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 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

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:

  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;

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 compiled 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 points.

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 capture_top is one.

The capture_last field contains the number of the most recently captured substring.

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.

RETURN VALUES

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 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: 21 January 2003
Copyright (c) 1997-2003 University of Cambridge.