PCRE - Perl-compatible regular expressions
int (*pcre_callout)(pcre_callout_block *);
PCRE provides a feature called "callout", which is a means of temporar‐
ily 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. 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
During matching, when PCRE reaches a callout point (and pcre_callout is
set), the external function is called. Its only argument is a pointer
to a pcre_callout block. This contains the following variables:
const char *subject;
The version field is an integer containing the version number of the
block format. The current version is zero. The version number may
change in future if additional fields are added, but the intention is
never to remove any of the existing fields.
The callout_number field contains the number of the callout, as com‐
piled into the pattern (that is, the number after ?C).
The offset_vector field is a pointer to the vector of offsets that was
passed by the caller to pcre_exec(). 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.
The subject and subject_length fields contain copies the values that
were passed to pcre_exec().
The start_match field contains the offset within the subject at which
the current match attempt started. If the pattern is not anchored, the
callout function may be called several times for different starting
The current_position field contains the offset within the subject of
the current match pointer.
The capture_top field contains one more than the number of the highest
numbered captured substring so far. If no substrings have been cap‐
tured, the value of capture_top is one.
The capture_last field contains the number of the most recently cap‐
The callout_data field contains a value that is passed to pcre_exec()
by the caller specifically so that it can be passed back in callouts.
It is passed in the pcre_callout field of the pcre_extra data struc‐
ture. If no such data was passed, the value of callout_data in a
pcre_callout block is NULL. There is a description of the pcre_extra
structure in the pcreapi documentation.
The callout function returns an integer. If the value is zero, matching
proceeds as normal. If the value is greater than zero, matching fails
at the current point, but backtracking to test other possibilities goes
ahead, just as if a lookahead assertion had failed. If the value is
less than zero, the match is abandoned, and pcre_exec() returns the
Negative values should normally be chosen from the set of
PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan‐
dard "no match" failure. The error number PCRE_ERROR_CALLOUT is
reserved for use by callout functions; it will never be used by PCRE
Last updated: 21 January 2003Copyright (c) 1997-2003 University of Cambridge.