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.TH PCRECALLOUT 3	.TH PCRECALLOUT 3 "12 November 2013" "PCRE 8.34"
.SH NAME	.SH NAME
PCRE - Perl-compatible regular expressions	PCRE - Perl-compatible regular expressions
.SH "PCRE CALLOUTS"	.SH SYNOPSIS
.rs	.rs
.sp	.sp
	.B #include <pcre.h>
	.PP
	.SM
.B int (pcre_callout)(pcre_callout_block );	.B int (pcre_callout)(pcre_callout_block );
.PP	.PP
	.B int (pcre16_callout)(pcre16_callout_block );
	.PP
	.B int (pcre32_callout)(pcre32_callout_block );
	.
	.SH DESCRIPTION
	.rs
	.sp
PCRE provides a feature called "callout", which is a means of temporarily	PCRE provides a feature called "callout", which is a means of temporarily
passing control to the caller of PCRE in the middle of pattern matching. The	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	caller of PCRE provides an external function by putting its entry point in the
global variable \fIpcre_callout\fP. By default, this variable contains NULL,	global variable \fIpcre_callout\fP (\fIpcre16_callout\fP for the 16-bit
which disables all calling out.	library, \fIpcre32_callout\fP for the 32-bit library). By default, this
	variable contains NULL, which disables all calling out.
.P	.P
Within a regular expression, (?C) indicates the points at which the external	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	function is to be called. Different callout points can be identified by putting
Line 19 For example, this pattern has two callout points:	Line 30 For example, this pattern has two callout points:
.sp	.sp
(?C1)abc(?C2)def	(?C1)abc(?C2)def
.sp	.sp
If the PCRE_AUTO_CALLOUT option bit is set when \fBpcre_compile()\fP or	If the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled, PCRE
\fBpcre_compile2()\fP is called, PCRE automatically inserts callouts, all with	automatically inserts callouts, all with number 255, before each item in the
number 255, before each item in the pattern. For example, if PCRE_AUTO_CALLOUT	pattern. For example, if PCRE_AUTO_CALLOUT is used with the pattern
is used with the pattern
.sp	.sp
A(\ed{2}\|--)	A(\ed{2}\|--)
.sp	.sp
Line 31 it is processed as if it were	Line 41 it is processed as if it were
(?C255)A(?C255)((?C255)\ed{2}(?C255)\|(?C255)-(?C255)-(?C255))(?C255)	(?C255)A(?C255)((?C255)\ed{2}(?C255)\|(?C255)-(?C255)-(?C255))(?C255)
.sp	.sp
Notice that there is a callout before and after each parenthesis and	Notice that there is a callout before and after each parenthesis and
alternation bar. Automatic callouts can be used for tracking the progress of	alternation bar. If the pattern contains a conditional group whose condition is
pattern matching. The	an assertion, an automatic callout is inserted immediately before the
	condition. Such a callout may also be inserted explicitly, for example:
	.sp
	(?(?C9)(?=a)ab\|de)
	.sp
	This applies only to assertion conditions (because they are themselves
	independent groups).
	.P
	Automatic callouts can be used for tracking the progress of pattern matching.
	The
.\" HREF	.\" HREF
\fBpcretest\fP	\fBpcretest\fP
.\"	.\"
command has an option that sets automatic callouts; when it is used, the output	program has a pattern qualifier (/C) that sets automatic callouts; when it is
indicates how the pattern is matched. This is useful information when you are	used, the output indicates how the pattern is being matched. This is useful
trying to optimize the performance of a particular pattern.	information when you are trying to optimize the performance of a particular
.P	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.
.	.
.	.
.SH "MISSING CALLOUTS"	.SH "MISSING CALLOUTS"
.rs	.rs
.sp	.sp
You should be aware that, because of optimizations in the way PCRE matches	You should be aware that, because of optimizations in the way PCRE compiles and
patterns by default, callouts sometimes do not happen. For example, if the	matches patterns, callouts sometimes do not happen exactly as you might expect.
pattern is	.P
	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 \fBpcretest\fP output when this pattern is anchored and
	then applied with automatic callouts to the string "aaaa" is:
.sp	.sp
	--->aaaa
	+0 ^ ^
	+1 ^ a+
	+3 ^ ^ [bc]
	No match
	.sp
	This indicates that when matching [bc] fails, there is no backtracking into a+
	and therefore the callouts that would be taken for the backtracks do not occur.
	You can disable the auto-possessify feature by passing PCRE_NO_AUTO_POSSESS
	to \fBpcre_compile()\fP, or starting the pattern with (*NO_AUTO_POSSESS). If
	this is done in \fBpcretest\fP (using the /O qualifier), the output changes to
	this:
	.sp
	--->aaaa
	+0 ^ ^
	+1 ^ a+
	+3 ^ ^ [bc]
	+3 ^ ^ [bc]
	+3 ^ ^ [bc]
	+3 ^^ [bc]
	No match
	.sp
	This time, when matching [bc] fails, the matcher backtracks into a+ and tries
	again, repeatedly, until a+ itself fails.
	.P
	Other optimizations that provide fast "no match" results also affect callouts.
	For example, if the pattern is
	.sp
ab(?C4)cd	ab(?C4)cd
.sp	.sp
PCRE knows that any matching string must contain the letter "d". If the subject	PCRE knows that any matching string must contain the letter "d". If the subject
Line 65 if the subject is not long enough, or, for unanchored	Line 113 if the subject is not long enough, or, for unanchored
been scanned far enough.	been scanned far enough.
.P	.P
You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE	You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
option to \fBpcre_compile()\fP, \fBpcre_exec()\fP, or \fBpcre_dfa_exec()\fP,	option to the matching function, or by starting the pattern with
or by starting the pattern with (*NO_START_OPT). This slows down the matching	(*NO_START_OPT). This slows down the matching process, but does ensure that
process, but does ensure that callouts such as the example above are obeyed.	callouts such as the example above are obeyed.
.	.
.	.
.SH "THE CALLOUT INTERFACE"	.SH "THE CALLOUT INTERFACE"
.rs	.rs
.sp	.sp
During matching, when PCRE reaches a callout point, the external function	During matching, when PCRE reaches a callout point, the external function
defined by \fIpcre_callout\fP is called (if it is set). This applies to both	defined by \fIpcre_callout\fP or \fIpcre[16\|32]_callout\fP is called (if it is
the \fBpcre_exec()\fP and the \fBpcre_dfa_exec()\fP matching functions. The	set). This applies to both normal and DFA matching. The only argument to the
only argument to the callout function is a pointer to a \fBpcre_callout\fP	callout function is a pointer to a \fBpcre_callout\fP or
block. This structure contains the following fields:	\fBpcre[16\|32]_callout\fP block. These structures contains the following
	fields:
.sp	.sp
int \fIversion\fP;	int \fIversion\fP;
int \fIcallout_number\fP;	int \fIcallout_number\fP;
int *\fIoffset_vector\fP;	int *\fIoffset_vector\fP;
const char *\fIsubject\fP;	const char *\fIsubject\fP; (8-bit version)
int \fIsubject_length\fP;	PCRE_SPTR16 \fIsubject\fP; (16-bit version)
int \fIstart_match\fP;	PCRE_SPTR32 \fIsubject\fP; (32-bit version)
int \fIcurrent_position\fP;	int \fIsubject_length\fP;
int \fIcapture_top\fP;	int \fIstart_match\fP;
int \fIcapture_last\fP;	int \fIcurrent_position\fP;
void *\fIcallout_data\fP;	int \fIcapture_top\fP;
int \fIpattern_position\fP;	int \fIcapture_last\fP;
int \fInext_item_length\fP;	.sp
const unsigned char *\fImark\fP;	int \fIpattern_position\fP;
	int \fInext_item_length\fP;
	const unsigned char *\fImark\fP; (8-bit version)
	const PCRE_UCHAR16 *\fImark\fP; (16-bit version)
	const PCRE_UCHAR32 *\fImark\fP; (32-bit version)
.sp	.sp
The \fIversion\fP field is an integer containing the version number of the	The \fIversion\fP field is an integer containing the version number of the
block format. The initial version was 0; the current version is 2. The version	block format. The initial version was 0; the current version is 2. The version
Line 103 into the pattern (that is, the number after ?C for man	Line 156 into the pattern (that is, the number after ?C for man
automatically generated callouts).	automatically generated callouts).
.P	.P
The \fIoffset_vector\fP field is a pointer to the vector of offsets that was	The \fIoffset_vector\fP field is a pointer to the vector of offsets that was
passed by the caller to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. When	passed by the caller to the matching function. When \fBpcre_exec()\fP or
\fBpcre_exec()\fP is used, the contents can be inspected in order to extract	\fBpcre[16\|32]_exec()\fP is used, the contents can be inspected, in order to
substrings that have been matched so far, in the same way as for extracting	extract substrings that have been matched so far, in the same way as for
substrings after a match has completed. For \fBpcre_dfa_exec()\fP this field is	extracting substrings after a match has completed. For the DFA matching
not useful.	functions, this field is not useful.
.P	.P
The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values	The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values
that were passed to \fBpcre_exec()\fP.	that were passed to the matching function.
.P	.P
The \fIstart_match\fP field normally contains the offset within the subject at	The \fIstart_match\fP field normally contains the offset within the subject at
which the current match attempt started. However, if the escape sequence \eK	which the current match attempt started. However, if the escape sequence \eK
Line 122 in the subject.	Line 175 in the subject.
The \fIcurrent_position\fP field contains the offset within the subject of the	The \fIcurrent_position\fP field contains the offset within the subject of the
current match pointer.	current match pointer.
.P	.P
When the \fBpcre_exec()\fP function is used, the \fIcapture_top\fP field	When the \fBpcre_exec()\fP or \fBpcre[16\|32]_exec()\fP is used, the
contains one more than the number of the highest numbered captured substring so	\fIcapture_top\fP field contains one more than the number of the highest
far. If no substrings have been captured, the value of \fIcapture_top\fP is	numbered captured substring so far. If no substrings have been captured, the
one. This is always the case when \fBpcre_dfa_exec()\fP is used, because it	value of \fIcapture_top\fP is one. This is always the case when the DFA
does not support captured substrings.	functions are used, because they do not support captured substrings.
.P	.P
The \fIcapture_last\fP field contains the number of the most recently captured	The \fIcapture_last\fP field contains the number of the most recently captured
substring. If no substrings have been captured, its value is -1. This is always	substring. However, when a recursion exits, the value reverts to what it was
the case when \fBpcre_dfa_exec()\fP is used.	outside the recursion, as do the values of all captured substrings. If no
	substrings have been captured, the value of \fIcapture_last\fP is -1. This is
	always the case for the DFA matching functions.
.P	.P
The \fIcallout_data\fP field contains a value that is passed to	The \fIcallout_data\fP field contains a value that is passed to a matching
\fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP specifically so that it can be	function specifically so that it can be passed back in callouts. It is passed
passed back in callouts. It is passed in the \fIpcre_callout\fP field of the	in the \fIcallout_data\fP field of a \fBpcre_extra\fP or \fBpcre[16\|32]_extra\fP
\fBpcre_extra\fP data structure. If no such data was passed, the value of	data structure. If no such data was passed, the value of \fIcallout_data\fP in
\fIcallout_data\fP in a \fBpcre_callout\fP block is NULL. There is a	a callout block is NULL. There is a description of the \fBpcre_extra\fP
description of the \fBpcre_extra\fP structure in the	structure in the
.\" HREF	.\" HREF
\fBpcreapi\fP	\fBpcreapi\fP
.\"	.\"
documentation.	documentation.
.P	.P
The \fIpattern_position\fP field is present from version 1 of the	The \fIpattern_position\fP field is present from version 1 of the callout
\fIpcre_callout\fP structure. It contains the offset to the next item to be	structure. It contains the offset to the next item to be matched in the pattern
matched in the pattern string.	string.
.P	.P
The \fInext_item_length\fP field is present from version 1 of the	The \fInext_item_length\fP field is present from version 1 of the callout
\fIpcre_callout\fP structure. It contains the length of the next item to be	structure. It contains the length of the next item to be matched in the pattern
matched in the pattern string. When the callout immediately precedes an	string. When the callout immediately precedes an alternation bar, a closing
alternation bar, a closing parenthesis, or the end of the pattern, the length	parenthesis, or the end of the pattern, the length is zero. When the callout
is zero. When the callout precedes an opening parenthesis, the length is that	precedes an opening parenthesis, the length is that of the entire subpattern.
of the entire subpattern.
.P	.P
The \fIpattern_position\fP and \fInext_item_length\fP fields are intended to	The \fIpattern_position\fP and \fInext_item_length\fP fields are intended to
help in distinguishing between different automatic callouts, which all have the	help in distinguishing between different automatic callouts, which all have the
same callout number. However, they are set for all callouts.	same callout number. However, they are set for all callouts.
.P	.P
The \fImark\fP field is present from version 2 of the \fIpcre_callout\fP	The \fImark\fP field is present from version 2 of the callout structure. In
structure. In callouts from \fBpcre_exec()\fP it contains a pointer to the	callouts from \fBpcre_exec()\fP or \fBpcre[16\|32]_exec()\fP it contains a
zero-terminated name of the most recently passed (MARK), (PRUNE), or (*THEN)	pointer to the zero-terminated name of the most recently passed (*MARK),
item in the match, or NULL if no such items have been passed. Instances of	(PRUNE), or (THEN) item in the match, or NULL if no such items have been
(PRUNE) or (THEN) without a name do not obliterate a previous (*MARK). In	passed. Instances of (PRUNE) or (THEN) without a name do not obliterate a
callouts from \fBpcre_dfa_exec()\fP this field always contains NULL.	previous (*MARK). In callouts from the DFA matching functions this field always
	contains NULL.
.	.
.	.
.SH "RETURN VALUES"	.SH "RETURN VALUES"
Line 173 The external callout function returns an integer to PC	Line 228 The external callout function returns an integer to PC
matching proceeds as normal. If the value is greater than zero, matching fails	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	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	ahead, just as if a lookahead assertion had failed. If the value is less than
zero, the match is abandoned, and \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP	zero, the match is abandoned, the matching function returns the negative value.
returns the negative value.
.P	.P
Negative values should normally be chosen from the set of PCRE_ERROR_xxx	Negative values should normally be chosen from the set of PCRE_ERROR_xxx
values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.	values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
Line 196 Cambridge CB2 3QH, England.	Line 250 Cambridge CB2 3QH, England.
.rs	.rs
.sp	.sp
.nf	.nf
Last updated: 30 November 2011	Last updated: 12 November 2013
Copyright (c) 1997-2011 University of Cambridge.	Copyright (c) 1997-2013 University of Cambridge.
.fi	.fi

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