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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 const unsigned char *<i>mark</i>;
109 </pre>
110 The <i>version</i> field is an integer containing the version number of the
111 block format. The initial version was 0; the current version is 2. The version
112 number will change again in future if additional fields are added, but the
113 intention is never to remove any of the existing fields.
114 </P>
115 <P>
116 The <i>callout_number</i> field contains the number of the callout, as compiled
117 into the pattern (that is, the number after ?C for manual callouts, and 255 for
118 automatically generated callouts).
119 </P>
120 <P>
121 The <i>offset_vector</i> field is a pointer to the vector of offsets that was
122 passed by the caller to <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. When
123 <b>pcre_exec()</b> is used, the contents can be inspected in order to extract
124 substrings that have been matched so far, in the same way as for extracting
125 substrings after a match has completed. For <b>pcre_dfa_exec()</b> this field is
126 not useful.
127 </P>
128 <P>
129 The <i>subject</i> and <i>subject_length</i> fields contain copies of the values
130 that were passed to <b>pcre_exec()</b>.
131 </P>
132 <P>
133 The <i>start_match</i> field normally contains the offset within the subject at
134 which the current match attempt started. However, if the escape sequence \K
135 has been encountered, this value is changed to reflect the modified starting
136 point. If the pattern is not anchored, the callout function may be called
137 several times from the same point in the pattern for different starting points
138 in the subject.
139 </P>
140 <P>
141 The <i>current_position</i> field contains the offset within the subject of the
142 current match pointer.
143 </P>
144 <P>
145 When the <b>pcre_exec()</b> function is used, the <i>capture_top</i> field
146 contains one more than the number of the highest numbered captured substring so
147 far. If no substrings have been captured, the value of <i>capture_top</i> is
148 one. This is always the case when <b>pcre_dfa_exec()</b> is used, because it
149 does not support captured substrings.
150 </P>
151 <P>
152 The <i>capture_last</i> field contains the number of the most recently captured
153 substring. If no substrings have been captured, its value is -1. This is always
154 the case when <b>pcre_dfa_exec()</b> is used.
155 </P>
156 <P>
157 The <i>callout_data</i> field contains a value that is passed to
158 <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b> specifically so that it can be
159 passed back in callouts. It is passed in the <i>pcre_callout</i> field of the
160 <b>pcre_extra</b> data structure. If no such data was passed, the value of
161 <i>callout_data</i> in a <b>pcre_callout</b> block is NULL. There is a
162 description of the <b>pcre_extra</b> structure in the
163 <a href="pcreapi.html"><b>pcreapi</b></a>
164 documentation.
165 </P>
166 <P>
167 The <i>pattern_position</i> field is present from version 1 of the
168 <i>pcre_callout</i> structure. It contains the offset to the next item to be
169 matched in the pattern string.
170 </P>
171 <P>
172 The <i>next_item_length</i> field is present from version 1 of the
173 <i>pcre_callout</i> structure. It contains the length of the next item to be
174 matched in the pattern string. When the callout immediately precedes an
175 alternation bar, a closing parenthesis, or the end of the pattern, the length
176 is zero. When the callout precedes an opening parenthesis, the length is that
177 of the entire subpattern.
178 </P>
179 <P>
180 The <i>pattern_position</i> and <i>next_item_length</i> fields are intended to
181 help in distinguishing between different automatic callouts, which all have the
182 same callout number. However, they are set for all callouts.
183 </P>
184 <P>
185 The <i>mark</i> field is present from version 2 of the <i>pcre_callout</i>
186 structure. In callouts from <b>pcre_exec()</b> it contains a pointer to the
187 zero-terminated name of the most recently passed (*MARK) item in the match, or
188 NULL if there are no (*MARK)s in the current matching path. In callouts from
189 <b>pcre_dfa_exec()</b> this field always contains NULL.
190 </P>
191 <br><a name="SEC4" href="#TOC1">RETURN VALUES</a><br>
192 <P>
193 The external callout function returns an integer to PCRE. If the value is zero,
194 matching proceeds as normal. If the value is greater than zero, matching fails
195 at the current point, but the testing of other matching possibilities goes
196 ahead, just as if a lookahead assertion had failed. If the value is less than
197 zero, the match is abandoned, and <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>
198 returns the negative value.
199 </P>
200 <P>
201 Negative values should normally be chosen from the set of PCRE_ERROR_xxx
202 values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
203 The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
204 it will never be used by PCRE itself.
205 </P>
206 <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
207 <P>
208 Philip Hazel
209 <br>
210 University Computing Service
211 <br>
212 Cambridge CB2 3QH, England.
213 <br>
214 </P>
215 <br><a name="SEC6" href="#TOC1">REVISION</a><br>
216 <P>
217 Last updated: 31 July 2011
218 <br>
219 Copyright &copy; 1997-2011 University of Cambridge.
220 <br>
221 <p>
222 Return to the <a href="index.html">PCRE index page</a>.
223 </p>

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