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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">SYNOPSIS</a>
17 <li><a name="TOC2" href="#SEC2">DESCRIPTION</a>
18 <li><a name="TOC3" href="#SEC3">MISSING CALLOUTS</a>
19 <li><a name="TOC4" href="#SEC4">THE CALLOUT INTERFACE</a>
20 <li><a name="TOC5" href="#SEC5">RETURN VALUES</a>
21 <li><a name="TOC6" href="#SEC6">AUTHOR</a>
22 <li><a name="TOC7" href="#SEC7">REVISION</a>
23 </ul>
24 <br><a name="SEC1" href="#TOC1">SYNOPSIS</a><br>
25 <P>
26 <b>#include &#60;pcre.h&#62;</b>
27 </P>
28 <P>
29 <b>int (*pcre_callout)(pcre_callout_block *);</b>
30 </P>
31 <P>
32 <b>int (*pcre16_callout)(pcre16_callout_block *);</b>
33 </P>
34 <P>
35 <b>int (*pcre32_callout)(pcre32_callout_block *);</b>
36 </P>
37 <br><a name="SEC2" href="#TOC1">DESCRIPTION</a><br>
38 <P>
39 PCRE provides a feature called "callout", which is a means of temporarily
40 passing control to the caller of PCRE in the middle of pattern matching. The
41 caller of PCRE provides an external function by putting its entry point in the
42 global variable <i>pcre_callout</i> (<i>pcre16_callout</i> for the 16-bit
43 library, <i>pcre32_callout</i> for the 32-bit library). By default, this
44 variable contains NULL, which disables all calling out.
45 </P>
46 <P>
47 Within a regular expression, (?C) indicates the points at which the external
48 function is to be called. Different callout points can be identified by putting
49 a number less than 256 after the letter C. The default value is zero.
50 For example, this pattern has two callout points:
51 <pre>
52 (?C1)abc(?C2)def
53 </pre>
54 If the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled, PCRE
55 automatically inserts callouts, all with number 255, before each item in the
56 pattern. For example, if PCRE_AUTO_CALLOUT is used with the pattern
57 <pre>
58 A(\d{2}|--)
59 </pre>
60 it is processed as if it were
61 <br>
62 <br>
63 (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
64 <br>
65 <br>
66 Notice that there is a callout before and after each parenthesis and
67 alternation bar. If the pattern contains a conditional group whose condition is
68 an assertion, an automatic callout is inserted immediately before the
69 condition. Such a callout may also be inserted explicitly, for example:
70 <pre>
71 (?(?C9)(?=a)ab|de)
72 </pre>
73 This applies only to assertion conditions (because they are themselves
74 independent groups).
75 </P>
76 <P>
77 Automatic callouts can be used for tracking the progress of pattern matching.
78 The
79 <a href="pcretest.html"><b>pcretest</b></a>
80 command has an option that sets automatic callouts; when it is used, the output
81 indicates how the pattern is matched. This is useful information when you are
82 trying to optimize the performance of a particular pattern.
83 </P>
84 <br><a name="SEC3" href="#TOC1">MISSING CALLOUTS</a><br>
85 <P>
86 You should be aware that, because of optimizations in the way PCRE matches
87 patterns by default, callouts sometimes do not happen. For example, if the
88 pattern is
89 <pre>
90 ab(?C4)cd
91 </pre>
92 PCRE knows that any matching string must contain the letter "d". If the subject
93 string is "abyz", the lack of "d" means that matching doesn't ever start, and
94 the callout is never reached. However, with "abyd", though the result is still
95 no match, the callout is obeyed.
96 </P>
97 <P>
98 If the pattern is studied, PCRE knows the minimum length of a matching string,
99 and will immediately give a "no match" return without actually running a match
100 if the subject is not long enough, or, for unanchored patterns, if it has
101 been scanned far enough.
102 </P>
103 <P>
104 You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
105 option to the matching function, or by starting the pattern with
106 (*NO_START_OPT). This slows down the matching process, but does ensure that
107 callouts such as the example above are obeyed.
108 </P>
109 <br><a name="SEC4" href="#TOC1">THE CALLOUT INTERFACE</a><br>
110 <P>
111 During matching, when PCRE reaches a callout point, the external function
112 defined by <i>pcre_callout</i> or <i>pcre[16|32]_callout</i> is called
113 (if it is set). This applies to both normal and DFA matching. The only
114 argument to the callout function is a pointer to a <b>pcre_callout</b>
115 or <b>pcre[16|32]_callout</b> block.
116 These structures contains the following fields:
117 <pre>
118 int <i>version</i>;
119 int <i>callout_number</i>;
120 int *<i>offset_vector</i>;
121 const char *<i>subject</i>; (8-bit version)
122 PCRE_SPTR16 <i>subject</i>; (16-bit version)
123 PCRE_SPTR32 <i>subject</i>; (32-bit version)
124 int <i>subject_length</i>;
125 int <i>start_match</i>;
126 int <i>current_position</i>;
127 int <i>capture_top</i>;
128 int <i>capture_last</i>;
129 void *<i>callout_data</i>;
130 int <i>pattern_position</i>;
131 int <i>next_item_length</i>;
132 const unsigned char *<i>mark</i>; (8-bit version)
133 const PCRE_UCHAR16 *<i>mark</i>; (16-bit version)
134 const PCRE_UCHAR32 *<i>mark</i>; (32-bit version)
135 </pre>
136 The <i>version</i> field is an integer containing the version number of the
137 block format. The initial version was 0; the current version is 2. The version
138 number will change again in future if additional fields are added, but the
139 intention is never to remove any of the existing fields.
140 </P>
141 <P>
142 The <i>callout_number</i> field contains the number of the callout, as compiled
143 into the pattern (that is, the number after ?C for manual callouts, and 255 for
144 automatically generated callouts).
145 </P>
146 <P>
147 The <i>offset_vector</i> field is a pointer to the vector of offsets that was
148 passed by the caller to the matching function. When <b>pcre_exec()</b> or
149 <b>pcre[16|32]_exec()</b> is used, the contents can be inspected, in order to
150 extract substrings that have been matched so far, in the same way as for
151 extracting substrings after a match has completed. For the DFA matching
152 functions, this field is not useful.
153 </P>
154 <P>
155 The <i>subject</i> and <i>subject_length</i> fields contain copies of the values
156 that were passed to the matching function.
157 </P>
158 <P>
159 The <i>start_match</i> field normally contains the offset within the subject at
160 which the current match attempt started. However, if the escape sequence \K
161 has been encountered, this value is changed to reflect the modified starting
162 point. If the pattern is not anchored, the callout function may be called
163 several times from the same point in the pattern for different starting points
164 in the subject.
165 </P>
166 <P>
167 The <i>current_position</i> field contains the offset within the subject of the
168 current match pointer.
169 </P>
170 <P>
171 When the <b>pcre_exec()</b> or <b>pcre[16|32]_exec()</b> is used, the
172 <i>capture_top</i> field contains one more than the number of the highest
173 numbered captured substring so far. If no substrings have been captured, the
174 value of <i>capture_top</i> is one. This is always the case when the DFA
175 functions are used, because they do not support captured substrings.
176 </P>
177 <P>
178 The <i>capture_last</i> field contains the number of the most recently captured
179 substring. However, when a recursion exits, the value reverts to what it was
180 outside the recursion, as do the values of all captured substrings. If no
181 substrings have been captured, the value of <i>capture_last</i> is -1. This is
182 always the case for the DFA matching functions.
183 </P>
184 <P>
185 The <i>callout_data</i> field contains a value that is passed to a matching
186 function specifically so that it can be passed back in callouts. It is passed
187 in the <i>callout_data</i> field of a <b>pcre_extra</b> or <b>pcre[16|32]_extra</b>
188 data structure. If no such data was passed, the value of <i>callout_data</i> in
189 a callout block is NULL. There is a description of the <b>pcre_extra</b>
190 structure in the
191 <a href="pcreapi.html"><b>pcreapi</b></a>
192 documentation.
193 </P>
194 <P>
195 The <i>pattern_position</i> field is present from version 1 of the callout
196 structure. It contains the offset to the next item to be matched in the pattern
197 string.
198 </P>
199 <P>
200 The <i>next_item_length</i> field is present from version 1 of the callout
201 structure. It contains the length of the next item to be matched in the pattern
202 string. When the callout immediately precedes an alternation bar, a closing
203 parenthesis, or the end of the pattern, the length is zero. When the callout
204 precedes an opening parenthesis, the length is that of the entire subpattern.
205 </P>
206 <P>
207 The <i>pattern_position</i> and <i>next_item_length</i> fields are intended to
208 help in distinguishing between different automatic callouts, which all have the
209 same callout number. However, they are set for all callouts.
210 </P>
211 <P>
212 The <i>mark</i> field is present from version 2 of the callout structure. In
213 callouts from <b>pcre_exec()</b> or <b>pcre[16|32]_exec()</b> it contains a
214 pointer to the zero-terminated name of the most recently passed (*MARK),
215 (*PRUNE), or (*THEN) item in the match, or NULL if no such items have been
216 passed. Instances of (*PRUNE) or (*THEN) without a name do not obliterate a
217 previous (*MARK). In callouts from the DFA matching functions this field always
218 contains NULL.
219 </P>
220 <br><a name="SEC5" href="#TOC1">RETURN VALUES</a><br>
221 <P>
222 The external callout function returns an integer to PCRE. If the value is zero,
223 matching proceeds as normal. If the value is greater than zero, matching fails
224 at the current point, but the testing of other matching possibilities goes
225 ahead, just as if a lookahead assertion had failed. If the value is less than
226 zero, the match is abandoned, the matching function returns the negative value.
227 </P>
228 <P>
229 Negative values should normally be chosen from the set of PCRE_ERROR_xxx
230 values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
231 The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
232 it will never be used by PCRE itself.
233 </P>
234 <br><a name="SEC6" href="#TOC1">AUTHOR</a><br>
235 <P>
236 Philip Hazel
237 <br>
238 University Computing Service
239 <br>
240 Cambridge CB2 3QH, England.
241 <br>
242 </P>
243 <br><a name="SEC7" href="#TOC1">REVISION</a><br>
244 <P>
245 Last updated: 03 March 2013
246 <br>
247 Copyright &copy; 1997-2013 University of Cambridge.
248 <br>
249 <p>
250 Return to the <a href="index.html">PCRE index page</a>.
251 </p>

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