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PCRECALLOUT(3)							PCRECALLOUT(3)

NAME
       PCRE - Perl-compatible regular expressions

PCRE CALLOUTS

       int (*pcre_callout)(pcre_callout_block *);

       int (*pcre16_callout)(pcre16_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).  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. 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.

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_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 pcre16_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  pcre16_call‐
       out block.  These structures contains the following fields:

	 int	       version;
	 int	       callout_number;
	 int	      *offset_vector;
	 const char   *subject;		  (8-bit version)
	 PCRE_SPTR16   subject;		  (16-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)

       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
       pcre16_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 pcre16_exec() is used,  the  capture_top	 field
       contains one more than the number of the highest numbered captured sub‐
       string so far. If no substrings have been captured, the value  of  cap‐
       ture_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. If no substrings have been captured, its value is -1.
       This is always the case for the DFA matching functions.

       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  pcre16_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	struc‐
       ture 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 pcre16_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 functions 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: 08 Janurary 2012
       Copyright (c) 1997-2012 University of Cambridge.

PCRE 8.30			08 January 2012			PCRECALLOUT(3)
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