PCRECALLOUT(3)                                                  PCRECALLOUT(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

       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

         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 patterns by default, 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.

       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:

         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 automatically generated callouts).

       The offset_vector field is a pointer to the vector of offsets that was
       passed by the caller 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 not useful.

       The subject and subject_length fields contain copies of the values that
       were passed to pcre_exec().

       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 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() 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 captured, 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 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 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.

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 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 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.

AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION

       Last updated: 21 November 2010
       Copyright (c) 1997-2010 University of Cambridge.



                                                                PCRECALLOUT(3)
