doc/html/pcrecallout.html

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<title>pcrecallout specification</title>
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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_compile()</b>, <b>pcre_exec()</b>, or <b>pcre_dfa_exec()</b>,
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.
</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>;
  const unsigned char *<i>mark</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 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>
<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>
<P>
The <i>mark</i> field is present from version 2 of the <i>pcre_callout</i>
structure. In callouts from <b>pcre_exec()</b> 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
<b>pcre_dfa_exec()</b> this field always contains NULL.
</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: 31 July 2011
<br>
Copyright &copy; 1997-2011 University of Cambridge.
<br>
<p>
Return to the <a href="index.html">PCRE index page</a>.
</p>
1	nigel	63	<html>
2			<head>
3			<title>pcrecallout specification</title>
4			</head>
5			<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6	nigel	75	<h1>pcrecallout man page</h1>
7			<p>
8			Return to the <a href="index.html">PCRE index page</a>.
9			</p>
10	ph10	111	<p>
11	nigel	75	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	ph10	111	<br>
15	nigel	63	<ul>
16			<li><a name="TOC1" href="#SEC1">PCRE CALLOUTS</a>
17	nigel	75	<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	ph10	99	<li><a name="TOC5" href="#SEC5">AUTHOR</a>
21			<li><a name="TOC6" href="#SEC6">REVISION</a>
22	nigel	63	</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	nigel	75	<pre>
40	ph10	155	(?C1)abc(?C2)def
41	nigel	75	</pre>
42	ph10	461	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	nigel	75	<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	nigel	63	</P>
63	nigel	75	<br><a name="SEC2" href="#TOC1">MISSING CALLOUTS</a><br>
64	nigel	63	<P>
65	nigel	75	You should be aware that, because of optimizations in the way PCRE matches
66	ph10	392	patterns by default, callouts sometimes do not happen. For example, if the
67			pattern is
68	nigel	63	<pre>
69	nigel	75	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	nigel	63	</P>
76	ph10	392	<P>
77	ph10	461	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	ph10	392	You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
84	ph10	579	option to <b>pcre_compile()</b>, <b>pcre_exec()</b>, or <b>pcre_dfa_exec()</b>,
85			or by starting the pattern with (*NO_START_OPT). This slows down the matching
86			process, but does ensure that callouts such as the example above are obeyed.
87	ph10	392	</P>
88	nigel	75	<br><a name="SEC3" href="#TOC1">THE CALLOUT INTERFACE</a><br>
89	nigel	63	<P>
90	nigel	75	During matching, when PCRE reaches a callout point, the external function
91	nigel	77	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	nigel	63	<pre>
96	ph10	654	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			const unsigned char *<i>mark</i>;
109	nigel	75	</pre>
110	nigel	63	The <i>version</i> field is an integer containing the version number of the
111	ph10	654	block format. The initial version was 0; the current version is 2. The version
112	nigel	75	number will change again in future if additional fields are added, but the
113			intention is never to remove any of the existing fields.
114	nigel	63	</P>
115			<P>
116			The <i>callout_number</i> field contains the number of the callout, as compiled
117	nigel	75	into the pattern (that is, the number after ?C for manual callouts, and 255 for
118			automatically generated callouts).
119	nigel	63	</P>
120			<P>
121			The <i>offset_vector</i> field is a pointer to the vector of offsets that was
122	nigel	77	passed by the caller to <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. When
123			<b>pcre_exec()</b> is used, the contents can be inspected in order to extract
124			substrings that have been matched so far, in the same way as for extracting
125			substrings after a match has completed. For <b>pcre_dfa_exec()</b> this field is
126			not useful.
127	nigel	63	</P>
128			<P>
129	nigel	75	The <i>subject</i> and <i>subject_length</i> fields contain copies of the values
130	nigel	63	that were passed to <b>pcre_exec()</b>.
131			</P>
132			<P>
133	ph10	172	The <i>start_match</i> field normally contains the offset within the subject at
134			which the current match attempt started. However, if the escape sequence \K
135			has been encountered, this value is changed to reflect the modified starting
136			point. If the pattern is not anchored, the callout function may be called
137			several times from the same point in the pattern for different starting points
138			in the subject.
139	nigel	63	</P>
140			<P>
141			The <i>current_position</i> field contains the offset within the subject of the
142			current match pointer.
143			</P>
144			<P>
145	nigel	77	When the <b>pcre_exec()</b> function is used, the <i>capture_top</i> field
146			contains one more than the number of the highest numbered captured substring so
147			far. If no substrings have been captured, the value of <i>capture_top</i> is
148			one. This is always the case when <b>pcre_dfa_exec()</b> is used, because it
149			does not support captured substrings.
150	nigel	63	</P>
151			<P>
152			The <i>capture_last</i> field contains the number of the most recently captured
153	nigel	77	substring. If no substrings have been captured, its value is -1. This is always
154			the case when <b>pcre_dfa_exec()</b> is used.
155	nigel	63	</P>
156			<P>
157			The <i>callout_data</i> field contains a value that is passed to
158	nigel	77	<b>pcre_exec()</b> or <b>pcre_dfa_exec()</b> specifically so that it can be
159			passed back in callouts. It is passed in the <i>pcre_callout</i> field of the
160			<b>pcre_extra</b> data structure. If no such data was passed, the value of
161			<i>callout_data</i> in a <b>pcre_callout</b> block is NULL. There is a
162			description of the <b>pcre_extra</b> structure in the
163	nigel	75	<a href="pcreapi.html"><b>pcreapi</b></a>
164			documentation.
165	nigel	63	</P>
166			<P>
167	nigel	75	The <i>pattern_position</i> field is present from version 1 of the
168			<i>pcre_callout</i> structure. It contains the offset to the next item to be
169			matched in the pattern string.
170	nigel	63	</P>
171			<P>
172	nigel	75	The <i>next_item_length</i> field is present from version 1 of the
173			<i>pcre_callout</i> structure. It contains the length of the next item to be
174			matched in the pattern string. When the callout immediately precedes an
175			alternation bar, a closing parenthesis, or the end of the pattern, the length
176			is zero. When the callout precedes an opening parenthesis, the length is that
177			of the entire subpattern.
178			</P>
179			<P>
180			The <i>pattern_position</i> and <i>next_item_length</i> fields are intended to
181			help in distinguishing between different automatic callouts, which all have the
182			same callout number. However, they are set for all callouts.
183			</P>
184	ph10	654	<P>
185			The <i>mark</i> field is present from version 2 of the <i>pcre_callout</i>
186			structure. In callouts from <b>pcre_exec()</b> it contains a pointer to the
187			zero-terminated name of the most recently passed (*MARK) item in the match, or
188			NULL if there are no (*MARK)s in the current matching path. In callouts from
189			<b>pcre_dfa_exec()</b> this field always contains NULL.
190			</P>
191	nigel	75	<br><a name="SEC4" href="#TOC1">RETURN VALUES</a><br>
192			<P>
193			The external callout function returns an integer to PCRE. If the value is zero,
194			matching proceeds as normal. If the value is greater than zero, matching fails
195	nigel	77	at the current point, but the testing of other matching possibilities goes
196			ahead, just as if a lookahead assertion had failed. If the value is less than
197	ph10	461	zero, the match is abandoned, and <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>
198	nigel	77	returns the negative value.
199	nigel	75	</P>
200			<P>
201	nigel	63	Negative values should normally be chosen from the set of PCRE_ERROR_xxx
202			values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
203			The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
204			it will never be used by PCRE itself.
205			</P>
206	ph10	99	<br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
207	nigel	63	<P>
208	ph10	99	Philip Hazel
209	nigel	63	<br>
210	ph10	99	University Computing Service
211			<br>
212			Cambridge CB2 3QH, England.
213			<br>
214			</P>
215			<br><a name="SEC6" href="#TOC1">REVISION</a><br>
216			<P>
217	ph10	654	Last updated: 31 July 2011
218	ph10	99	<br>
219	ph10	654	Copyright © 1997-2011 University of Cambridge.
220	ph10	99	<br>
221	nigel	75	<p>
222			Return to the <a href="index.html">PCRE index page</a>.
223			</p>
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