PCRECALLOUT(3)             Library Functions Manual             PCRECALLOUT(3)

       PCRE - Perl-compatible regular expressions


       #include <pcre.h>

       int (*pcre_callout)(pcre_callout_block *);

       int (*pcre16_callout)(pcre16_callout_block *);

       int (*pcre32_callout)(pcre32_callout_block *);


       PCRE provides a feature called "callout", which is a means of temporar-
       ily 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 (pcre16_callout for
       the 16-bit library, pcre32_callout for the 32-bit library). By default,
       this variable contains NULL, which disables all calling out.

       Within a regular expression, (?C) indicates the points at which the ex-
       ternal  function is to be called. Different callout points can be iden-
       tified by putting a number less than 256 after the letter  C.  The  de-
       fault value is zero.  For example, this pattern has two callout points:


       If  the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled,
       PCRE automatically inserts callouts, all with number 255,  before  each
       item in the pattern. For example, if PCRE_AUTO_CALLOUT is used with the


       it is processed as if it were


       Notice that there is a callout before and after  each  parenthesis  and
       alternation bar. If the pattern contains a conditional group whose con-
       dition is an assertion, an automatic callout  is  inserted  immediately
       before  the  condition. Such a callout may also be inserted explicitly,
       for example:


       This applies only to assertion conditions (because they are  themselves
       independent groups).

       Automatic  callouts  can  be  used for tracking the progress of pattern
       matching.  The pcretest program has a pattern qualifier (/C) that  sets
       automatic  callouts; when it is used, the output indicates how the pat-
       tern is being 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 com-
       piles and matches patterns, callouts sometimes do not happen exactly as
       you might expect.

       At  compile time, PCRE "auto-possessifies" repeated items when it knows
       that what follows cannot be part of the repeat. For example, a+[bc]  is
       compiled  as  if it were a++[bc]. The pcretest output when this pattern
       is anchored and then applied with  automatic  callouts  to  the  string
       "aaaa" is:

          +0 ^        ^
          +1 ^        a+
          +3 ^   ^    [bc]
         No match

       This  indicates that when matching [bc] fails, there is no backtracking
       into a+ and therefore the callouts that would be taken  for  the  back-
       tracks  do  not  occur.  You can disable the auto-possessify feature by
       passing PCRE_NO_AUTO_POSSESS to pcre_compile(), or starting the pattern
       with  (*NO_AUTO_POSSESS).  If  this  is  done in pcretest (using the /O
       qualifier), the output changes to this:

          +0 ^        ^
          +1 ^        a+
          +3 ^   ^    [bc]
          +3 ^  ^     [bc]
          +3 ^ ^      [bc]
          +3 ^^       [bc]
         No match

       This time, when matching [bc] fails, the matcher backtracks into a+ and
       tries again, repeatedly, until a+ itself fails.

       Other  optimizations  that  provide fast "no match" results also affect
       callouts.  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_OPTI-
       MIZE  option  to the matching function, 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 func-
       tion defined by pcre_callout or pcre[16|32]_callout is called (if it is
       set).  This  applies to both normal and DFA matching. The only argument
       to  the  callout  function  is  a  pointer   to   a   pcre_callout   or
       pcre[16|32]_callout  block.  These  structures  contains  the following

         int           version;
         int           callout_number;
         int          *offset_vector;
         const char   *subject;           (8-bit version)
         PCRE_SPTR16   subject;           (16-bit version)
         PCRE_SPTR32   subject;           (32-bit version)
         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;       (8-bit version)
         const PCRE_UCHAR16  *mark;       (16-bit version)
         const PCRE_UCHAR32  *mark;       (32-bit version)

       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 com-
       piled into the pattern (that is, the number after ?C for  manual  call-
       outs, 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 the  matching  function.  When  pcre_exec()  or
       pcre[16|32]_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 the DFA
       matching functions, this field is not useful.

       The subject and subject_length fields contain copies of the values that
       were passed to the matching function.

       The  start_match  field normally contains the offset within the subject
       at which the current match attempt started. However, if the escape  se-
       quence  \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()  or  pcre[16|32]_exec() is used, the capture_top
       field contains one more than the number of the  highest  numbered  cap-
       tured  substring so far. If no substrings have been captured, the value
       of capture_top is one. This is always the case when the  DFA  functions
       are used, because they do not support captured substrings.

       The  capture_last  field  contains the number of the most recently cap-
       tured substring. However, when a recursion exits, the value reverts  to
       what  it  was  outside  the recursion, as do the values of all captured
       substrings. If no substrings have been  captured,  the  value  of  cap-
       ture_last  is  -1.  This  is always the case for the DFA matching func-

       The callout_data field contains a value that is passed  to  a  matching
       function  specifically so that it can be passed back in callouts. It is
       passed in the callout_data field of a pcre_extra  or  pcre[16|32]_extra
       data  structure.  If no such data was passed, the value of callout_data
       in a 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 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 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 mark field is present from version 2 of the callout  structure.  In
       callouts  from  pcre_exec() or pcre[16|32]_exec() it contains 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 the DFA matching func-
       tions 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 possibilities goes ahead, just as if a lookahead assertion had
       failed. If the value is less than zero, the  match  is  abandoned,  the
       matching function returns the negative value.

       Negative  values  should  normally  be  chosen from the set of PCRE_ER-
       ROR_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: 12 November 2013
       Copyright (c) 1997-2013 University of Cambridge.

PCRE 8.34                      12 November 2013                 PCRECALLOUT(3)
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