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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 This HTML document has been generated automatically from the original man page.
7 If there is any nonsense in it, please consult the man page, in case the
8 conversion went wrong.<br>
9 <ul>
10 <li><a name="TOC1" href="#SEC1">PCRE CALLOUTS</a>
11 <li><a name="TOC2" href="#SEC2">RETURN VALUES</a>
12 </ul>
13 <br><a name="SEC1" href="#TOC1">PCRE CALLOUTS</a><br>
14 <P>
15 <b>int (*pcre_callout)(pcre_callout_block *);</b>
16 </P>
17 <P>
18 PCRE provides a feature called "callout", which is a means of temporarily
19 passing control to the caller of PCRE in the middle of pattern matching. The
20 caller of PCRE provides an external function by putting its entry point in the
21 global variable <i>pcre_callout</i>. By default, this variable contains NULL,
22 which disables all calling out.
23 </P>
24 <P>
25 Within a regular expression, (?C) indicates the points at which the external
26 function is to be called. Different callout points can be identified by putting
27 a number less than 256 after the letter C. The default value is zero.
28 For example, this pattern has two callout points:
29 </P>
30 <P>
31 <pre>
32 (?C1)\dabc(?C2)def
33 </PRE>
34 </P>
35 <P>
36 During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is
37 set), the external function is called. Its only argument is a pointer to a
38 <b>pcre_callout</b> block. This contains the following variables:
39 </P>
40 <P>
41 <pre>
42 int <i>version</i>;
43 int <i>callout_number</i>;
44 int *<i>offset_vector</i>;
45 const char *<i>subject</i>;
46 int <i>subject_length</i>;
47 int <i>start_match</i>;
48 int <i>current_position</i>;
49 int <i>capture_top</i>;
50 int <i>capture_last</i>;
51 void *<i>callout_data</i>;
52 </PRE>
53 </P>
54 <P>
55 The <i>version</i> field is an integer containing the version number of the
56 block format. The current version is zero. The version number may change in
57 future if additional fields are added, but the intention is never to remove any
58 of the existing fields.
59 </P>
60 <P>
61 The <i>callout_number</i> field contains the number of the callout, as compiled
62 into the pattern (that is, the number after ?C).
63 </P>
64 <P>
65 The <i>offset_vector</i> field is a pointer to the vector of offsets that was
66 passed by the caller to <b>pcre_exec()</b>. The contents can be inspected in
67 order to extract substrings that have been matched so far, in the same way as
68 for extracting substrings after a match has completed.
69 </P>
70 <P>
71 The <i>subject</i> and <i>subject_length</i> fields contain copies the values
72 that were passed to <b>pcre_exec()</b>.
73 </P>
74 <P>
75 The <i>start_match</i> field contains the offset within the subject at which the
76 current match attempt started. If the pattern is not anchored, the callout
77 function may be called several times for different starting points.
78 </P>
79 <P>
80 The <i>current_position</i> field contains the offset within the subject of the
81 current match pointer.
82 </P>
83 <P>
84 The <i>capture_top</i> field contains the number of the highest captured
85 substring so far.
86 </P>
87 <P>
88 The <i>capture_last</i> field contains the number of the most recently captured
89 substring.
90 </P>
91 <P>
92 The <i>callout_data</i> field contains a value that is passed to
93 <b>pcre_exec()</b> by the caller specifically so that it can be passed back in
94 callouts. It is passed in the <i>pcre_callout</i> field of the <b>pcre_extra</b>
95 data structure. If no such data was passed, the value of <i>callout_data</i> in
96 a <b>pcre_callout</b> block is NULL. There is a description of the
97 <b>pcre_extra</b> structure in the <b>pcreapi</b> documentation.
98 </P>
99 <br><a name="SEC2" href="#TOC1">RETURN VALUES</a><br>
100 <P>
101 The callout function returns an integer. If the value is zero, matching
102 proceeds as normal. If the value is greater than zero, matching fails at the
103 current point, but backtracking to test other possibilities goes ahead, just as
104 if a lookahead assertion had failed. If the value is less than zero, the match
105 is abandoned, and <b>pcre_exec()</b> returns the value.
106 </P>
107 <P>
108 Negative values should normally be chosen from the set of PCRE_ERROR_xxx
109 values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
110 The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
111 it will never be used by PCRE itself.
112 </P>
113 <P>
114 Last updated: 21 January 2003
115 <br>
116 Copyright &copy; 1997-2003 University of Cambridge.

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