955 that an integer variable can hold. However, when using the traditional 8956 matching function, PCRE uses recursion to handle subpatterns and indef- 8957 inite repetition. This means that the available stack space may limit 8958 the size of a subject string that can be processed by certain patterns. 8959 For a discussion of stack issues, see the pcrestack documentation. 8960 8961 8962AUTHOR 8963 8964 Philip Hazel 8965 University Computing Service 8966 Cambridge CB2 3QH, England. 8967 8968 8969REVISION 8970 8971 Last updated: 04 May 2012 8972 Copyright (c) 1997-2012 University of Cambridge. 8973------------------------------------------------------------------------------ 8974 8975 8976PCRESTACK(3) PCRESTACK(3) 8977 8978 8979NAME 8980 PCRE - Perl-compatible regular expressions 8981 8982 8983PCRE DISCUSSION OF STACK USAGE 8984 8985 When you call pcre[16]_exec(), it makes use of an internal function 8986 called match(). This calls itself recursively at branch points in the 8987 pattern, in order to remember the state of the match so that it can 8988 back up and try a different alternative if the first one fails. As 8989 matching proceeds deeper and deeper into the tree of possibilities, the 8990 recursion depth increases. The match() function is also called in other 8991 circumstances, for example, whenever a parenthesized sub-pattern is 8992 entered, and in certain cases of repetition. 8993 8994 Not all calls of match() increase the recursion depth; for an item such 8995 as a* it may be called several times at the same level, after matching 8996 different numbers of a's. Furthermore, in a number of cases where the 8997 result of the recursive call would immediately be passed back as the 8998 result of the current call (a "tail recursion"), the function is just 8999 restarted instead. 9000 9001 The above comments apply when pcre[16]_exec() is run in its normal 9002 interpretive manner. If the pattern was studied with the 9003 PCRE_STUDY_JIT_COMPILE option, and just-in-time compiling was success- 9004 ful, and the options passed to pcre[16]_exec() were not incompatible, 9005 the matching process uses the JIT-compiled code instead of the match() 9006 function. In this case, the memory requirements are handled entirely 9007 differently. See the pcrejit documentation for details. 9008 9009 The pcre[16]_dfa_exec() function operates in an entirely different way, 9010 and uses recursion only when there is a regular expression recursion or 9011 subroutine call in the pattern. This includes the processing of asser- 9012 tion and "once-only" subpatterns, which are handled like subroutine 9013 calls. Normally, these are never very deep, and the limit on the com- 9014 plexity of pcre[16]_dfa_exec() is controlled by the amount of workspace 9015 it is given. However, it is possible to write patterns with runaway 9016 infinite recursions; such patterns will cause pcre[16]_dfa_exec() to 9017 run out of stack. At present, there is no protection against this. 9018 9019 The comments that follow do NOT apply to pcre[16]_dfa_exec(); they are 9020 relevant only for pcre[16]_exec() without the JIT optimization. 9021 9022 Reducing pcre[16]_exec()'s stack usage 9023 9024 Each time that match() is actually called recursively, it uses memory 9025 from the process stack. For certain kinds of pattern and data, very 9026 large amounts of stack may be needed, despite the recognition of "tail 9027 recursion". You can often reduce the amount of recursion, and there- 9028 fore the amount of stack used, by modifying the pattern that is being 9029 matched. Consider, for example, this pattern: 9030 9031 ([^<]|<(?!inet))+ 9032 9033 It matches from wherever it starts until it encounters "<inet" or the 9034 end of the data, and is the kind of pattern that might be used when 9035 processing an XML file. Each iteration of the outer parentheses matches 9036 either one character that is not "<" or a "<" that is not followed by 9037 "inet". However, each time a parenthesis is processed, a recursion 9038 occurs, so this formulation uses a stack frame for each matched charac- 9039 ter. For a long string, a lot of stack is required. Consider now this 9040 rewritten pattern, which matches exactly the same strings: 9041 9042 ([^<]++|<(?!inet))+ 9043 9044 This uses very much less stack, because runs of characters that do not 9045 contain "<" are "swallowed" in one item inside the parentheses. Recur- 9046 sion happens only when a "<" character that is not followed by "inet" 9047 is encountered (and we assume this is relatively rare). A possessive 9048 quantifier is used to stop any backtracking into the runs of non-"<" 9049 characters, but that is not related to stack usage. 9050 9051 This example shows that one way of avoiding stack problems when match- 9052 ing long subject strings is to write repeated parenthesized subpatterns 9053 to match more than one character whenever possible. 9054 9055 Compiling PCRE to use heap instead of stack for pcre[16]_exec() 9056 9057 In environments where stack memory is constrained, you might want to 9058 compile PCRE to use heap memory instead of stack for remembering back- 9059 up points when pcre[16]_exec() is running. This makes it run a lot more 9060 slowly, however. Details of how to do this are given in the pcrebuild 9061 documentation. When built in this way, instead of using the stack, PCRE 9062 obtains and frees memory by calling the functions that are pointed to 9063 by the pcre[16]_stack_malloc and pcre[16]_stack_free variables. By 9064 default, these point to malloc() and free(), but you can replace the 9065 pointers to cause PCRE to use your own functions. Since the block sizes 9066 are always the same, and are always freed in reverse order, it may be 9067 possible to implement customized memory handlers that are more effi- 9068 cient than the standard functions. 9069 9070 Limiting pcre[16]_exec()'s stack usage 9071 9072 You can set limits on the number of times that match() is called, both 9073 in total and recursively. If a limit is exceeded, pcre[16]_exec() 9074 returns an error code. Setting suitable limits should prevent it from 9075 running out of stack. The default values of the limits are very large, 9076 and unlikely ever to operate. They can be changed when PCRE is built, 9077 and they can also be set when pcre[16]_exec() is called. For details of 9078 these interfaces, see the pcrebuild documentation and the section on 9079 extra data for pcre[16]_exec() in the pcreapi documentation. 9080 9081 As a very rough rule of thumb, you should reckon on about 500 bytes per 9082 recursion. Thus, if you want to limit your stack usage to 8Mb, you 9083 should set the limit at 16000 recursions. A 64Mb stack, on the other 9084 hand, can support around 128000 recursions. 9085 9086 In Unix-like environments, the pcretest test program has a command line 9087 option (-S) that can be used to increase the size of its stack. As long 9088 as the stack is large enough, another option (-M) can be used to find 9089 the smallest limits that allow a particular pattern to match a given 9090 subject string. This is done by calling pcre[16]_exec() repeatedly with 9091 different limits. 9092 9093 Obtaining an estimate of stack usage 9094 9095 The actual amount of stack used per recursion can vary quite a lot, 9096 depending on the compiler that was used to build PCRE and the optimiza- 9097 tion or debugging options that were set for it. The rule of thumb value 9098 of 500 bytes mentioned above may be larger or smaller than what is 9099 actually needed. A better approximation can be obtained by running this 9100 command: 9101 9102 pcretest -m -C 9103 9104 The -C option causes pcretest to output information about the options 9105 with which PCRE was compiled. When -m is also given (before -C), infor- 9106 mation about stack use is given in a line like this: 9107 9108 Match recursion uses stack: approximate frame size = 640 bytes 9109 9110 The value is approximate because some recursions need a bit more (up to 9111 perhaps 16 more bytes). 9112 9113 If the above command is given when PCRE is compiled to use the heap 9114 instead of the stack for recursion, the value that is output is the 9115 size of each block that is obtained from the heap. 9116 9117 Changing stack size in Unix-like systems 9118 9119 In Unix-like environments, there is not often a problem with the stack 9120 unless very long strings are involved, though the default limit on 9121 stack size varies from system to system. Values from 8Mb to 64Mb are 9122 common. You can find your default limit by running the command: 9123 9124 ulimit -s 9125 9126 Unfortunately, the effect of running out of stack is often SIGSEGV, 9127 though sometimes a more explicit error message is given. You can nor- 9128 mally increase the limit on stack size by code such as this: 9129 9130 struct rlimit rlim; 9131 getrlimit(RLIMIT_STACK, &rlim); 9132 rlim.rlim_cur = 100*1024*1024; 9133 setrlimit(RLIMIT_STACK, &rlim); 9134 9135 This reads the current limits (soft and hard) using getrlimit(), then 9136 attempts to increase the soft limit to 100Mb using setrlimit(). You 9137 must do this before calling pcre[16]_exec(). 9138 9139 Changing stack size in Mac OS X 9140 9141 Using setrlimit(), as described above, should also work on Mac OS X. It 9142 is also possible to set a stack size when linking a program. There is a 9143 discussion about stack sizes in Mac OS X at this web site: 9144 http://developer.apple.com/qa/qa2005/qa1419.html. 9145 9146 9147AUTHOR 9148 9149 Philip Hazel 9150 University Computing Service 9151 Cambridge CB2 3QH, England. 9152 9153 9154REVISION 9155 9156 Last updated: 21 January 2012 9157 Copyright (c) 1997-2012 University of Cambridge. 9158------------------------------------------------------------------------------ 9159 9160 9161