1<html> 2<head> 3<title>pcrecompat specification</title> 4</head> 5<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB"> 6<h1>pcrecompat man page</h1> 7<p> 8Return to the <a href="index.html">PCRE index page</a>. 9</p> 10<p> 11This page is part of the PCRE HTML documentation. It was generated automatically 12from the original man page. If there is any nonsense in it, please consult the 13man page, in case the conversion went wrong. 14<br> 15<br><b> 16DIFFERENCES BETWEEN PCRE AND PERL 17</b><br> 18<P> 19This document describes the differences in the ways that PCRE and Perl handle 20regular expressions. The differences described here are with respect to Perl 215.10. 22</P> 23<P> 241. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what 25it does have are given in the 26<a href="pcre.html#utf8support">section on UTF-8 support</a> 27in the main 28<a href="pcre.html"><b>pcre</b></a> 29page. 30</P> 31<P> 322. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits 33them, but they do not mean what you might think. For example, (?!a){3} does 34not assert that the next three characters are not "a". It just asserts that the 35next character is not "a" three times. 36</P> 37<P> 383. Capturing subpatterns that occur inside negative lookahead assertions are 39counted, but their entries in the offsets vector are never set. Perl sets its 40numerical variables from any such patterns that are matched before the 41assertion fails to match something (thereby succeeding), but only if the 42negative lookahead assertion contains just one branch. 43</P> 44<P> 454. Though binary zero characters are supported in the subject string, they are 46not allowed in a pattern string because it is passed as a normal C string, 47terminated by zero. The escape sequence \0 can be used in the pattern to 48represent a binary zero. 49</P> 50<P> 515. The following Perl escape sequences are not supported: \l, \u, \L, 52\U, and \N. In fact these are implemented by Perl's general string-handling 53and are not part of its pattern matching engine. If any of these are 54encountered by PCRE, an error is generated. 55</P> 56<P> 576. The Perl escape sequences \p, \P, and \X are supported only if PCRE is 58built with Unicode character property support. The properties that can be 59tested with \p and \P are limited to the general category properties such as 60Lu and Nd, script names such as Greek or Han, and the derived properties Any 61and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the 62Perl documentation says "Because Perl hides the need for the user to understand 63the internal representation of Unicode characters, there is no need to 64implement the somewhat messy concept of surrogates." 65</P> 66<P> 677. PCRE does support the \Q...\E escape for quoting substrings. Characters in 68between are treated as literals. This is slightly different from Perl in that $ 69and @ are also handled as literals inside the quotes. In Perl, they cause 70variable interpolation (but of course PCRE does not have variables). Note the 71following examples: 72<pre> 73 Pattern PCRE matches Perl matches 74 75 \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz 76 \Qabc\$xyz\E abc\$xyz abc\$xyz 77 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz 78</pre> 79The \Q...\E sequence is recognized both inside and outside character classes. 80</P> 81<P> 828. Fairly obviously, PCRE does not support the (?{code}) and (??{code}) 83constructions. However, there is support for recursive patterns. This is not 84available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout" 85feature allows an external function to be called during pattern matching. See 86the 87<a href="pcrecallout.html"><b>pcrecallout</b></a> 88documentation for details. 89</P> 90<P> 919. Subpatterns that are called recursively or as "subroutines" are always 92treated as atomic groups in PCRE. This is like Python, but unlike Perl. There 93is a discussion of an example that explains this in more detail in the 94<a href="pcrepattern.html#recursiondifference">section on recursion differences from Perl</a> 95in the 96<a href="pcrepattern.html"><b>pcrepattern</b></a> 97page. 98</P> 99<P> 10010. There are some differences that are concerned with the settings of captured 101strings when part of a pattern is repeated. For example, matching "aba" against 102the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b". 103</P> 104<P> 10511. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F), 106(*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the forms without an 107argument. PCRE does not support (*MARK). 108</P> 109<P> 11012. PCRE's handling of duplicate subpattern numbers and duplicate subpattern 111names is not as general as Perl's. This is a consequence of the fact the PCRE 112works internally just with numbers, using an external table to translate 113between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B), 114where the two capturing parentheses have the same number but different names, 115is not supported, and causes an error at compile time. If it were allowed, it 116would not be possible to distinguish which parentheses matched, because both 117names map to capturing subpattern number 1. To avoid this confusing situation, 118an error is given at compile time. 119</P> 120<P> 12113. PCRE provides some extensions to the Perl regular expression facilities. 122Perl 5.10 includes new features that are not in earlier versions of Perl, some 123of which (such as named parentheses) have been in PCRE for some time. This list 124is with respect to Perl 5.10: 125<br> 126<br> 127(a) Although lookbehind assertions in PCRE must match fixed length strings, 128each alternative branch of a lookbehind assertion can match a different length 129of string. Perl requires them all to have the same length. 130<br> 131<br> 132(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ 133meta-character matches only at the very end of the string. 134<br> 135<br> 136(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special 137meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored. 138(Perl can be made to issue a warning.) 139<br> 140<br> 141(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is 142inverted, that is, by default they are not greedy, but if followed by a 143question mark they are. 144<br> 145<br> 146(e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried 147only at the first matching position in the subject string. 148<br> 149<br> 150(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and 151PCRE_NO_AUTO_CAPTURE options for <b>pcre_exec()</b> have no Perl equivalents. 152<br> 153<br> 154(g) The \R escape sequence can be restricted to match only CR, LF, or CRLF 155by the PCRE_BSR_ANYCRLF option. 156<br> 157<br> 158(h) The callout facility is PCRE-specific. 159<br> 160<br> 161(i) The partial matching facility is PCRE-specific. 162<br> 163<br> 164(j) Patterns compiled by PCRE can be saved and re-used at a later time, even on 165different hosts that have the other endianness. 166<br> 167<br> 168(k) The alternative matching function (<b>pcre_dfa_exec()</b>) matches in a 169different way and is not Perl-compatible. 170<br> 171<br> 172(l) PCRE recognizes some special sequences such as (*CR) at the start of 173a pattern that set overall options that cannot be changed within the pattern. 174</P> 175<br><b> 176AUTHOR 177</b><br> 178<P> 179Philip Hazel 180<br> 181University Computing Service 182<br> 183Cambridge CB2 3QH, England. 184<br> 185</P> 186<br><b> 187REVISION 188</b><br> 189<P> 190Last updated: 04 October 2009 191<br> 192Copyright © 1997-2009 University of Cambridge. 193<br> 194<p> 195Return to the <a href="index.html">PCRE index page</a>. 196</p> 197