pcrecallout


       #include <pcre.h>

       int (*pcre_callout)(pcre_callout_block *);

       int (*pcre16_callout)(pcre16_callout_block *);

       int (*pcre32_callout)(pcre32_callout_block *);

DESCRIPTION

       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
       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 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
       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. 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:

         (?(?C9)(?=a)ab|de)

       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.

         --->aaaa
          +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:

         --->aaaa
          +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

         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_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.

THE CALLOUT INTERFACE

       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
       fields:

         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
       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() 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-
       tions.

       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  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.

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, the
       matching function 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 stan-
       dard "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: 12 November 2013
       Copyright (c) 1997-2013 University of Cambridge.

PCRE 8.34                      12 November 2013                 PCRECALLOUT(3)
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