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version 1.1.1.2, 2012/02/21 23:50:25	version 1.1.1.5, 2014/06/15 19:46:05
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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 );	.B int (pcre16_callout)(pcre16_callout_block );
.PP	.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 (\fIpcre16_callout\fP for the 16-bit	global variable \fIpcre_callout\fP (\fIpcre16_callout\fP for the 16-bit
library). By default, this variable contains NULL, which disables all calling	library, \fIpcre32_callout\fP for the 32-bit library). By default, this
out.	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 33 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 76 callouts such as the example above are obeyed.	Line 122 callouts such as the example above are obeyed.
.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 or \fIpcre16_callout\fP is called (if it is set).	defined by \fIpcre_callout\fP or \fIpcre[16\|32]_callout\fP is called (if it is
This applies to both normal and DFA matching. The only argument to the callout	set). This applies to both normal and DFA matching. The only argument to the
function is a pointer to a \fBpcre_callout\fP or \fBpcre16_callout\fP block.	callout function is a pointer to a \fBpcre_callout\fP or
These structures 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; (8-bit version)	const char *\fIsubject\fP; (8-bit version)
PCRE_SPTR16 \fIsubject\fP; (16-bit version)	PCRE_SPTR16 \fIsubject\fP; (16-bit version)
	PCRE_SPTR32 \fIsubject\fP; (32-bit version)
int \fIsubject_length\fP;	int \fIsubject_length\fP;
int \fIstart_match\fP;	int \fIstart_match\fP;
int \fIcurrent_position\fP;	int \fIcurrent_position\fP;
Line 96 These structures contains the following fields:	Line 144 These structures contains the following fields:
int \fInext_item_length\fP;	int \fInext_item_length\fP;
const unsigned char *\fImark\fP; (8-bit version)	const unsigned char *\fImark\fP; (8-bit version)
const PCRE_UCHAR16 *\fImark\fP; (16-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 108 automatically generated callouts).	Line 157 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 the matching function. When \fBpcre_exec()\fP or	passed by the caller to the matching function. When \fBpcre_exec()\fP or
\fBpcre16_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 the DFA matching functions, this	extracting substrings after a match has completed. For the DFA matching
field is 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 the matching function.	that were passed to the matching function.
Line 126 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 or \fBpcre16_exec()\fP is used, the	When the \fBpcre_exec()\fP or \fBpcre[16\|32]_exec()\fP is used, the
\fIcapture_top\fP field contains one more than the number of the highest	\fIcapture_top\fP field contains one more than the number of the highest
numbered captured substring so far. If no substrings have been captured, the	numbered captured substring so far. If no substrings have been captured, the
value of \fIcapture_top\fP is one. This is always the case when the DFA	value of \fIcapture_top\fP is one. This is always the case when the DFA
functions are used, because they do 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 for the DFA matching functions.	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 a matching	The \fIcallout_data\fP field contains a value that is passed to a matching
function specifically so that it can be passed back in callouts. It is passed	function specifically so that it can be passed back in callouts. It is passed
in the \fIcallout_data\fP field of a \fBpcre_extra\fP or \fBpcre16_extra\fP	in the \fIcallout_data\fP field of a \fBpcre_extra\fP or \fBpcre[16\|32]_extra\fP
data structure. If no such data was passed, the value of \fIcallout_data\fP in	data structure. If no such data was passed, the value of \fIcallout_data\fP in
a callout block is NULL. There is a description of the \fBpcre_extra\fP	a callout block is NULL. There is a description of the \fBpcre_extra\fP
structure in the	structure in the
Line 162 help in distinguishing between different automatic cal	Line 213 help in distinguishing between different automatic cal
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 callout structure. In	The \fImark\fP field is present from version 2 of the callout structure. In
callouts from \fBpcre_exec()\fP or \fBpcre16_exec()\fP it contains a pointer to	callouts from \fBpcre_exec()\fP or \fBpcre[16\|32]_exec()\fP it contains a
the zero-terminated name of the most recently passed (MARK), (PRUNE), or	pointer to the zero-terminated name of the most recently passed (*MARK),
(*THEN) item in the match, or NULL if no such items have been passed. Instances	(PRUNE), or (THEN) item in the match, or NULL if no such items have been
of (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 the DFA matching functions 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 198 Cambridge CB2 3QH, England.	Line 250 Cambridge CB2 3QH, England.
.rs	.rs
.sp	.sp
.nf	.nf
Last updated: 08 Janurary 2012	Last updated: 12 November 2013
Copyright (c) 1997-2012 University of Cambridge.	Copyright (c) 1997-2013 University of Cambridge.
.fi	.fi

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