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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 <br><a name="SEC2" href="#TOC1">MISSING CALLOUTS</a><br>
64 <P>
65 You should be aware that, because of optimizations in the way PCRE matches
66 patterns by default, callouts sometimes do not happen. For example, if the
67 pattern is
68 <pre>
69 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 </P>
76 <P>
77 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 You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
84 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 </P>
88 <br><a name="SEC3" href="#TOC1">THE CALLOUT INTERFACE</a><br>
89 <P>
90 During matching, when PCRE reaches a callout point, the external function
91 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 <pre>
96 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 </pre>
109 The <i>version</i> field is an integer containing the version number of the
110 block format. The initial version was 0; the current version is 1. The version
111 number will change again in future if additional fields are added, but the
112 intention is never to remove any of the existing fields.
113 </P>
114 <P>
115 The <i>callout_number</i> field contains the number of the callout, as compiled
116 into the pattern (that is, the number after ?C for manual callouts, and 255 for
117 automatically generated callouts).
118 </P>
119 <P>
120 The <i>offset_vector</i> field is a pointer to the vector of offsets that was
121 passed by the caller to <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. When
122 <b>pcre_exec()</b> is used, the contents can be inspected in order to extract
123 substrings that have been matched so far, in the same way as for extracting
124 substrings after a match has completed. For <b>pcre_dfa_exec()</b> this field is
125 not useful.
126 </P>
127 <P>
128 The <i>subject</i> and <i>subject_length</i> fields contain copies of the values
129 that were passed to <b>pcre_exec()</b>.
130 </P>
131 <P>
132 The <i>start_match</i> field normally contains the offset within the subject at
133 which the current match attempt started. However, if the escape sequence \K
134 has been encountered, this value is changed to reflect the modified starting
135 point. If the pattern is not anchored, the callout function may be called
136 several times from the same point in the pattern for different starting points
137 in the subject.
138 </P>
139 <P>
140 The <i>current_position</i> field contains the offset within the subject of the
141 current match pointer.
142 </P>
143 <P>
144 When the <b>pcre_exec()</b> function is used, the <i>capture_top</i> field
145 contains one more than the number of the highest numbered captured substring so
146 far. If no substrings have been captured, the value of <i>capture_top</i> is
147 one. This is always the case when <b>pcre_dfa_exec()</b> is used, because it
148 does not support captured substrings.
149 </P>
150 <P>
151 The <i>capture_last</i> field contains the number of the most recently captured
152 substring. If no substrings have been captured, its value is -1. This is always
153 the case when <b>pcre_dfa_exec()</b> is used.
154 </P>
155 <P>
156 The <i>callout_data</i> field contains a value that is passed to
157 <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b> specifically so that it can be
158 passed back in callouts. It is passed in the <i>pcre_callout</i> field of the
159 <b>pcre_extra</b> data structure. If no such data was passed, the value of
160 <i>callout_data</i> in a <b>pcre_callout</b> block is NULL. There is a
161 description of the <b>pcre_extra</b> structure in the
162 <a href="pcreapi.html"><b>pcreapi</b></a>
163 documentation.
164 </P>
165 <P>
166 The <i>pattern_position</i> field is present from version 1 of the
167 <i>pcre_callout</i> structure. It contains the offset to the next item to be
168 matched in the pattern string.
169 </P>
170 <P>
171 The <i>next_item_length</i> field is present from version 1 of the
172 <i>pcre_callout</i> structure. It contains the length of the next item to be
173 matched in the pattern string. When the callout immediately precedes an
174 alternation bar, a closing parenthesis, or the end of the pattern, the length
175 is zero. When the callout precedes an opening parenthesis, the length is that
176 of the entire subpattern.
177 </P>
178 <P>
179 The <i>pattern_position</i> and <i>next_item_length</i> fields are intended to
180 help in distinguishing between different automatic callouts, which all have the
181 same callout number. However, they are set for all callouts.
182 </P>
183 <br><a name="SEC4" href="#TOC1">RETURN VALUES</a><br>
184 <P>
185 The external callout function returns an integer to PCRE. If the value is zero,
186 matching proceeds as normal. If the value is greater than zero, matching fails
187 at the current point, but the testing of other matching possibilities goes
188 ahead, just as if a lookahead assertion had failed. If the value is less than
189 zero, the match is abandoned, and <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>
190 returns the negative value.
191 </P>
192 <P>
193 Negative values should normally be chosen from the set of PCRE_ERROR_xxx
194 values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
195 The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
196 it will never be used by PCRE itself.
197 </P>
198 <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
199 <P>
200 Philip Hazel
201 <br>
202 University Computing Service
203 <br>
204 Cambridge CB2 3QH, England.
205 <br>
206 </P>
207 <br><a name="SEC6" href="#TOC1">REVISION</a><br>
208 <P>
209 Last updated: 21 November 2010
210 <br>
211 Copyright &copy; 1997-2010 University of Cambridge.
212 <br>
213 <p>
214 Return to the <a href="index.html">PCRE index page</a>.
215 </p>

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