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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 program has a pattern qualifier (/C) that sets automatic callouts; when it is
81 used, the output indicates how the pattern is being matched. This is useful
82 information when you are trying to optimize the performance of a particular
83 pattern.
84 </P>
85 <br><a name="SEC3" href="#TOC1">MISSING CALLOUTS</a><br>
86 <P>
87 You should be aware that, because of optimizations in the way PCRE compiles and
88 matches patterns, callouts sometimes do not happen exactly as you might expect.
89 </P>
90 <P>
91 At compile time, PCRE "auto-possessifies" repeated items when it knows that
92 what follows cannot be part of the repeat. For example, a+[bc] is compiled as
93 if it were a++[bc]. The <b>pcretest</b> output when this pattern is anchored and
94 then applied with automatic callouts to the string "aaaa" is:
95 <pre>
96 ---&#62;aaaa
97 +0 ^ ^
98 +1 ^ a+
99 +3 ^ ^ [bc]
100 No match
101 </pre>
102 This indicates that when matching [bc] fails, there is no backtracking into a+
103 and therefore the callouts that would be taken for the backtracks do not occur.
104 You can disable the auto-possessify feature by passing PCRE_NO_AUTO_POSSESS
105 to <b>pcre_compile()</b>, or starting the pattern with (*NO_AUTO_POSSESS). If
106 this is done in <b>pcretest</b> (using the /O qualifier), the output changes to
107 this:
108 <pre>
109 ---&#62;aaaa
110 +0 ^ ^
111 +1 ^ a+
112 +3 ^ ^ [bc]
113 +3 ^ ^ [bc]
114 +3 ^ ^ [bc]
115 +3 ^^ [bc]
116 No match
117 </pre>
118 This time, when matching [bc] fails, the matcher backtracks into a+ and tries
119 again, repeatedly, until a+ itself fails.
120 </P>
121 <P>
122 Other optimizations that provide fast "no match" results also affect callouts.
123 For example, if the pattern is
124 <pre>
125 ab(?C4)cd
126 </pre>
127 PCRE knows that any matching string must contain the letter "d". If the subject
128 string is "abyz", the lack of "d" means that matching doesn't ever start, and
129 the callout is never reached. However, with "abyd", though the result is still
130 no match, the callout is obeyed.
131 </P>
132 <P>
133 If the pattern is studied, PCRE knows the minimum length of a matching string,
134 and will immediately give a "no match" return without actually running a match
135 if the subject is not long enough, or, for unanchored patterns, if it has
136 been scanned far enough.
137 </P>
138 <P>
139 You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
140 option to the matching function, or by starting the pattern with
141 (*NO_START_OPT). This slows down the matching process, but does ensure that
142 callouts such as the example above are obeyed.
143 </P>
144 <br><a name="SEC4" href="#TOC1">THE CALLOUT INTERFACE</a><br>
145 <P>
146 During matching, when PCRE reaches a callout point, the external function
147 defined by <i>pcre_callout</i> or <i>pcre[16|32]_callout</i> is called (if it is
148 set). This applies to both normal and DFA matching. The only argument to the
149 callout function is a pointer to a <b>pcre_callout</b> or
150 <b>pcre[16|32]_callout</b> block. These structures contains the following
151 fields:
152 <pre>
153 int <i>version</i>;
154 int <i>callout_number</i>;
155 int *<i>offset_vector</i>;
156 const char *<i>subject</i>; (8-bit version)
157 PCRE_SPTR16 <i>subject</i>; (16-bit version)
158 PCRE_SPTR32 <i>subject</i>; (32-bit version)
159 int <i>subject_length</i>;
160 int <i>start_match</i>;
161 int <i>current_position</i>;
162 int <i>capture_top</i>;
163 int <i>capture_last</i>;
164 void *<i>callout_data</i>;
165 int <i>pattern_position</i>;
166 int <i>next_item_length</i>;
167 const unsigned char *<i>mark</i>; (8-bit version)
168 const PCRE_UCHAR16 *<i>mark</i>; (16-bit version)
169 const PCRE_UCHAR32 *<i>mark</i>; (32-bit version)
170 </pre>
171 The <i>version</i> field is an integer containing the version number of the
172 block format. The initial version was 0; the current version is 2. The version
173 number will change again in future if additional fields are added, but the
174 intention is never to remove any of the existing fields.
175 </P>
176 <P>
177 The <i>callout_number</i> field contains the number of the callout, as compiled
178 into the pattern (that is, the number after ?C for manual callouts, and 255 for
179 automatically generated callouts).
180 </P>
181 <P>
182 The <i>offset_vector</i> field is a pointer to the vector of offsets that was
183 passed by the caller to the matching function. When <b>pcre_exec()</b> or
184 <b>pcre[16|32]_exec()</b> is used, the contents can be inspected, in order to
185 extract substrings that have been matched so far, in the same way as for
186 extracting substrings after a match has completed. For the DFA matching
187 functions, this field is not useful.
188 </P>
189 <P>
190 The <i>subject</i> and <i>subject_length</i> fields contain copies of the values
191 that were passed to the matching function.
192 </P>
193 <P>
194 The <i>start_match</i> field normally contains the offset within the subject at
195 which the current match attempt started. However, if the escape sequence \K
196 has been encountered, this value is changed to reflect the modified starting
197 point. If the pattern is not anchored, the callout function may be called
198 several times from the same point in the pattern for different starting points
199 in the subject.
200 </P>
201 <P>
202 The <i>current_position</i> field contains the offset within the subject of the
203 current match pointer.
204 </P>
205 <P>
206 When the <b>pcre_exec()</b> or <b>pcre[16|32]_exec()</b> is used, the
207 <i>capture_top</i> field contains one more than the number of the highest
208 numbered captured substring so far. If no substrings have been captured, the
209 value of <i>capture_top</i> is one. This is always the case when the DFA
210 functions are used, because they do not support captured substrings.
211 </P>
212 <P>
213 The <i>capture_last</i> field contains the number of the most recently captured
214 substring. However, when a recursion exits, the value reverts to what it was
215 outside the recursion, as do the values of all captured substrings. If no
216 substrings have been captured, the value of <i>capture_last</i> is -1. This is
217 always the case for the DFA matching functions.
218 </P>
219 <P>
220 The <i>callout_data</i> field contains a value that is passed to a matching
221 function specifically so that it can be passed back in callouts. It is passed
222 in the <i>callout_data</i> field of a <b>pcre_extra</b> or <b>pcre[16|32]_extra</b>
223 data structure. If no such data was passed, the value of <i>callout_data</i> in
224 a callout block is NULL. There is a description of the <b>pcre_extra</b>
225 structure in the
226 <a href="pcreapi.html"><b>pcreapi</b></a>
227 documentation.
228 </P>
229 <P>
230 The <i>pattern_position</i> field is present from version 1 of the callout
231 structure. It contains the offset to the next item to be matched in the pattern
232 string.
233 </P>
234 <P>
235 The <i>next_item_length</i> field is present from version 1 of the callout
236 structure. It contains the length of the next item to be matched in the pattern
237 string. When the callout immediately precedes an alternation bar, a closing
238 parenthesis, or the end of the pattern, the length is zero. When the callout
239 precedes an opening parenthesis, the length is that of the entire subpattern.
240 </P>
241 <P>
242 The <i>pattern_position</i> and <i>next_item_length</i> fields are intended to
243 help in distinguishing between different automatic callouts, which all have the
244 same callout number. However, they are set for all callouts.
245 </P>
246 <P>
247 The <i>mark</i> field is present from version 2 of the callout structure. In
248 callouts from <b>pcre_exec()</b> or <b>pcre[16|32]_exec()</b> it contains a
249 pointer to the zero-terminated name of the most recently passed (*MARK),
250 (*PRUNE), or (*THEN) item in the match, or NULL if no such items have been
251 passed. Instances of (*PRUNE) or (*THEN) without a name do not obliterate a
252 previous (*MARK). In callouts from the DFA matching functions this field always
253 contains NULL.
254 </P>
255 <br><a name="SEC5" href="#TOC1">RETURN VALUES</a><br>
256 <P>
257 The external callout function returns an integer to PCRE. If the value is zero,
258 matching proceeds as normal. If the value is greater than zero, matching fails
259 at the current point, but the testing of other matching possibilities goes
260 ahead, just as if a lookahead assertion had failed. If the value is less than
261 zero, the match is abandoned, the matching function returns the negative value.
262 </P>
263 <P>
264 Negative values should normally be chosen from the set of PCRE_ERROR_xxx
265 values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
266 The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
267 it will never be used by PCRE itself.
268 </P>
269 <br><a name="SEC6" href="#TOC1">AUTHOR</a><br>
270 <P>
271 Philip Hazel
272 <br>
273 University Computing Service
274 <br>
275 Cambridge CB2 3QH, England.
276 <br>
277 </P>
278 <br><a name="SEC7" href="#TOC1">REVISION</a><br>
279 <P>
280 Last updated: 12 November 2013
281 <br>
282 Copyright &copy; 1997-2013 University of Cambridge.
283 <br>
284 <p>
285 Return to the <a href="index.html">PCRE index page</a>.
286 </p>

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