doc/html/pcrecallout.html

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<head>
<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>
<b>int (*pcre16_callout)(pcre16_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> (<i>pcre16_callout</i> for the 16-bit 
library). 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 a pattern is compiled, 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>
<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.
</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 the matching function, 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> or <i>pcre16_callout</i> is called (if it is set).
This applies to both normal and DFA matching. The only argument to the callout
function is a pointer to a <b>pcre_callout</b> or <b>pcre16_callout</b> block.
These structures contains the following fields:
<pre>
  int           <i>version</i>;
  int           <i>callout_number</i>;
  int          *<i>offset_vector</i>;
  const char   *<i>subject</i>;           (8-bit version)
  PCRE_SPTR16   <i>subject</i>;           (16-bit version) 
  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>;       (8-bit version)
  const PCRE_UCHAR16  *<i>mark</i>;       (16-bit version)
</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 the matching function. When <b>pcre_exec()</b> or 
<b>pcre16_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 the DFA matching functions, 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 the matching function.
</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> or <b>pcre16_exec()</b> 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 the DFA
functions are used, because they do 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 for the DFA matching functions.
</P>
<P>
The <i>callout_data</i> field contains a value that is passed to a matching
function specifically so that it can be passed back in callouts. It is passed
in the <i>callout_data</i> field of a <b>pcre_extra</b> or <b>pcre16_extra</b>
data structure. If no such data was passed, the value of <i>callout_data</i> in
a callout 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 callout
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 callout
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 callout structure. In
callouts from <b>pcre_exec()</b> or <b>pcre16_exec()</b> it contains a pointer to
the zero-terminated name of the most recently passed (*MARK), (*PRUNE), or
(*THEN) item in the match, or NULL if no such items have been passed. Instances
of (*PRUNE) or (*THEN) without a name do not obliterate a previous (*MARK). In
callouts from the DFA matching functions 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, the matching function 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: 08 Janurary 2012
<br>
Copyright &copy; 1997-2012 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	ph10	869	<b>int (pcre16_callout)(pcre16_callout_block );</b>
29			</P>
30			<P>
31	nigel	63	PCRE provides a feature called "callout", which is a means of temporarily
32			passing control to the caller of PCRE in the middle of pattern matching. The
33			caller of PCRE provides an external function by putting its entry point in the
34	ph10	869	global variable <i>pcre_callout</i> (<i>pcre16_callout</i> for the 16-bit
35			library). By default, this variable contains NULL, which disables all calling
36			out.
37	nigel	63	</P>
38			<P>
39			Within a regular expression, (?C) indicates the points at which the external
40			function is to be called. Different callout points can be identified by putting
41			a number less than 256 after the letter C. The default value is zero.
42			For example, this pattern has two callout points:
43	nigel	75	<pre>
44	ph10	155	(?C1)abc(?C2)def
45	nigel	75	</pre>
46	ph10	869	If the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled, PCRE
47			automatically inserts callouts, all with number 255, before each item in the
48			pattern. For example, if PCRE_AUTO_CALLOUT is used with the pattern
49	nigel	75	<pre>
50			A(\d{2}\|--)
51			</pre>
52			it is processed as if it were
53			<br>
54			<br>
55			(?C255)A(?C255)((?C255)\d{2}(?C255)\|(?C255)-(?C255)-(?C255))(?C255)
56			<br>
57			<br>
58			Notice that there is a callout before and after each parenthesis and
59			alternation bar. Automatic callouts can be used for tracking the progress of
60			pattern matching. The
61			<a href="pcretest.html"><b>pcretest</b></a>
62			command has an option that sets automatic callouts; when it is used, the output
63			indicates how the pattern is matched. This is useful information when you are
64			trying to optimize the performance of a particular pattern.
65	nigel	63	</P>
66	ph10	691	<P>
67			The use of callouts in a pattern makes it ineligible for optimization by the
68			just-in-time compiler. Studying such a pattern with the PCRE_STUDY_JIT_COMPILE
69			option always fails.
70			</P>
71	nigel	75	<br><a name="SEC2" href="#TOC1">MISSING CALLOUTS</a><br>
72	nigel	63	<P>
73	nigel	75	You should be aware that, because of optimizations in the way PCRE matches
74	ph10	392	patterns by default, callouts sometimes do not happen. For example, if the
75			pattern is
76	nigel	63	<pre>
77	nigel	75	ab(?C4)cd
78			</pre>
79			PCRE knows that any matching string must contain the letter "d". If the subject
80			string is "abyz", the lack of "d" means that matching doesn't ever start, and
81			the callout is never reached. However, with "abyd", though the result is still
82			no match, the callout is obeyed.
83	nigel	63	</P>
84	ph10	392	<P>
85	ph10	461	If the pattern is studied, PCRE knows the minimum length of a matching string,
86			and will immediately give a "no match" return without actually running a match
87			if the subject is not long enough, or, for unanchored patterns, if it has
88			been scanned far enough.
89			</P>
90			<P>
91	ph10	392	You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
92	ph10	869	option to the matching function, or by starting the pattern with
93			(*NO_START_OPT). This slows down the matching process, but does ensure that
94			callouts such as the example above are obeyed.
95	ph10	392	</P>
96	nigel	75	<br><a name="SEC3" href="#TOC1">THE CALLOUT INTERFACE</a><br>
97	nigel	63	<P>
98	nigel	75	During matching, when PCRE reaches a callout point, the external function
99	ph10	869	defined by <i>pcre_callout</i> or <i>pcre16_callout</i> is called (if it is set).
100			This applies to both normal and DFA matching. The only argument to the callout
101			function is a pointer to a <b>pcre_callout</b> or <b>pcre16_callout</b> block.
102			These structures contains the following fields:
103	nigel	63	<pre>
104	ph10	869	int <i>version</i>;
105			int <i>callout_number</i>;
106			int *<i>offset_vector</i>;
107			const char *<i>subject</i>; (8-bit version)
108			PCRE_SPTR16 <i>subject</i>; (16-bit version)
109			int <i>subject_length</i>;
110			int <i>start_match</i>;
111			int <i>current_position</i>;
112			int <i>capture_top</i>;
113			int <i>capture_last</i>;
114			void *<i>callout_data</i>;
115			int <i>pattern_position</i>;
116			int <i>next_item_length</i>;
117			const unsigned char *<i>mark</i>; (8-bit version)
118			const PCRE_UCHAR16 *<i>mark</i>; (16-bit version)
119	nigel	75	</pre>
120	nigel	63	The <i>version</i> field is an integer containing the version number of the
121	ph10	654	block format. The initial version was 0; the current version is 2. The version
122	nigel	75	number will change again in future if additional fields are added, but the
123			intention is never to remove any of the existing fields.
124	nigel	63	</P>
125			<P>
126			The <i>callout_number</i> field contains the number of the callout, as compiled
127	nigel	75	into the pattern (that is, the number after ?C for manual callouts, and 255 for
128			automatically generated callouts).
129	nigel	63	</P>
130			<P>
131			The <i>offset_vector</i> field is a pointer to the vector of offsets that was
132	ph10	869	passed by the caller to the matching function. When <b>pcre_exec()</b> or
133			<b>pcre16_exec()</b> is used, the contents can be inspected, in order to extract
134	nigel	77	substrings that have been matched so far, in the same way as for extracting
135	ph10	869	substrings after a match has completed. For the DFA matching functions, this
136			field is not useful.
137	nigel	63	</P>
138			<P>
139	nigel	75	The <i>subject</i> and <i>subject_length</i> fields contain copies of the values
140	ph10	869	that were passed to the matching function.
141	nigel	63	</P>
142			<P>
143	ph10	172	The <i>start_match</i> field normally contains the offset within the subject at
144			which the current match attempt started. However, if the escape sequence \K
145			has been encountered, this value is changed to reflect the modified starting
146			point. If the pattern is not anchored, the callout function may be called
147			several times from the same point in the pattern for different starting points
148			in the subject.
149	nigel	63	</P>
150			<P>
151			The <i>current_position</i> field contains the offset within the subject of the
152			current match pointer.
153			</P>
154			<P>
155	ph10	869	When the <b>pcre_exec()</b> or <b>pcre16_exec()</b> is used, the
156			<i>capture_top</i> field contains one more than the number of the highest
157			numbered captured substring so far. If no substrings have been captured, the
158			value of <i>capture_top</i> is one. This is always the case when the DFA
159			functions are used, because they do not support captured substrings.
160	nigel	63	</P>
161			<P>
162			The <i>capture_last</i> field contains the number of the most recently captured
163	nigel	77	substring. If no substrings have been captured, its value is -1. This is always
164	ph10	869	the case for the DFA matching functions.
165	nigel	63	</P>
166			<P>
167	ph10	869	The <i>callout_data</i> field contains a value that is passed to a matching
168			function specifically so that it can be passed back in callouts. It is passed
169			in the <i>callout_data</i> field of a <b>pcre_extra</b> or <b>pcre16_extra</b>
170			data structure. If no such data was passed, the value of <i>callout_data</i> in
171			a callout block is NULL. There is a description of the <b>pcre_extra</b>
172			structure in the
173	nigel	75	<a href="pcreapi.html"><b>pcreapi</b></a>
174			documentation.
175	nigel	63	</P>
176			<P>
177	ph10	869	The <i>pattern_position</i> field is present from version 1 of the callout
178			structure. It contains the offset to the next item to be matched in the pattern
179			string.
180	nigel	63	</P>
181			<P>
182	ph10	869	The <i>next_item_length</i> field is present from version 1 of the callout
183			structure. It contains the length of the next item to be matched in the pattern
184			string. When the callout immediately precedes an alternation bar, a closing
185			parenthesis, or the end of the pattern, the length is zero. When the callout
186			precedes an opening parenthesis, the length is that of the entire subpattern.
187	nigel	75	</P>
188			<P>
189			The <i>pattern_position</i> and <i>next_item_length</i> fields are intended to
190			help in distinguishing between different automatic callouts, which all have the
191			same callout number. However, they are set for all callouts.
192			</P>
193	ph10	654	<P>
194	ph10	869	The <i>mark</i> field is present from version 2 of the callout structure. In
195			callouts from <b>pcre_exec()</b> or <b>pcre16_exec()</b> it contains a pointer to
196			the zero-terminated name of the most recently passed (MARK), (PRUNE), or
197			(*THEN) item in the match, or NULL if no such items have been passed. Instances
198			of (PRUNE) or (THEN) without a name do not obliterate a previous (*MARK). In
199			callouts from the DFA matching functions this field always contains NULL.
200	ph10	654	</P>
201	nigel	75	<br><a name="SEC4" href="#TOC1">RETURN VALUES</a><br>
202			<P>
203			The external callout function returns an integer to PCRE. If the value is zero,
204			matching proceeds as normal. If the value is greater than zero, matching fails
205	nigel	77	at the current point, but the testing of other matching possibilities goes
206			ahead, just as if a lookahead assertion had failed. If the value is less than
207	ph10	869	zero, the match is abandoned, the matching function returns the negative value.
208	nigel	75	</P>
209			<P>
210	nigel	63	Negative values should normally be chosen from the set of PCRE_ERROR_xxx
211			values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
212			The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
213			it will never be used by PCRE itself.
214			</P>
215	ph10	99	<br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
216	nigel	63	<P>
217	ph10	99	Philip Hazel
218	nigel	63	<br>
219	ph10	99	University Computing Service
220			<br>
221			Cambridge CB2 3QH, England.
222			<br>
223			</P>
224			<br><a name="SEC6" href="#TOC1">REVISION</a><br>
225			<P>
226	ph10	869	Last updated: 08 Janurary 2012
227	ph10	99	<br>
228	ph10	869	Copyright © 1997-2012 University of Cambridge.
229	ph10	99	<br>
230	nigel	75	<p>
231			Return to the <a href="index.html">PCRE index page</a>.
232			</p>
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