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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> is called,
43 PCRE automatically inserts callouts, all with number 255, before each item in
44 the pattern. For example, if PCRE_AUTO_CALLOUT is used with the pattern
45 <pre>
46 A(\d{2}|--)
47 </pre>
48 it is processed as if it were
49 <br>
50 <br>
51 (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
52 <br>
53 <br>
54 Notice that there is a callout before and after each parenthesis and
55 alternation bar. Automatic callouts can be used for tracking the progress of
56 pattern matching. The
57 <a href="pcretest.html"><b>pcretest</b></a>
58 command has an option that sets automatic callouts; when it is used, the output
59 indicates how the pattern is matched. This is useful information when you are
60 trying to optimize the performance of a particular pattern.
61 </P>
62 <br><a name="SEC2" href="#TOC1">MISSING CALLOUTS</a><br>
63 <P>
64 You should be aware that, because of optimizations in the way PCRE matches
65 patterns by default, callouts sometimes do not happen. For example, if the
66 pattern is
67 <pre>
68 ab(?C4)cd
69 </pre>
70 PCRE knows that any matching string must contain the letter "d". If the subject
71 string is "abyz", the lack of "d" means that matching doesn't ever start, and
72 the callout is never reached. However, with "abyd", though the result is still
73 no match, the callout is obeyed.
74 </P>
75 <P>
76 You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
77 option to <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. This slows down the
78 matching process, but does ensure that callouts such as the example above are
79 obeyed.
80 </P>
81 <br><a name="SEC3" href="#TOC1">THE CALLOUT INTERFACE</a><br>
82 <P>
83 During matching, when PCRE reaches a callout point, the external function
84 defined by <i>pcre_callout</i> is called (if it is set). This applies to both
85 the <b>pcre_exec()</b> and the <b>pcre_dfa_exec()</b> matching functions. The
86 only argument to the callout function is a pointer to a <b>pcre_callout</b>
87 block. This structure contains the following fields:
88 <pre>
89 int <i>version</i>;
90 int <i>callout_number</i>;
91 int *<i>offset_vector</i>;
92 const char *<i>subject</i>;
93 int <i>subject_length</i>;
94 int <i>start_match</i>;
95 int <i>current_position</i>;
96 int <i>capture_top</i>;
97 int <i>capture_last</i>;
98 void *<i>callout_data</i>;
99 int <i>pattern_position</i>;
100 int <i>next_item_length</i>;
101 </pre>
102 The <i>version</i> field is an integer containing the version number of the
103 block format. The initial version was 0; the current version is 1. The version
104 number will change again in future if additional fields are added, but the
105 intention is never to remove any of the existing fields.
106 </P>
107 <P>
108 The <i>callout_number</i> field contains the number of the callout, as compiled
109 into the pattern (that is, the number after ?C for manual callouts, and 255 for
110 automatically generated callouts).
111 </P>
112 <P>
113 The <i>offset_vector</i> field is a pointer to the vector of offsets that was
114 passed by the caller to <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. When
115 <b>pcre_exec()</b> is used, the contents can be inspected in order to extract
116 substrings that have been matched so far, in the same way as for extracting
117 substrings after a match has completed. For <b>pcre_dfa_exec()</b> this field is
118 not useful.
119 </P>
120 <P>
121 The <i>subject</i> and <i>subject_length</i> fields contain copies of the values
122 that were passed to <b>pcre_exec()</b>.
123 </P>
124 <P>
125 The <i>start_match</i> field normally contains the offset within the subject at
126 which the current match attempt started. However, if the escape sequence \K
127 has been encountered, this value is changed to reflect the modified starting
128 point. If the pattern is not anchored, the callout function may be called
129 several times from the same point in the pattern for different starting points
130 in the subject.
131 </P>
132 <P>
133 The <i>current_position</i> field contains the offset within the subject of the
134 current match pointer.
135 </P>
136 <P>
137 When the <b>pcre_exec()</b> function is used, the <i>capture_top</i> field
138 contains one more than the number of the highest numbered captured substring so
139 far. If no substrings have been captured, the value of <i>capture_top</i> is
140 one. This is always the case when <b>pcre_dfa_exec()</b> is used, because it
141 does not support captured substrings.
142 </P>
143 <P>
144 The <i>capture_last</i> field contains the number of the most recently captured
145 substring. If no substrings have been captured, its value is -1. This is always
146 the case when <b>pcre_dfa_exec()</b> is used.
147 </P>
148 <P>
149 The <i>callout_data</i> field contains a value that is passed to
150 <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b> specifically so that it can be
151 passed back in callouts. It is passed in the <i>pcre_callout</i> field of the
152 <b>pcre_extra</b> data structure. If no such data was passed, the value of
153 <i>callout_data</i> in a <b>pcre_callout</b> block is NULL. There is a
154 description of the <b>pcre_extra</b> structure in the
155 <a href="pcreapi.html"><b>pcreapi</b></a>
156 documentation.
157 </P>
158 <P>
159 The <i>pattern_position</i> field is present from version 1 of the
160 <i>pcre_callout</i> structure. It contains the offset to the next item to be
161 matched in the pattern string.
162 </P>
163 <P>
164 The <i>next_item_length</i> field is present from version 1 of the
165 <i>pcre_callout</i> structure. It contains the length of the next item to be
166 matched in the pattern string. When the callout immediately precedes an
167 alternation bar, a closing parenthesis, or the end of the pattern, the length
168 is zero. When the callout precedes an opening parenthesis, the length is that
169 of the entire subpattern.
170 </P>
171 <P>
172 The <i>pattern_position</i> and <i>next_item_length</i> fields are intended to
173 help in distinguishing between different automatic callouts, which all have the
174 same callout number. However, they are set for all callouts.
175 </P>
176 <br><a name="SEC4" href="#TOC1">RETURN VALUES</a><br>
177 <P>
178 The external callout function returns an integer to PCRE. If the value is zero,
179 matching proceeds as normal. If the value is greater than zero, matching fails
180 at the current point, but the testing of other matching possibilities goes
181 ahead, just as if a lookahead assertion had failed. If the value is less than
182 zero, the match is abandoned, and <b>pcre_exec()</b> (or <b>pcre_dfa_exec()</b>)
183 returns the negative value.
184 </P>
185 <P>
186 Negative values should normally be chosen from the set of PCRE_ERROR_xxx
187 values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
188 The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
189 it will never be used by PCRE itself.
190 </P>
191 <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
192 <P>
193 Philip Hazel
194 <br>
195 University Computing Service
196 <br>
197 Cambridge CB2 3QH, England.
198 <br>
199 </P>
200 <br><a name="SEC6" href="#TOC1">REVISION</a><br>
201 <P>
202 Last updated: 15 March 2009
203 <br>
204 Copyright &copy; 1997-2009 University of Cambridge.
205 <br>
206 <p>
207 Return to the <a href="index.html">PCRE index page</a>.
208 </p>

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