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<h1>pcrecallout man page</h1>
<p>
Return to the <a href="index.html">PCRE index page</a>.
</p>
<p>
This page is part of the PCRE HTML documentation. It was generated automatically
from the original man page. If there is any nonsense in it, please consult the
man page, in case the conversion went wrong.
<br>
<ul>
<li><a name="TOC1" href="#SEC1">PCRE CALLOUTS</a>
<li><a name="TOC2" href="#SEC2">MISSING CALLOUTS</a>
<li><a name="TOC3" href="#SEC3">THE CALLOUT INTERFACE</a>
<li><a name="TOC4" href="#SEC4">RETURN VALUES</a>
<li><a name="TOC5" href="#SEC5">AUTHOR</a>
<li><a name="TOC6" href="#SEC6">REVISION</a>
</ul>
<br><a name="SEC1" href="#TOC1">PCRE CALLOUTS</a><br>
<P>
<b>int (*pcre_callout)(pcre_callout_block *);</b>
</P>
<P>
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 <i>pcre_callout</i>. 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
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:
<pre>
  (?C1)abc(?C2)def
</pre>
If the PCRE_AUTO_CALLOUT option bit is set when <b>pcre_compile()</b> or
<b>pcre_compile2()</b> 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
<pre>
  A(\d{2}|--)
</pre>
it is processed as if it were
<br>
<br>
(?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
<br>
<br>
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
<a href="pcretest.html"><b>pcretest</b></a>
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>
<br><a name="SEC2" href="#TOC1">MISSING CALLOUTS</a><br>
<P>
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
<pre>
  ab(?C4)cd
</pre>
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>
<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>
<P>
You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
option to <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. This slows down the
matching process, but does ensure that callouts such as the example above are
obeyed.
</P>
<br><a name="SEC3" href="#TOC1">THE CALLOUT INTERFACE</a><br>
<P>
During matching, when PCRE reaches a callout point, the external function
defined by <i>pcre_callout</i> is called (if it is set). This applies to both
the <b>pcre_exec()</b> and the <b>pcre_dfa_exec()</b> matching functions. The
only argument to the callout function is a pointer to a <b>pcre_callout</b>
block. This structure contains the following fields:
<pre>
  int          <i>version</i>;
  int          <i>callout_number</i>;
  int         *<i>offset_vector</i>;
  const char  *<i>subject</i>;
  int          <i>subject_length</i>;
  int          <i>start_match</i>;
  int          <i>current_position</i>;
  int          <i>capture_top</i>;
  int          <i>capture_last</i>;
  void        *<i>callout_data</i>;
  int          <i>pattern_position</i>;
  int          <i>next_item_length</i>;
</pre>
The <i>version</i> 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>
<P>
The <i>callout_number</i> 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>
<P>
The <i>offset_vector</i> field is a pointer to the vector of offsets that was
passed by the caller to <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. When
<b>pcre_exec()</b> 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 <b>pcre_dfa_exec()</b> this field is
not useful.
</P>
<P>
The <i>subject</i> and <i>subject_length</i> fields contain copies of the values
that were passed to <b>pcre_exec()</b>.
</P>
<P>
The <i>start_match</i> field normally contains the offset within the subject at
which the current match attempt started. However, if the escape sequence \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.
</P>
<P>
The <i>current_position</i> field contains the offset within the subject of the
current match pointer.
</P>
<P>
When the <b>pcre_exec()</b> function is used, the <i>capture_top</i> field
contains one more than the number of the highest numbered captured substring so
far. If no substrings have been captured, the value of <i>capture_top</i> is
one. This is always the case when <b>pcre_dfa_exec()</b> is used, because it
does not support captured substrings.
</P>
<P>
The <i>capture_last</i> 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 <b>pcre_dfa_exec()</b> is used.
</P>
<P>
The <i>callout_data</i> field contains a value that is passed to
<b>pcre_exec()</b> or <b>pcre_dfa_exec()</b> specifically so that it can be
passed back in callouts. It is passed in the <i>pcre_callout</i> field of the
<b>pcre_extra</b> data structure. If no such data was passed, the value of
<i>callout_data</i> in a <b>pcre_callout</b> block is NULL. There is a
description of the <b>pcre_extra</b> structure in the
<a href="pcreapi.html"><b>pcreapi</b></a>
documentation.
</P>
<P>
The <i>pattern_position</i> field is present from version 1 of the
<i>pcre_callout</i> structure. It contains the offset to the next item to be
matched in the pattern string.
</P>
<P>
The <i>next_item_length</i> field is present from version 1 of the
<i>pcre_callout</i> 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>
<P>
The <i>pattern_position</i> and <i>next_item_length</i> 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>
<br><a name="SEC4" href="#TOC1">RETURN VALUES</a><br>
<P>
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 <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>
returns the negative value.
</P>
<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.
</P>
<br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
<P>
Philip Hazel
<br>
University Computing Service
<br>
Cambridge CB2 3QH, England.
<br>
</P>
<br><a name="SEC6" href="#TOC1">REVISION</a><br>
<P>
Last updated: 29 September 2009
<br>
Copyright &copy; 1997-2009 University of Cambridge.
<br>
<p>
Return to the <a href="index.html">PCRE index page</a>.
</p>
1	<html>
2	<head>
3	<title>pcrecallout specification</title>
4	</head>
5	<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6	<h1>pcrecallout man page</h1>
7	<p>
8	Return to the <a href="index.html">PCRE index page</a>.
9	</p>
10	<p>
11	This page is part of the PCRE HTML documentation. It was generated automatically
12	from the original man page. If there is any nonsense in it, please consult the
13	man page, in case the conversion went wrong.
14	<br>
15	<ul>
16	<li><a name="TOC1" href="#SEC1">PCRE CALLOUTS</a>
17	<li><a name="TOC2" href="#SEC2">MISSING CALLOUTS</a>
18	<li><a name="TOC3" href="#SEC3">THE CALLOUT INTERFACE</a>
19	<li><a name="TOC4" href="#SEC4">RETURN VALUES</a>
20	<li><a name="TOC5" href="#SEC5">AUTHOR</a>
21	<li><a name="TOC6" href="#SEC6">REVISION</a>
22	</ul>
23	<br><a name="SEC1" href="#TOC1">PCRE CALLOUTS</a><br>
24	<P>
25	<b>int (pcre_callout)(pcre_callout_block );</b>
26	</P>
27	<P>
28	PCRE provides a feature called "callout", which is a means of temporarily
29	passing control to the caller of PCRE in the middle of pattern matching. The
30	caller of PCRE provides an external function by putting its entry point in the
31	global variable <i>pcre_callout</i>. By default, this variable contains NULL,
32	which disables all calling out.
33	</P>
34	<P>
35	Within a regular expression, (?C) indicates the points at which the external
36	function is to be called. Different callout points can be identified by putting
37	a number less than 256 after the letter C. The default value is zero.
38	For example, this pattern has two callout points:
39	<pre>
40	(?C1)abc(?C2)def
41	</pre>
42	If the PCRE_AUTO_CALLOUT option bit is set when <b>pcre_compile()</b> or
43	<b>pcre_compile2()</b> is called, PCRE automatically inserts callouts, all with
44	number 255, before each item in the pattern. For example, if PCRE_AUTO_CALLOUT
45	is used with the pattern
46	<pre>
47	A(\d{2}\|--)
48	</pre>
49	it is processed as if it were
50	<br>
51	<br>
52	(?C255)A(?C255)((?C255)\d{2}(?C255)\|(?C255)-(?C255)-(?C255))(?C255)
53	<br>
54	<br>
55	Notice that there is a callout before and after each parenthesis and
56	alternation bar. Automatic callouts can be used for tracking the progress of
57	pattern matching. The
58	<a href="pcretest.html"><b>pcretest</b></a>
59	command has an option that sets automatic callouts; when it is used, the output
60	indicates how the pattern is matched. This is useful information when you are
61	trying to optimize the performance of a particular pattern.
62	</P>
63	<br><a name="SEC2" href="#TOC1">MISSING CALLOUTS</a><br>
64	<P>
65	You should be aware that, because of optimizations in the way PCRE matches
66	patterns by default, callouts sometimes do not happen. For example, if the
67	pattern is
68	<pre>
69	ab(?C4)cd
70	</pre>
71	PCRE knows that any matching string must contain the letter "d". If the subject
72	string is "abyz", the lack of "d" means that matching doesn't ever start, and
73	the callout is never reached. However, with "abyd", though the result is still
74	no match, the callout is obeyed.
75	</P>
76	<P>
77	If the pattern is studied, PCRE knows the minimum length of a matching string,
78	and will immediately give a "no match" return without actually running a match
79	if the subject is not long enough, or, for unanchored patterns, if it has
80	been scanned far enough.
81	</P>
82	<P>
83	You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
84	option to <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. This slows down the
85	matching process, but does ensure that callouts such as the example above are
86	obeyed.
87	</P>
88	<br><a name="SEC3" href="#TOC1">THE CALLOUT INTERFACE</a><br>
89	<P>
90	During matching, when PCRE reaches a callout point, the external function
91	defined by <i>pcre_callout</i> is called (if it is set). This applies to both
92	the <b>pcre_exec()</b> and the <b>pcre_dfa_exec()</b> matching functions. The
93	only argument to the callout function is a pointer to a <b>pcre_callout</b>
94	block. This structure contains the following fields:
95	<pre>
96	int <i>version</i>;
97	int <i>callout_number</i>;
98	int *<i>offset_vector</i>;
99	const char *<i>subject</i>;
100	int <i>subject_length</i>;
101	int <i>start_match</i>;
102	int <i>current_position</i>;
103	int <i>capture_top</i>;
104	int <i>capture_last</i>;
105	void *<i>callout_data</i>;
106	int <i>pattern_position</i>;
107	int <i>next_item_length</i>;
108	</pre>
109	The <i>version</i> field is an integer containing the version number of the
110	block format. The initial version was 0; the current version is 1. The version
111	number will change again in future if additional fields are added, but the
112	intention is never to remove any of the existing fields.
113	</P>
114	<P>
115	The <i>callout_number</i> field contains the number of the callout, as compiled
116	into the pattern (that is, the number after ?C for manual callouts, and 255 for
117	automatically generated callouts).
118	</P>
119	<P>
120	The <i>offset_vector</i> field is a pointer to the vector of offsets that was
121	passed by the caller to <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. When
122	<b>pcre_exec()</b> is used, the contents can be inspected in order to extract
123	substrings that have been matched so far, in the same way as for extracting
124	substrings after a match has completed. For <b>pcre_dfa_exec()</b> this field is
125	not useful.
126	</P>
127	<P>
128	The <i>subject</i> and <i>subject_length</i> fields contain copies of the values
129	that were passed to <b>pcre_exec()</b>.
130	</P>
131	<P>
132	The <i>start_match</i> field normally contains the offset within the subject at
133	which the current match attempt started. However, if the escape sequence \K
134	has been encountered, this value is changed to reflect the modified starting
135	point. If the pattern is not anchored, the callout function may be called
136	several times from the same point in the pattern for different starting points
137	in the subject.
138	</P>
139	<P>
140	The <i>current_position</i> field contains the offset within the subject of the
141	current match pointer.
142	</P>
143	<P>
144	When the <b>pcre_exec()</b> function is used, the <i>capture_top</i> field
145	contains one more than the number of the highest numbered captured substring so
146	far. If no substrings have been captured, the value of <i>capture_top</i> is
147	one. This is always the case when <b>pcre_dfa_exec()</b> is used, because it
148	does not support captured substrings.
149	</P>
150	<P>
151	The <i>capture_last</i> field contains the number of the most recently captured
152	substring. If no substrings have been captured, its value is -1. This is always
153	the case when <b>pcre_dfa_exec()</b> is used.
154	</P>
155	<P>
156	The <i>callout_data</i> field contains a value that is passed to
157	<b>pcre_exec()</b> or <b>pcre_dfa_exec()</b> specifically so that it can be
158	passed back in callouts. It is passed in the <i>pcre_callout</i> field of the
159	<b>pcre_extra</b> data structure. If no such data was passed, the value of
160	<i>callout_data</i> in a <b>pcre_callout</b> block is NULL. There is a
161	description of the <b>pcre_extra</b> structure in the
162	<a href="pcreapi.html"><b>pcreapi</b></a>
163	documentation.
164	</P>
165	<P>
166	The <i>pattern_position</i> field is present from version 1 of the
167	<i>pcre_callout</i> structure. It contains the offset to the next item to be
168	matched in the pattern string.
169	</P>
170	<P>
171	The <i>next_item_length</i> field is present from version 1 of the
172	<i>pcre_callout</i> structure. It contains the length of the next item to be
173	matched in the pattern string. When the callout immediately precedes an
174	alternation bar, a closing parenthesis, or the end of the pattern, the length
175	is zero. When the callout precedes an opening parenthesis, the length is that
176	of the entire subpattern.
177	</P>
178	<P>
179	The <i>pattern_position</i> and <i>next_item_length</i> fields are intended to
180	help in distinguishing between different automatic callouts, which all have the
181	same callout number. However, they are set for all callouts.
182	</P>
183	<br><a name="SEC4" href="#TOC1">RETURN VALUES</a><br>
184	<P>
185	The external callout function returns an integer to PCRE. If the value is zero,
186	matching proceeds as normal. If the value is greater than zero, matching fails
187	at the current point, but the testing of other matching possibilities goes
188	ahead, just as if a lookahead assertion had failed. If the value is less than
189	zero, the match is abandoned, and <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>
190	returns the negative value.
191	</P>
192	<P>
193	Negative values should normally be chosen from the set of PCRE_ERROR_xxx
194	values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
195	The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
196	it will never be used by PCRE itself.
197	</P>
198	<br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
199	<P>
200	Philip Hazel
201	<br>
202	University Computing Service
203	<br>
204	Cambridge CB2 3QH, England.
205	<br>
206	</P>
207	<br><a name="SEC6" href="#TOC1">REVISION</a><br>
208	<P>
209	Last updated: 29 September 2009
210	<br>
211	Copyright © 1997-2009 University of Cambridge.
212	<br>
213	<p>
214	Return to the <a href="index.html">PCRE index page</a>.
215	</p>
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