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PCRE(3) PCRE(3)
NAME
PCRE - Perl-compatible regular expressions
PCRE CALLOUTS
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 func-
tion is to be called. Different callout points can be identified by putting a num-
ber less than 256 after the letter C. The default value is zero. For example, this
pattern has two callout points:
(?C1)eabc(?C2)def
If the PCRE_AUTO_CALLOUT option bit is set when pcre_compile() is called, PCRE
automatically inserts callouts, all with number 255, before each item in the pat-
tern. For example, if PCRE_AUTO_CALLOUT is used with the pattern
A(\d{2}|--)
it is processed as if it were
(?C255)A(?C255)((?C255)\d{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 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.
MISSING CALLOUTS
You should be aware that, because of optimizations in the way PCRE matches pat-
terns, callouts sometimes do not happen. For example, if the pattern is
ab(?C4)cd
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.
THE CALLOUT INTERFACE
During matching, when PCRE reaches a callout point, the external function defined
by pcre_callout is called (if it is set). The only argument is a pointer to a
pcre_callout block. This structure contains the following fields:
int version;
int callout_number;
int *offset_vector;
const char *subject;
int subject_length;
int start_match;
int current_position;
int capture_top;
int capture_last;
void *callout_data;
int pattern_position;
int next_item_length;
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 existing fields.
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 automati-
cally generated callouts).
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 sub-
strings 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 of 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 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.
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 cap-
ture_top is one.
The capture_last field contains the number of the most recently captured substring.
If no substrings have been captured, its value is -1.
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 structure. 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 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 paren-
thesis, the length is that of the entire subpattern.
The pattern_position and next_item_length fields are intended to help in distin-
guishing between different automatic callouts, which all have the same callout num-
ber. However, they are set for all callouts.
RETURN VALUES
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 backtracking to test 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() returns the negative value.
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.
Last updated: 09 September 2004
Copyright (c) 1997-2004 University of Cambridge.
PCRE(3)
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On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
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