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PCRECALLOUT(3)		   Library Functions Manual		PCRECALLOUT(3)

NAME
       PCRE - Perl-compatible regular expressions

SYNOPSIS
       #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 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:

	 (?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.

MISSING	CALLOUTS
       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:

	 --->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	       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-
       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  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.

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_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.

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)

NAME | SYNOPSIS | DESCRIPTION | MISSING CALLOUTS | THE CALLOUT INTERFACE | RETURN VALUES | AUTHOR | REVISION

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