int (*pcre16_callout)(pcre16_callout_block *);
PCRE provides a feature called "callout", which is a means of temporarily passing control to the caller of PCRE in the middle of pattern matching. The caller of PCRE provides an external function by putting its entry point in the global variable pcre_callout (pcre16_callout for the 16-bit library). By default, this variable contains NULL, which disables all calling out.
Within a regular expression, (?C) indicates the points at which the external
function is to be called. Different callout points can be identified by putting
a number less than 256 after the letter C. The default value is zero.
For example, this pattern has two callout points:
(?C1)abc(?C2)def
If the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled, PCRE
automatically inserts callouts, all with number 255, before each item in the
pattern. For example, if PCRE_AUTO_CALLOUT is used with the pattern
A(\ed{2}|--)
it is processed as if it were
(?C255)A(?C255)((?C255)\ed{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
Notice that there is a callout before and after each parenthesis and
alternation bar. Automatic callouts can be used for tracking the progress of
pattern matching. The
HREF
pcretest
command has an option that sets automatic callouts; when it is used, the output
indicates how the pattern is matched. This is useful information when you are
trying to optimize the performance of a particular pattern.
The use of callouts in a pattern makes it ineligible for optimization by the just-in-time compiler. Studying such a pattern with the PCRE_STUDY_JIT_COMPILE option always fails. . .
If the pattern is studied, PCRE knows the minimum length of a matching string, and will immediately give a "no match" return without actually running a match if the subject is not long enough, or, for unanchored patterns, if it has been scanned far enough.
You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE option to the matching function, or by starting the pattern with (*NO_START_OPT). This slows down the matching process, but does ensure that callouts such as the example above are obeyed. . .
The callout_number field contains the number of the callout, as compiled into the pattern (that is, the number after ?C for manual callouts, and 255 for automatically generated callouts).
The offset_vector field is a pointer to the vector of offsets that was passed by the caller to the matching function. When pcre_exec() or pcre16_exec() is used, the contents can be inspected, in order to extract substrings that have been matched so far, in the same way as for extracting substrings after a match has completed. For the DFA matching functions, this field is not useful.
The subject and subject_length fields contain copies of the values that were passed to the matching function.
The start_match field normally contains the offset within the subject at which the current match attempt started. However, if the escape sequence \eK has been encountered, this value is changed to reflect the modified starting point. If the pattern is not anchored, the callout function may be called several times from the same point in the pattern for different starting points in the subject.
The current_position field contains the offset within the subject of the current match pointer.
When the pcre_exec() or pcre16_exec() is used, the capture_top field contains one more than the number of the highest numbered captured substring so far. If no substrings have been captured, the value of capture_top is one. This is always the case when the DFA functions are used, because they do not support captured substrings.
The capture_last field contains the number of the most recently captured substring. If no substrings have been captured, its value is -1. This is always the case for the DFA matching functions.
The callout_data field contains a value that is passed to a matching
function specifically so that it can be passed back in callouts. It is passed
in the callout_data field of a pcre_extra or pcre16_extra
data structure. If no such data was passed, the value of callout_data in
a callout block is NULL. There is a description of the pcre_extra
structure in the
HREF
pcreapi
documentation.
The pattern_position field is present from version 1 of the callout structure. It contains the offset to the next item to be matched in the pattern string.
The next_item_length field is present from version 1 of the callout structure. It contains the length of the next item to be matched in the pattern string. When the callout immediately precedes an alternation bar, a closing parenthesis, or the end of the pattern, the length is zero. When the callout precedes an opening parenthesis, the length is that of the entire subpattern.
The pattern_position and next_item_length fields are intended to help in distinguishing between different automatic callouts, which all have the same callout number. However, they are set for all callouts.
The mark field is present from version 2 of the callout structure. In callouts from pcre_exec() or pcre16_exec() it contains a pointer to the zero-terminated name of the most recently passed (*MARK), (*PRUNE), or (*THEN) item in the match, or NULL if no such items have been passed. Instances of (*PRUNE) or (*THEN) without a name do not obliterate a previous (*MARK). In callouts from the DFA matching functions this field always contains NULL. . .
Negative values should normally be chosen from the set of PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure. The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions; it will never be used by PCRE itself. . .
Philip Hazel University Computing Service Cambridge CB2 3QH, England.. .
Last updated: 08 Janurary 2012 Copyright (c) 1997-2012 University of Cambridge.