PCRE - Perl-compatible regular expressions
int (*pcre_callout)(pcre_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. 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
If the PCRE_AUTO_CALLOUT option bit is set when pcre_compile() or pcre_compile2() is
called, 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
it is processed as if it were
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 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.
You should be aware that, because of optimizations in the way PCRE matches patterns by
default, callouts sometimes do not happen. For example, if the pattern is
PCRE knows that any matching string must contain the letter "d". If the subject string is
"abyz", the lack of "d" means that matching doesn't ever start, and the callout is never
reached. However, with "abyd", though the result is still no match, the callout is obeyed.
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
pcre_compile(), pcre_exec(), or pcre_dfa_exec(), 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 INTERFACE
During matching, when PCRE reaches a callout point, the external function defined by
pcre_callout is called (if it is set). This applies to both the pcre_exec() and the
pcre_dfa_exec() matching functions. The only argument to the callout function is a pointer
to a pcre_callout block. This structure contains the following fields:
const char *subject;
The version field is an integer containing the version number of the block format. The
initial version was 0; the current version is 1. The version number will change again in
future if additional fields are added, but the intention is never to remove any of the
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
The offset_vector field is a pointer to the vector of offsets that was passed by the call-
er to pcre_exec() or pcre_dfa_exec(). When pcre_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 pcre_dfa_exec() this field is
The subject and subject_length fields contain copies of the values that were passed to
The start_match field normally contains the offset within the subject at which the current
match attempt started. However, if the escape sequence \K 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 dif-
ferent starting points in the subject.
The current_position field contains the offset within the subject of the current match
When the pcre_exec() function 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 cap-
tured, the value of capture_top is one. This is always the case when pcre_dfa_exec() is
used, because it does 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 when
pcre_dfa_exec() is used.
The callout_data field contains a value that is passed to pcre_exec() or pcre_dfa_exec()
specifically so that it can be passed back in callouts. It is passed in the pcre_callout
field of the pcre_extra data structure. If no such data was passed, the value of call-
out_data in a pcre_callout block is NULL. There is a description of the pcre_extra struc-
ture in the pcreapi documentation.
The pattern_position field is present from version 1 of the pcre_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 pcre_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 external callout function returns an integer to PCRE. If the value is zero, matching
proceeds as normal. If the value is greater than zero, matching fails at the current
point, but the testing of other matching 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() or pcre_dfa_exec() returns the negative value.
Negative values should normally be chosen from the set of PCRE_ERROR_xxx values. In par-
ticular, 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
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Last updated: 21 November 2010
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