trunk/doc/pcrecallout.3

.TH PCRECALLOUT 3
.SH NAME
PCRE - Perl-compatible regular expressions
.SH "PCRE CALLOUTS"
.rs
.sp
.B int (*pcre_callout)(pcre_callout_block *);
.PP
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
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,
which disables all calling out.
.P
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:
.sp
  (?C1)abc(?C2)def
.sp
If the PCRE_AUTO_CALLOUT option bit is set when \fBpcre_compile()\fP or
\fBpcre_compile2()\fP is called, 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
.sp
  A(\ed{2}|--)
.sp
it is processed as if it were
.sp
(?C255)A(?C255)((?C255)\ed{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
.sp
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
.\" HREF
\fBpcretest\fP
.\"
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.
.P
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"
.rs
.sp
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
.sp
  ab(?C4)cd
.sp
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.
.P
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.
.P
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,
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.
.
.
.SH "THE CALLOUT INTERFACE"
.rs
.sp
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
the \fBpcre_exec()\fP and the \fBpcre_dfa_exec()\fP matching functions. The
only argument to the callout function is a pointer to a \fBpcre_callout\fP
block. This structure contains the following fields:
.sp
  int         \fIversion\fP;
  int         \fIcallout_number\fP;
  int        *\fIoffset_vector\fP;
  const char *\fIsubject\fP;
  int         \fIsubject_length\fP;
  int         \fIstart_match\fP;
  int         \fIcurrent_position\fP;
  int         \fIcapture_top\fP;
  int         \fIcapture_last\fP;
  void       *\fIcallout_data\fP;
  int         \fIpattern_position\fP;
  int         \fInext_item_length\fP;
  const unsigned char *\fImark\fP;
.sp
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
number will change again in future if additional fields are added, but the
intention is never to remove any of the existing fields.
.P
The \fIcallout_number\fP field contains the number of the callout, as compiled
into the pattern (that is, the number after ?C for manual callouts, and 255 for
automatically generated callouts).
.P
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
\fBpcre_exec()\fP 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 \fBpcre_dfa_exec()\fP this field is
not useful.
.P
The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values
that were passed to \fBpcre_exec()\fP.
.P
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
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.
.P
The \fIcurrent_position\fP field contains the offset within the subject of the
current match pointer.
.P
When the \fBpcre_exec()\fP function is used, the \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 value of \fIcapture_top\fP is
one. This is always the case when \fBpcre_dfa_exec()\fP is used, because it
does not support captured substrings.
.P
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
the case when \fBpcre_dfa_exec()\fP is used.
.P
The \fIcallout_data\fP field contains a value that is passed to
\fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP specifically so that it can be
passed back in callouts. It is passed in the \fIpcre_callout\fP field of the
\fBpcre_extra\fP data structure. If no such data was passed, the value of
\fIcallout_data\fP in a \fBpcre_callout\fP block is NULL. There is a
description of the \fBpcre_extra\fP structure in the
.\" HREF
\fBpcreapi\fP
.\"
documentation.
.P
The \fIpattern_position\fP field is present from version 1 of the
\fIpcre_callout\fP structure. It contains the offset to the next item to be
matched in the pattern string.
.P
The \fInext_item_length\fP field is present from version 1 of the
\fIpcre_callout\fP 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.
.P
The \fIpattern_position\fP and \fInext_item_length\fP 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.
.P
The \fImark\fP field is present from version 2 of the \fIpcre_callout\fP
structure. In callouts from \fBpcre_exec()\fP it contains a pointer to the
zero-terminated name of the most recently passed (*MARK) item in the match, or
NULL if there are no (*MARK)s in the current matching path. In callouts from
\fBpcre_dfa_exec()\fP this field always contains NULL.
.
.
.SH "RETURN VALUES"
.rs
.sp
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, and \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP
returns the negative value.
.P
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.
The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
it will never be used by PCRE itself.
.
.
.SH AUTHOR
.rs
.sp
.nf
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
.fi
.
.
.SH REVISION
.rs
.sp
.nf
Last updated: 26 August 2011
Copyright (c) 1997-2011 University of Cambridge.
.fi
1	.TH PCRECALLOUT 3
2	.SH NAME
3	PCRE - Perl-compatible regular expressions
4	.SH "PCRE CALLOUTS"
5	.rs
6	.sp
7	.B int (pcre_callout)(pcre_callout_block );
8	.PP
9	PCRE provides a feature called "callout", which is a means of temporarily
10	passing control to the caller of PCRE in the middle of pattern matching. The
11	caller of PCRE provides an external function by putting its entry point in the
12	global variable \fIpcre_callout\fP. By default, this variable contains NULL,
13	which disables all calling out.
14	.P
15	Within a regular expression, (?C) indicates the points at which the external
16	function is to be called. Different callout points can be identified by putting
17	a number less than 256 after the letter C. The default value is zero.
18	For example, this pattern has two callout points:
19	.sp
20	(?C1)abc(?C2)def
21	.sp
22	If the PCRE_AUTO_CALLOUT option bit is set when \fBpcre_compile()\fP or
23	\fBpcre_compile2()\fP is called, PCRE automatically inserts callouts, all with
24	number 255, before each item in the pattern. For example, if PCRE_AUTO_CALLOUT
25	is used with the pattern
26	.sp
27	A(\ed{2}\|--)
28	.sp
29	it is processed as if it were
30	.sp
31	(?C255)A(?C255)((?C255)\ed{2}(?C255)\|(?C255)-(?C255)-(?C255))(?C255)
32	.sp
33	Notice that there is a callout before and after each parenthesis and
34	alternation bar. Automatic callouts can be used for tracking the progress of
35	pattern matching. The
36	.\" HREF
37	\fBpcretest\fP
38	.\"
39	command has an option that sets automatic callouts; when it is used, the output
40	indicates how the pattern is matched. This is useful information when you are
41	trying to optimize the performance of a particular pattern.
42	.P
43	The use of callouts in a pattern makes it ineligible for optimization by the
44	just-in-time compiler. Studying such a pattern with the PCRE_STUDY_JIT_COMPILE
45	option always fails.
46	.
47	.
48	.SH "MISSING CALLOUTS"
49	.rs
50	.sp
51	You should be aware that, because of optimizations in the way PCRE matches
52	patterns by default, callouts sometimes do not happen. For example, if the
53	pattern is
54	.sp
55	ab(?C4)cd
56	.sp
57	PCRE knows that any matching string must contain the letter "d". If the subject
58	string is "abyz", the lack of "d" means that matching doesn't ever start, and
59	the callout is never reached. However, with "abyd", though the result is still
60	no match, the callout is obeyed.
61	.P
62	If the pattern is studied, PCRE knows the minimum length of a matching string,
63	and will immediately give a "no match" return without actually running a match
64	if the subject is not long enough, or, for unanchored patterns, if it has
65	been scanned far enough.
66	.P
67	You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
68	option to \fBpcre_compile()\fP, \fBpcre_exec()\fP, or \fBpcre_dfa_exec()\fP,
69	or by starting the pattern with (*NO_START_OPT). This slows down the matching
70	process, but does ensure that callouts such as the example above are obeyed.
71	.
72	.
73	.SH "THE CALLOUT INTERFACE"
74	.rs
75	.sp
76	During matching, when PCRE reaches a callout point, the external function
77	defined by \fIpcre_callout\fP is called (if it is set). This applies to both
78	the \fBpcre_exec()\fP and the \fBpcre_dfa_exec()\fP matching functions. The
79	only argument to the callout function is a pointer to a \fBpcre_callout\fP
80	block. This structure contains the following fields:
81	.sp
82	int \fIversion\fP;
83	int \fIcallout_number\fP;
84	int *\fIoffset_vector\fP;
85	const char *\fIsubject\fP;
86	int \fIsubject_length\fP;
87	int \fIstart_match\fP;
88	int \fIcurrent_position\fP;
89	int \fIcapture_top\fP;
90	int \fIcapture_last\fP;
91	void *\fIcallout_data\fP;
92	int \fIpattern_position\fP;
93	int \fInext_item_length\fP;
94	const unsigned char *\fImark\fP;
95	.sp
96	The \fIversion\fP field is an integer containing the version number of the
97	block format. The initial version was 0; the current version is 2. The version
98	number will change again in future if additional fields are added, but the
99	intention is never to remove any of the existing fields.
100	.P
101	The \fIcallout_number\fP field contains the number of the callout, as compiled
102	into the pattern (that is, the number after ?C for manual callouts, and 255 for
103	automatically generated callouts).
104	.P
105	The \fIoffset_vector\fP field is a pointer to the vector of offsets that was
106	passed by the caller to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. When
107	\fBpcre_exec()\fP is used, the contents can be inspected in order to extract
108	substrings that have been matched so far, in the same way as for extracting
109	substrings after a match has completed. For \fBpcre_dfa_exec()\fP this field is
110	not useful.
111	.P
112	The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values
113	that were passed to \fBpcre_exec()\fP.
114	.P
115	The \fIstart_match\fP field normally contains the offset within the subject at
116	which the current match attempt started. However, if the escape sequence \eK
117	has been encountered, this value is changed to reflect the modified starting
118	point. If the pattern is not anchored, the callout function may be called
119	several times from the same point in the pattern for different starting points
120	in the subject.
121	.P
122	The \fIcurrent_position\fP field contains the offset within the subject of the
123	current match pointer.
124	.P
125	When the \fBpcre_exec()\fP function is used, the \fIcapture_top\fP field
126	contains one more than the number of the highest numbered captured substring so
127	far. If no substrings have been captured, the value of \fIcapture_top\fP is
128	one. This is always the case when \fBpcre_dfa_exec()\fP is used, because it
129	does not support captured substrings.
130	.P
131	The \fIcapture_last\fP field contains the number of the most recently captured
132	substring. If no substrings have been captured, its value is -1. This is always
133	the case when \fBpcre_dfa_exec()\fP is used.
134	.P
135	The \fIcallout_data\fP field contains a value that is passed to
136	\fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP specifically so that it can be
137	passed back in callouts. It is passed in the \fIpcre_callout\fP field of the
138	\fBpcre_extra\fP data structure. If no such data was passed, the value of
139	\fIcallout_data\fP in a \fBpcre_callout\fP block is NULL. There is a
140	description of the \fBpcre_extra\fP structure in the
141	.\" HREF
142	\fBpcreapi\fP
143	.\"
144	documentation.
145	.P
146	The \fIpattern_position\fP field is present from version 1 of the
147	\fIpcre_callout\fP structure. It contains the offset to the next item to be
148	matched in the pattern string.
149	.P
150	The \fInext_item_length\fP field is present from version 1 of the
151	\fIpcre_callout\fP structure. It contains the length of the next item to be
152	matched in the pattern string. When the callout immediately precedes an
153	alternation bar, a closing parenthesis, or the end of the pattern, the length
154	is zero. When the callout precedes an opening parenthesis, the length is that
155	of the entire subpattern.
156	.P
157	The \fIpattern_position\fP and \fInext_item_length\fP fields are intended to
158	help in distinguishing between different automatic callouts, which all have the
159	same callout number. However, they are set for all callouts.
160	.P
161	The \fImark\fP field is present from version 2 of the \fIpcre_callout\fP
162	structure. In callouts from \fBpcre_exec()\fP it contains a pointer to the
163	zero-terminated name of the most recently passed (*MARK) item in the match, or
164	NULL if there are no (*MARK)s in the current matching path. In callouts from
165	\fBpcre_dfa_exec()\fP this field always contains NULL.
166	.
167	.
168	.SH "RETURN VALUES"
169	.rs
170	.sp
171	The external callout function returns an integer to PCRE. If the value is zero,
172	matching proceeds as normal. If the value is greater than zero, matching fails
173	at the current point, but the testing of other matching possibilities goes
174	ahead, just as if a lookahead assertion had failed. If the value is less than
175	zero, the match is abandoned, and \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP
176	returns the negative value.
177	.P
178	Negative values should normally be chosen from the set of PCRE_ERROR_xxx
179	values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
180	The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
181	it will never be used by PCRE itself.
182	.
183	.
184	.SH AUTHOR
185	.rs
186	.sp
187	.nf
188	Philip Hazel
189	University Computing Service
190	Cambridge CB2 3QH, England.
191	.fi
192	.
193	.
194	.SH REVISION
195	.rs
196	.sp
197	.nf
198	Last updated: 26 August 2011
199	Copyright (c) 1997-2011 University of Cambridge.
200	.fi
Name	Value
svn:eol-style	native
svn:keywords	"Author Date Id Revision Url"