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.
.
.
.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_exec()\fP or \fBpcre_dfa_exec()\fP. 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;
.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 1. 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.
.
.
.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: 29 September 2009
Copyright (c) 1997-2009 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	.
43	.
44	.SH "MISSING CALLOUTS"
45	.rs
46	.sp
47	You should be aware that, because of optimizations in the way PCRE matches
48	patterns by default, callouts sometimes do not happen. For example, if the
49	pattern is
50	.sp
51	ab(?C4)cd
52	.sp
53	PCRE knows that any matching string must contain the letter "d". If the subject
54	string is "abyz", the lack of "d" means that matching doesn't ever start, and
55	the callout is never reached. However, with "abyd", though the result is still
56	no match, the callout is obeyed.
57	.P
58	If the pattern is studied, PCRE knows the minimum length of a matching string,
59	and will immediately give a "no match" return without actually running a match
60	if the subject is not long enough, or, for unanchored patterns, if it has
61	been scanned far enough.
62	.P
63	You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
64	option to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. This slows down the
65	matching process, but does ensure that callouts such as the example above are
66	obeyed.
67	.
68	.
69	.SH "THE CALLOUT INTERFACE"
70	.rs
71	.sp
72	During matching, when PCRE reaches a callout point, the external function
73	defined by \fIpcre_callout\fP is called (if it is set). This applies to both
74	the \fBpcre_exec()\fP and the \fBpcre_dfa_exec()\fP matching functions. The
75	only argument to the callout function is a pointer to a \fBpcre_callout\fP
76	block. This structure contains the following fields:
77	.sp
78	int \fIversion\fP;
79	int \fIcallout_number\fP;
80	int *\fIoffset_vector\fP;
81	const char *\fIsubject\fP;
82	int \fIsubject_length\fP;
83	int \fIstart_match\fP;
84	int \fIcurrent_position\fP;
85	int \fIcapture_top\fP;
86	int \fIcapture_last\fP;
87	void *\fIcallout_data\fP;
88	int \fIpattern_position\fP;
89	int \fInext_item_length\fP;
90	.sp
91	The \fIversion\fP field is an integer containing the version number of the
92	block format. The initial version was 0; the current version is 1. The version
93	number will change again in future if additional fields are added, but the
94	intention is never to remove any of the existing fields.
95	.P
96	The \fIcallout_number\fP field contains the number of the callout, as compiled
97	into the pattern (that is, the number after ?C for manual callouts, and 255 for
98	automatically generated callouts).
99	.P
100	The \fIoffset_vector\fP field is a pointer to the vector of offsets that was
101	passed by the caller to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. When
102	\fBpcre_exec()\fP is used, the contents can be inspected in order to extract
103	substrings that have been matched so far, in the same way as for extracting
104	substrings after a match has completed. For \fBpcre_dfa_exec()\fP this field is
105	not useful.
106	.P
107	The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values
108	that were passed to \fBpcre_exec()\fP.
109	.P
110	The \fIstart_match\fP field normally contains the offset within the subject at
111	which the current match attempt started. However, if the escape sequence \eK
112	has been encountered, this value is changed to reflect the modified starting
113	point. If the pattern is not anchored, the callout function may be called
114	several times from the same point in the pattern for different starting points
115	in the subject.
116	.P
117	The \fIcurrent_position\fP field contains the offset within the subject of the
118	current match pointer.
119	.P
120	When the \fBpcre_exec()\fP function is used, the \fIcapture_top\fP field
121	contains one more than the number of the highest numbered captured substring so
122	far. If no substrings have been captured, the value of \fIcapture_top\fP is
123	one. This is always the case when \fBpcre_dfa_exec()\fP is used, because it
124	does not support captured substrings.
125	.P
126	The \fIcapture_last\fP field contains the number of the most recently captured
127	substring. If no substrings have been captured, its value is -1. This is always
128	the case when \fBpcre_dfa_exec()\fP is used.
129	.P
130	The \fIcallout_data\fP field contains a value that is passed to
131	\fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP specifically so that it can be
132	passed back in callouts. It is passed in the \fIpcre_callout\fP field of the
133	\fBpcre_extra\fP data structure. If no such data was passed, the value of
134	\fIcallout_data\fP in a \fBpcre_callout\fP block is NULL. There is a
135	description of the \fBpcre_extra\fP structure in the
136	.\" HREF
137	\fBpcreapi\fP
138	.\"
139	documentation.
140	.P
141	The \fIpattern_position\fP field is present from version 1 of the
142	\fIpcre_callout\fP structure. It contains the offset to the next item to be
143	matched in the pattern string.
144	.P
145	The \fInext_item_length\fP field is present from version 1 of the
146	\fIpcre_callout\fP structure. It contains the length of the next item to be
147	matched in the pattern string. When the callout immediately precedes an
148	alternation bar, a closing parenthesis, or the end of the pattern, the length
149	is zero. When the callout precedes an opening parenthesis, the length is that
150	of the entire subpattern.
151	.P
152	The \fIpattern_position\fP and \fInext_item_length\fP fields are intended to
153	help in distinguishing between different automatic callouts, which all have the
154	same callout number. However, they are set for all callouts.
155	.
156	.
157	.SH "RETURN VALUES"
158	.rs
159	.sp
160	The external callout function returns an integer to PCRE. If the value is zero,
161	matching proceeds as normal. If the value is greater than zero, matching fails
162	at the current point, but the testing of other matching possibilities goes
163	ahead, just as if a lookahead assertion had failed. If the value is less than
164	zero, the match is abandoned, and \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP
165	returns the negative value.
166	.P
167	Negative values should normally be chosen from the set of PCRE_ERROR_xxx
168	values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
169	The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
170	it will never be used by PCRE itself.
171	.
172	.
173	.SH AUTHOR
174	.rs
175	.sp
176	.nf
177	Philip Hazel
178	University Computing Service
179	Cambridge CB2 3QH, England.
180	.fi
181	.
182	.
183	.SH REVISION
184	.rs
185	.sp
186	.nf
187	Last updated: 29 September 2009
188	Copyright (c) 1997-2009 University of Cambridge.
189	.fi
Name	Value
svn:eol-style	native
svn:keywords	"Author Date Id Revision Url"