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PCRE2CALLOUT(3)		   Library Functions Manual	       PCRE2CALLOUT(3)

NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

SYNOPSIS

       #include	<pcre2.h>

       int (*pcre2_callout)(pcre2_callout_block	*, void	*);

       int pcre2_callout_enumerate(const pcre2_code *code,
	 int (*callback)(pcre2_callout_enumerate_block *, void *),
	 void *user_data);

DESCRIPTION

       PCRE2  provides	a feature called "callout", which is a means of	tempo-
       rarily passing control to the caller of PCRE2 in	the middle of  pattern
       matching.  The caller of	PCRE2 provides an external function by putting
       its entry point in a match  context  (see  pcre2_set_callout()  in  the
       pcre2api	documentation).

       When  using the pcre2_substitute() function, an additional callout fea-
       ture is available. This does a callout after each change	to the subject
       string and is described in the pcre2api documentation; the rest of this
       document	is concerned with callouts during pattern matching.

       Within a	regular	expression, (?C<arg>) indicates	a point	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.	Alternatively, the argument may	be a delimited
       string. The starting delimiter must be one of ` ' " ^ % # $ {  and  the
       ending delimiter	is the same as the start, except for {,	where the end-
       ing delimiter is	}. If  the  ending  delimiter  is  needed  within  the
       string,	it  must be doubled. For example, this pattern has two callout
       points:

	 (?C1)abc(?C"some ""arbitrary""	text")def

       If the PCRE2_AUTO_CALLOUT option	bit is set when	a pattern is compiled,
       PCRE2  automatically inserts callouts, all with number 255, before each
       item in the pattern except for immediately before or after an  explicit
       callout.	For example, if	PCRE2_AUTO_CALLOUT is used with	the pattern

	 A(?C3)B

       it is processed as if it	were

	 (?C255)A(?C3)B(?C255)

       Here is a more complicated example:

	 A(\d{2}|--)

       With PCRE2_AUTO_CALLOUT,	this pattern 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)  (?(?C%text%)(?!=d)ab|de)

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

       Callouts	can be useful for tracking the progress	of  pattern  matching.
       The pcre2test program has a pattern qualifier (/auto_callout) that sets
       automatic callouts.  When any callouts are  present,  the  output  from
       pcre2test  indicates  how  the pattern is being matched.	This is	useful
       information when	you are	trying to optimize the performance of  a  par-
       ticular pattern.

MISSING	CALLOUTS

       You  should  be	aware  that, because of	optimizations in the way PCRE2
       compiles	and matches patterns, callouts sometimes do not	happen exactly
       as you might expect.

   Auto-possessification

       At compile time,	PCRE2 "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 pcre2test output when this pattern
       is compiled with	PCRE2_ANCHORED and PCRE2_AUTO_CALLOUT and then applied
       to the string "aaaa" is:

	 --->aaaa
	  +0 ^	      a+
	  +2 ^	 ^    [bc]
	 No match

       This  indicates that when matching [bc] fails, there is no backtracking
       into a+ (because	it is being treated as a++) and	therefore the callouts
       that  would  be	taken for the backtracks do not	occur. You can disable
       the  auto-possessify  feature  by  passing   PCRE2_NO_AUTO_POSSESS   to
       pcre2_compile(),	 or  starting  the pattern with	(*NO_AUTO_POSSESS). In
       this case, the output changes to	this:

	 --->aaaa
	  +0 ^	      a+
	  +2 ^	 ^    [bc]
	  +2 ^	^     [bc]
	  +2 ^ ^      [bc]
	  +2 ^^	      [bc]
	 No match

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

   Automatic .*	anchoring

       By default, an optimization is applied when .* is the first significant
       item in a pattern. If PCRE2_DOTALL is set, so that the  dot  can	 match
       any  character,	the pattern is automatically anchored. If PCRE2_DOTALL
       is not set, a match can start only after	an internal newline or at  the
       beginning of the	subject, and pcre2_compile() remembers this. If	a pat-
       tern has	more than one top-level	branch,	automatic anchoring occurs  if
       all branches are	anchorable.

       This  optimization is disabled, however,	if .* is in an atomic group or
       if there	is a backreference to the capture group	in which  it  appears.
       It  is  also disabled if	the pattern contains (*PRUNE) or (*SKIP). How-
       ever, the presence of callouts does not affect it.

       For example, if the pattern .*\d	is  compiled  with  PCRE2_AUTO_CALLOUT
       and applied to the string "aa", the pcre2test output is:

	 --->aa
	  +0 ^	    .*
	  +2 ^ ^    \d
	  +2 ^^	    \d
	  +2 ^	    \d
	 No match

       This  shows  that all match attempts start at the beginning of the sub-
       ject. In	other words, the pattern is anchored. You can disable this op-
       timization  by  passing	PCRE2_NO_DOTSTAR_ANCHOR	to pcre2_compile(), or
       starting	the pattern with (*NO_DOTSTAR_ANCHOR). In this case, the  out-
       put changes to:

	 --->aa
	  +0 ^	    .*
	  +2 ^ ^    \d
	  +2 ^^	    \d
	  +2 ^	    \d
	  +0  ^	    .*
	  +2  ^^    \d
	  +2  ^	    \d
	 No match

       This  shows more	match attempts,	starting at the	second subject charac-
       ter.  Another optimization, described in	the next section,  means  that
       there is	no subsequent attempt to match with an empty subject.

   Other optimizations

       Other  optimizations  that  provide fast	"no match" results also	affect
       callouts.  For example, if the pattern is

	 ab(?C4)cd

       PCRE2 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.

       For  most  patterns  PCRE2  also	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 PCRE2_NO_START_OPTI-
       MIZE  option  to	 pcre2_compile(),  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	PCRE2  reaches a callout point,	if an external
       function	is provided in the match context, it is	called.	 This  applies
       to  both	normal,	DFA, and JIT matching. The first argument to the call-
       out function is a pointer to a pcre2_callout block. The second argument
       is  the	void * callout data that was supplied when the callout was set
       up by calling pcre2_set_callout() (see the pcre2api documentation). The
       callout	block structure	contains the following fields, not necessarily
       in this order:

	 uint32_t      version;
	 uint32_t      callout_number;
	 uint32_t      capture_top;
	 uint32_t      capture_last;
	 uint32_t      callout_flags;
	 PCRE2_SIZE   *offset_vector;
	 PCRE2_SPTR    mark;
	 PCRE2_SPTR    subject;
	 PCRE2_SIZE    subject_length;
	 PCRE2_SIZE    start_match;
	 PCRE2_SIZE    current_position;
	 PCRE2_SIZE    pattern_position;
	 PCRE2_SIZE    next_item_length;
	 PCRE2_SIZE    callout_string_offset;
	 PCRE2_SIZE    callout_string_length;
	 PCRE2_SPTR    callout_string;

       The version field contains the version number of	the block format.  The
       current	version	 is  2;	the three callout string fields	were added for
       version 1, and the callout_flags	field for version 2. If	you are	 writ-
       ing  an	application  that  might  use an earlier release of PCRE2, you
       should check the	version	number before accessing	any of	these  fields.
       The  version  number  will increase in future if	more fields are	added,
       but the intention is never to remove any	of the existing	fields.

   Fields for numerical	callouts

       For a numerical callout,	callout_string	is  NULL,  and	callout_number
       contains	 the  number  of  the callout, in the range 0-255. This	is the
       number that follows (?C for callouts that part of the  pattern;	it  is
       255 for automatically generated callouts.

   Fields for string callouts

       For  callouts with string arguments, callout_number is always zero, and
       callout_string points to	the string that	is contained within  the  com-
       piled pattern. Its length is given by callout_string_length. Duplicated
       ending delimiters that were present in the original pattern string have
       been turned into	single characters, but there is	no other processing of
       the callout string argument. An additional code unit containing	binary
       zero  is	 present  after	the string, but	is not included	in the length.
       The delimiter that was used to start the	string is also	stored	within
       the  pattern, immediately before	the string itself. You can access this
       delimiter as callout_string[-1] if you need it.

       The callout_string_offset field is the code unit	offset to the start of
       the callout argument string within the original pattern string. This is
       provided	for the	benefit	of applications	such as	script languages  that
       might need to report errors in the callout string within	the pattern.

   Fields for all callouts

       The  remaining  fields in the callout block are the same	for both kinds
       of callout.

       The offset_vector field is a pointer to a vector	of  capturing  offsets
       (the "ovector").	You may	read the elements in this vector, but you must
       not change any of them.

       For calls to pcre2_match(), the offset_vector field is not  (since  re-
       lease  10.30)  a	 pointer  to the actual	ovector	that was passed	to the
       matching	function in the	match data block. Instead it points to an  in-
       ternal  ovector	of  a  size large enough to hold all possible captured
       substrings in the pattern. Note that whenever a recursion or subroutine
       call  within  a pattern completes, the capturing	state is reset to what
       it was before.

       The capture_last	field contains the number of the  most	recently  cap-
       tured  substring,  and the capture_top field contains one more than the
       number of the highest numbered captured substring so far.  If  no  sub-
       strings	have yet been captured,	the value of capture_last is 0 and the
       value of	capture_top is 1. The values of	these  fields  do  not	always
       differ	by   one;  for	example,  when	the  callout  in  the  pattern
       ((a)(b))(?C2) is	taken, capture_last is 1 but capture_top is 4.

       The contents of ovector[2] to  ovector[<capture_top>*2-1]  can  be  in-
       spected	in  order to extract substrings	that have been matched so far,
       in the same way as extracting substrings	after a	match  has  completed.
       The  values in ovector[0] and ovector[1]	are always PCRE2_UNSET because
       the match is by definition not complete.	Substrings that	have not  been
       captured	 but whose numbers are less than capture_top also have both of
       their ovector slots set to PCRE2_UNSET.

       For DFA matching, the offset_vector field points	to  the	 ovector  that
       was  passed  to the matching function in	the match data block for call-
       outs at the top level, but to an	internal ovector during	the processing
       of  pattern  recursions,	lookarounds, and atomic	groups.	However, these
       ovectors	hold no	useful information because pcre2_dfa_match() does  not
       support	substring  capturing. The value	of capture_top is always 1 and
       the value of capture_last is always 0 for DFA matching.

       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.

       The pattern_position field contains the offset in the pattern string to
       the next	item to	be matched.

       The next_item_length field contains the length of the next item	to  be
       processed  in the pattern string. When the callout is at	the end	of the
       pattern,	the length is zero.  When  the	callout	 precedes  an  opening
       parenthesis, the	length includes	meta characters	that follow the	paren-
       thesis. For example, in a callout before	an assertion  such  as	(?=ab)
       the  length  is	3. For an an alternation bar or	a closing parenthesis,
       the length is one, unless a closing parenthesis is followed by a	 quan-
       tifier, in which	case its length	is included.  (This changed in release
       10.23. In earlier releases, before an opening  parenthesis  the	length
       was  that of the	entire group, and before an alternation	bar or a clos-
       ing parenthesis the length was zero.)

       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,  and
       are used	by pcre2test to	show the next item to be matched when display-
       ing callout information.

       In callouts from	pcre2_match() the mark field 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 function this field
       always contains NULL.

       The   callout_flags   field   is	  always   zero	  in   callouts	  from
       pcre2_dfa_match() or when JIT is	being used. When pcre2_match() without
       JIT is used, the	following bits may be set:

	 PCRE2_CALLOUT_STARTMATCH

       This  is	set for	the first callout after	the start of matching for each
       new starting position in	the subject.

	 PCRE2_CALLOUT_BACKTRACK

       This is set if there has	been a matching	backtrack since	 the  previous
       callout,	 or  since  the	start of matching if this is the first callout
       from a pcre2_match() run.

       Both bits are set when a	backtrack has caused a "bumpalong"  to	a  new
       starting	 position in the subject. Output from pcre2test	does not indi-
       cate the	presence of these bits unless the  callout_extra  modifier  is
       set.

       The information in the callout_flags field is provided so that applica-
       tions can track and tell	their users how	matching with backtracking  is
       done.  This  can	be useful when trying to optimize patterns, or just to
       understand how PCRE2 works. There is no	support	 in  pcre2_dfa_match()
       because	there is no backtracking in DFA	matching, and there is no sup-
       port in JIT because JIT is all about maximimizing matching performance.
       In both these cases the callout_flags field is always zero.

RETURN VALUES FROM CALLOUTS

       The external callout function returns an	integer	to PCRE2. 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 the
       matching	function returns the negative value.

       Negative	values should normally be chosen from  the  set	 of  PCRE2_ER-
       ROR_xxx	values.	 In  particular, PCRE2_ERROR_NOMATCH forces a standard
       "no match" failure. The error number  PCRE2_ERROR_CALLOUT  is  reserved
       for use by callout functions; it	will never be used by PCRE2 itself.

CALLOUT	ENUMERATION

       int pcre2_callout_enumerate(const pcre2_code *code,
	 int (*callback)(pcre2_callout_enumerate_block *, void *),
	 void *user_data);

       A script	language that supports the use of string arguments in callouts
       might like to scan all the callouts in a	 pattern  before  running  the
       match. This can be done by calling pcre2_callout_enumerate(). The first
       argument	is a pointer to	a compiled pattern, the	 second	 points	 to  a
       callback	 function,  and	the third is arbitrary user data. The callback
       function	is called for every callout in the pattern  in	the  order  in
       which they appear. Its first argument is	a pointer to a callout enumer-
       ation block, and	its second argument is the user_data  value  that  was
       passed  to  pcre2_callout_enumerate(). The data block contains the fol-
       lowing fields:

	 version		Block version number
	 pattern_position	Offset to next item in pattern
	 next_item_length	Length of next item in pattern
	 callout_number		Number for numbered callouts
	 callout_string_offset	Offset to string within	pattern
	 callout_string_length	Length of callout string
	 callout_string		Points to callout string or is NULL

       The version number is currently 0. It will increase if new  fields  are
       ever  added  to	the  block. The	remaining fields are the same as their
       namesakes in the	pcre2_callout block that is used for  callouts	during
       matching, as described above.

       Note  that  the	value  of pattern_position is unique for each callout.
       However,	if a callout occurs inside a group that	is quantified  with  a
       non-zero	minimum	or a fixed maximum, the	group is replicated inside the
       compiled	pattern. For example, a	pattern	such as	/(a){2}/  is  compiled
       as  if it were /(a)(a)/.	This means that	the callout will be enumerated
       more than once, but with	the same value for  pattern_position  in  each
       case.

       The callback function should normally return zero. If it	returns	a non-
       zero value, scanning the	pattern	stops, and that	value is returned from
       pcre2_callout_enumerate().

AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.

REVISION

       Last updated: 03	February 2019
       Copyright (c) 1997-2019 University of Cambridge.

PCRE2 10.33		       03 February 2019		       PCRE2CALLOUT(3)

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

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