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       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_exec() or  pcre_dfa_exec().  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;

       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.


       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


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


       Last updated: 29 September 2009
       Copyright (c) 1997-2009 University of Cambridge.