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PCRECALLOUT(3)                                                  PCRECALLOUT(3)

       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. Differ-
       ent 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:


       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.

       The use of callouts in a pattern makes it ineligible for optimization by the  just-in-time  compiler.  Studying
       such a pattern with the PCRE_STUDY_JIT_COMPILE option always fails.


       You  should be aware that, because of optimizations in the way PCRE matches patterns by default, callouts some-
       times 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.


       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;
         const unsigned char *mark;

       The version field is an integer containing the version number of the block format. The initial version  was  0;
       the  current  version  is 2. 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 paren-
       thesis, the length is that of the entire subpattern.

       The pattern_position and next_item_length fields are intended to help in distinguishing between different auto-
       matic callouts, which all have the same callout number. However, they are set for all callouts.

       The mark field is present from version 2 of the pcre_callout structure. In callouts from  pcre_exec()  it  con-
       tains  a  pointer to the zero-terminated name of the most recently passed (*MARK), (*PRUNE), or (*THEN) item in
       the match, or NULL if no such items have been passed. Instances of (*PRUNE) or (*THEN) without a  name  do  not
       obliterate a previous (*MARK). In callouts from pcre_dfa_exec() this field always contains NULL.


       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  possi-
       bilities  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.


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


       Last updated: 30 November 2011
       Copyright (c) 1997-2011 University of Cambridge.