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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">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 <P>
64 The use of callouts in a pattern makes it ineligible for optimization by the
65 just-in-time compiler. Studying such a pattern with the PCRE_STUDY_JIT_COMPILE
66 option always fails.
67 </P>
68 <br><a name="SEC2" href="#TOC1">MISSING CALLOUTS</a><br>
69 <P>
70 You should be aware that, because of optimizations in the way PCRE matches
71 patterns by default, callouts sometimes do not happen. For example, if the
72 pattern is
73 <pre>
74 ab(?C4)cd
75 </pre>
76 PCRE knows that any matching string must contain the letter "d". If the subject
77 string is "abyz", the lack of "d" means that matching doesn't ever start, and
78 the callout is never reached. However, with "abyd", though the result is still
79 no match, the callout is obeyed.
80 </P>
81 <P>
82 If the pattern is studied, PCRE knows the minimum length of a matching string,
83 and will immediately give a "no match" return without actually running a match
84 if the subject is not long enough, or, for unanchored patterns, if it has
85 been scanned far enough.
86 </P>
87 <P>
88 You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
89 option to <b>pcre_compile()</b>, <b>pcre_exec()</b>, or <b>pcre_dfa_exec()</b>,
90 or by starting the pattern with (*NO_START_OPT). This slows down the matching
91 process, but does ensure that callouts such as the example above are obeyed.
92 </P>
93 <br><a name="SEC3" href="#TOC1">THE CALLOUT INTERFACE</a><br>
94 <P>
95 During matching, when PCRE reaches a callout point, the external function
96 defined by <i>pcre_callout</i> is called (if it is set). This applies to both
97 the <b>pcre_exec()</b> and the <b>pcre_dfa_exec()</b> matching functions. The
98 only argument to the callout function is a pointer to a <b>pcre_callout</b>
99 block. This structure contains the following fields:
100 <pre>
101 int <i>version</i>;
102 int <i>callout_number</i>;
103 int *<i>offset_vector</i>;
104 const char *<i>subject</i>;
105 int <i>subject_length</i>;
106 int <i>start_match</i>;
107 int <i>current_position</i>;
108 int <i>capture_top</i>;
109 int <i>capture_last</i>;
110 void *<i>callout_data</i>;
111 int <i>pattern_position</i>;
112 int <i>next_item_length</i>;
113 const unsigned char *<i>mark</i>;
114 </pre>
115 The <i>version</i> field is an integer containing the version number of the
116 block format. The initial version was 0; the current version is 2. The version
117 number will change again in future if additional fields are added, but the
118 intention is never to remove any of the existing fields.
119 </P>
120 <P>
121 The <i>callout_number</i> field contains the number of the callout, as compiled
122 into the pattern (that is, the number after ?C for manual callouts, and 255 for
123 automatically generated callouts).
124 </P>
125 <P>
126 The <i>offset_vector</i> field is a pointer to the vector of offsets that was
127 passed by the caller to <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. When
128 <b>pcre_exec()</b> is used, the contents can be inspected in order to extract
129 substrings that have been matched so far, in the same way as for extracting
130 substrings after a match has completed. For <b>pcre_dfa_exec()</b> this field is
131 not useful.
132 </P>
133 <P>
134 The <i>subject</i> and <i>subject_length</i> fields contain copies of the values
135 that were passed to <b>pcre_exec()</b>.
136 </P>
137 <P>
138 The <i>start_match</i> field normally contains the offset within the subject at
139 which the current match attempt started. However, if the escape sequence \K
140 has been encountered, this value is changed to reflect the modified starting
141 point. If the pattern is not anchored, the callout function may be called
142 several times from the same point in the pattern for different starting points
143 in the subject.
144 </P>
145 <P>
146 The <i>current_position</i> field contains the offset within the subject of the
147 current match pointer.
148 </P>
149 <P>
150 When the <b>pcre_exec()</b> function is used, the <i>capture_top</i> field
151 contains one more than the number of the highest numbered captured substring so
152 far. If no substrings have been captured, the value of <i>capture_top</i> is
153 one. This is always the case when <b>pcre_dfa_exec()</b> is used, because it
154 does not support captured substrings.
155 </P>
156 <P>
157 The <i>capture_last</i> field contains the number of the most recently captured
158 substring. If no substrings have been captured, its value is -1. This is always
159 the case when <b>pcre_dfa_exec()</b> is used.
160 </P>
161 <P>
162 The <i>callout_data</i> field contains a value that is passed to
163 <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b> specifically so that it can be
164 passed back in callouts. It is passed in the <i>pcre_callout</i> field of the
165 <b>pcre_extra</b> data structure. If no such data was passed, the value of
166 <i>callout_data</i> in a <b>pcre_callout</b> block is NULL. There is a
167 description of the <b>pcre_extra</b> structure in the
168 <a href="pcreapi.html"><b>pcreapi</b></a>
169 documentation.
170 </P>
171 <P>
172 The <i>pattern_position</i> field is present from version 1 of the
173 <i>pcre_callout</i> structure. It contains the offset to the next item to be
174 matched in the pattern string.
175 </P>
176 <P>
177 The <i>next_item_length</i> field is present from version 1 of the
178 <i>pcre_callout</i> structure. It contains the length of the next item to be
179 matched in the pattern string. When the callout immediately precedes an
180 alternation bar, a closing parenthesis, or the end of the pattern, the length
181 is zero. When the callout precedes an opening parenthesis, the length is that
182 of the entire subpattern.
183 </P>
184 <P>
185 The <i>pattern_position</i> and <i>next_item_length</i> fields are intended to
186 help in distinguishing between different automatic callouts, which all have the
187 same callout number. However, they are set for all callouts.
188 </P>
189 <P>
190 The <i>mark</i> field is present from version 2 of the <i>pcre_callout</i>
191 structure. In callouts from <b>pcre_exec()</b> it contains a pointer to the
192 zero-terminated name of the most recently passed (*MARK), (*PRUNE), or (*THEN)
193 item in the match, or NULL if no such items have been passed. Instances of
194 (*PRUNE) or (*THEN) without a name do not obliterate a previous (*MARK). In
195 callouts from <b>pcre_dfa_exec()</b> this field always contains NULL.
196 </P>
197 <br><a name="SEC4" href="#TOC1">RETURN VALUES</a><br>
198 <P>
199 The external callout function returns an integer to PCRE. If the value is zero,
200 matching proceeds as normal. If the value is greater than zero, matching fails
201 at the current point, but the testing of other matching possibilities goes
202 ahead, just as if a lookahead assertion had failed. If the value is less than
203 zero, the match is abandoned, and <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>
204 returns the negative value.
205 </P>
206 <P>
207 Negative values should normally be chosen from the set of PCRE_ERROR_xxx
208 values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
209 The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
210 it will never be used by PCRE itself.
211 </P>
212 <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
213 <P>
214 Philip Hazel
215 <br>
216 University Computing Service
217 <br>
218 Cambridge CB2 3QH, England.
219 <br>
220 </P>
221 <br><a name="SEC6" href="#TOC1">REVISION</a><br>
222 <P>
223 Last updated: 30 November 2011
224 <br>
225 Copyright &copy; 1997-2011 University of Cambridge.
226 <br>
227 <p>
228 Return to the <a href="index.html">PCRE index page</a>.
229 </p>

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