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2: <head>
3: <title>pcrecallout specification</title>
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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>
1.1.1.3 misho 16: <li><a name="TOC1" href="#SEC1">SYNOPSIS</a>
17: <li><a name="TOC2" href="#SEC2">DESCRIPTION</a>
18: <li><a name="TOC3" href="#SEC3">MISSING CALLOUTS</a>
19: <li><a name="TOC4" href="#SEC4">THE CALLOUT INTERFACE</a>
20: <li><a name="TOC5" href="#SEC5">RETURN VALUES</a>
21: <li><a name="TOC6" href="#SEC6">AUTHOR</a>
22: <li><a name="TOC7" href="#SEC7">REVISION</a>
1.1 misho 23: </ul>
1.1.1.3 misho 24: <br><a name="SEC1" href="#TOC1">SYNOPSIS</a><br>
25: <P>
26: <b>#include <pcre.h></b>
27: </P>
1.1 misho 28: <P>
29: <b>int (*pcre_callout)(pcre_callout_block *);</b>
30: </P>
31: <P>
1.1.1.2 misho 32: <b>int (*pcre16_callout)(pcre16_callout_block *);</b>
33: </P>
34: <P>
1.1.1.3 misho 35: <b>int (*pcre32_callout)(pcre32_callout_block *);</b>
36: </P>
37: <br><a name="SEC2" href="#TOC1">DESCRIPTION</a><br>
38: <P>
1.1 misho 39: PCRE provides a feature called "callout", which is a means of temporarily
40: passing control to the caller of PCRE in the middle of pattern matching. The
41: caller of PCRE provides an external function by putting its entry point in the
1.1.1.2 misho 42: global variable <i>pcre_callout</i> (<i>pcre16_callout</i> for the 16-bit
1.1.1.3 misho 43: library, <i>pcre32_callout</i> for the 32-bit library). By default, this
44: variable contains NULL, which disables all calling out.
1.1 misho 45: </P>
46: <P>
47: Within a regular expression, (?C) indicates the points at which the external
48: function is to be called. Different callout points can be identified by putting
49: a number less than 256 after the letter C. The default value is zero.
50: For example, this pattern has two callout points:
51: <pre>
52: (?C1)abc(?C2)def
53: </pre>
1.1.1.2 misho 54: If the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled, PCRE
55: automatically inserts callouts, all with number 255, before each item in the
56: pattern. For example, if PCRE_AUTO_CALLOUT is used with the pattern
1.1 misho 57: <pre>
58: A(\d{2}|--)
59: </pre>
60: it is processed as if it were
61: <br>
62: <br>
63: (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
64: <br>
65: <br>
66: Notice that there is a callout before and after each parenthesis and
1.1.1.3 misho 67: alternation bar. If the pattern contains a conditional group whose condition is
68: an assertion, an automatic callout is inserted immediately before the
69: condition. Such a callout may also be inserted explicitly, for example:
70: <pre>
71: (?(?C9)(?=a)ab|de)
72: </pre>
73: This applies only to assertion conditions (because they are themselves
74: independent groups).
75: </P>
76: <P>
77: Automatic callouts can be used for tracking the progress of pattern matching.
78: The
1.1 misho 79: <a href="pcretest.html"><b>pcretest</b></a>
1.1.1.4 ! misho 80: program has a pattern qualifier (/C) that sets automatic callouts; when it is
! 81: used, the output indicates how the pattern is being matched. This is useful
! 82: information when you are trying to optimize the performance of a particular
! 83: pattern.
1.1 misho 84: </P>
1.1.1.3 misho 85: <br><a name="SEC3" href="#TOC1">MISSING CALLOUTS</a><br>
1.1 misho 86: <P>
1.1.1.4 ! misho 87: You should be aware that, because of optimizations in the way PCRE compiles and
! 88: matches patterns, callouts sometimes do not happen exactly as you might expect.
! 89: </P>
! 90: <P>
! 91: At compile time, PCRE "auto-possessifies" repeated items when it knows that
! 92: what follows cannot be part of the repeat. For example, a+[bc] is compiled as
! 93: if it were a++[bc]. The <b>pcretest</b> output when this pattern is anchored and
! 94: then applied with automatic callouts to the string "aaaa" is:
! 95: <pre>
! 96: --->aaaa
! 97: +0 ^ ^
! 98: +1 ^ a+
! 99: +3 ^ ^ [bc]
! 100: No match
! 101: </pre>
! 102: This indicates that when matching [bc] fails, there is no backtracking into a+
! 103: and therefore the callouts that would be taken for the backtracks do not occur.
! 104: You can disable the auto-possessify feature by passing PCRE_NO_AUTO_POSSESS
! 105: to <b>pcre_compile()</b>, or starting the pattern with (*NO_AUTO_POSSESS). If
! 106: this is done in <b>pcretest</b> (using the /O qualifier), the output changes to
! 107: this:
! 108: <pre>
! 109: --->aaaa
! 110: +0 ^ ^
! 111: +1 ^ a+
! 112: +3 ^ ^ [bc]
! 113: +3 ^ ^ [bc]
! 114: +3 ^ ^ [bc]
! 115: +3 ^^ [bc]
! 116: No match
! 117: </pre>
! 118: This time, when matching [bc] fails, the matcher backtracks into a+ and tries
! 119: again, repeatedly, until a+ itself fails.
! 120: </P>
! 121: <P>
! 122: Other optimizations that provide fast "no match" results also affect callouts.
! 123: For example, if the pattern is
1.1 misho 124: <pre>
125: ab(?C4)cd
126: </pre>
127: PCRE knows that any matching string must contain the letter "d". If the subject
128: string is "abyz", the lack of "d" means that matching doesn't ever start, and
129: the callout is never reached. However, with "abyd", though the result is still
130: no match, the callout is obeyed.
131: </P>
132: <P>
133: If the pattern is studied, PCRE knows the minimum length of a matching string,
134: and will immediately give a "no match" return without actually running a match
135: if the subject is not long enough, or, for unanchored patterns, if it has
136: been scanned far enough.
137: </P>
138: <P>
139: You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
1.1.1.2 misho 140: option to the matching function, or by starting the pattern with
141: (*NO_START_OPT). This slows down the matching process, but does ensure that
142: callouts such as the example above are obeyed.
1.1 misho 143: </P>
1.1.1.3 misho 144: <br><a name="SEC4" href="#TOC1">THE CALLOUT INTERFACE</a><br>
1.1 misho 145: <P>
146: During matching, when PCRE reaches a callout point, the external function
1.1.1.4 ! misho 147: defined by <i>pcre_callout</i> or <i>pcre[16|32]_callout</i> is called (if it is
! 148: set). This applies to both normal and DFA matching. The only argument to the
! 149: callout function is a pointer to a <b>pcre_callout</b> or
! 150: <b>pcre[16|32]_callout</b> block. These structures contains the following
! 151: fields:
1.1 misho 152: <pre>
1.1.1.2 misho 153: int <i>version</i>;
154: int <i>callout_number</i>;
155: int *<i>offset_vector</i>;
156: const char *<i>subject</i>; (8-bit version)
157: PCRE_SPTR16 <i>subject</i>; (16-bit version)
1.1.1.3 misho 158: PCRE_SPTR32 <i>subject</i>; (32-bit version)
1.1.1.2 misho 159: int <i>subject_length</i>;
160: int <i>start_match</i>;
161: int <i>current_position</i>;
162: int <i>capture_top</i>;
163: int <i>capture_last</i>;
164: void *<i>callout_data</i>;
165: int <i>pattern_position</i>;
166: int <i>next_item_length</i>;
167: const unsigned char *<i>mark</i>; (8-bit version)
168: const PCRE_UCHAR16 *<i>mark</i>; (16-bit version)
1.1.1.3 misho 169: const PCRE_UCHAR32 *<i>mark</i>; (32-bit version)
1.1 misho 170: </pre>
171: The <i>version</i> field is an integer containing the version number of the
172: block format. The initial version was 0; the current version is 2. The version
173: number will change again in future if additional fields are added, but the
174: intention is never to remove any of the existing fields.
175: </P>
176: <P>
177: The <i>callout_number</i> field contains the number of the callout, as compiled
178: into the pattern (that is, the number after ?C for manual callouts, and 255 for
179: automatically generated callouts).
180: </P>
181: <P>
182: The <i>offset_vector</i> field is a pointer to the vector of offsets that was
1.1.1.2 misho 183: passed by the caller to the matching function. When <b>pcre_exec()</b> or
1.1.1.3 misho 184: <b>pcre[16|32]_exec()</b> is used, the contents can be inspected, in order to
185: extract substrings that have been matched so far, in the same way as for
186: extracting substrings after a match has completed. For the DFA matching
187: functions, this field is not useful.
1.1 misho 188: </P>
189: <P>
190: The <i>subject</i> and <i>subject_length</i> fields contain copies of the values
1.1.1.2 misho 191: that were passed to the matching function.
1.1 misho 192: </P>
193: <P>
194: The <i>start_match</i> field normally contains the offset within the subject at
195: which the current match attempt started. However, if the escape sequence \K
196: has been encountered, this value is changed to reflect the modified starting
197: point. If the pattern is not anchored, the callout function may be called
198: several times from the same point in the pattern for different starting points
199: in the subject.
200: </P>
201: <P>
202: The <i>current_position</i> field contains the offset within the subject of the
203: current match pointer.
204: </P>
205: <P>
1.1.1.3 misho 206: When the <b>pcre_exec()</b> or <b>pcre[16|32]_exec()</b> is used, the
1.1.1.2 misho 207: <i>capture_top</i> field contains one more than the number of the highest
208: numbered captured substring so far. If no substrings have been captured, the
209: value of <i>capture_top</i> is one. This is always the case when the DFA
210: functions are used, because they do not support captured substrings.
1.1 misho 211: </P>
212: <P>
213: The <i>capture_last</i> field contains the number of the most recently captured
1.1.1.3 misho 214: substring. However, when a recursion exits, the value reverts to what it was
215: outside the recursion, as do the values of all captured substrings. If no
216: substrings have been captured, the value of <i>capture_last</i> is -1. This is
217: always the case for the DFA matching functions.
1.1 misho 218: </P>
219: <P>
1.1.1.2 misho 220: The <i>callout_data</i> field contains a value that is passed to a matching
221: function specifically so that it can be passed back in callouts. It is passed
1.1.1.3 misho 222: in the <i>callout_data</i> field of a <b>pcre_extra</b> or <b>pcre[16|32]_extra</b>
1.1.1.2 misho 223: data structure. If no such data was passed, the value of <i>callout_data</i> in
224: a callout block is NULL. There is a description of the <b>pcre_extra</b>
225: structure in the
1.1 misho 226: <a href="pcreapi.html"><b>pcreapi</b></a>
227: documentation.
228: </P>
229: <P>
1.1.1.2 misho 230: The <i>pattern_position</i> field is present from version 1 of the callout
231: structure. It contains the offset to the next item to be matched in the pattern
232: string.
1.1 misho 233: </P>
234: <P>
1.1.1.2 misho 235: The <i>next_item_length</i> field is present from version 1 of the callout
236: structure. It contains the length of the next item to be matched in the pattern
237: string. When the callout immediately precedes an alternation bar, a closing
238: parenthesis, or the end of the pattern, the length is zero. When the callout
239: precedes an opening parenthesis, the length is that of the entire subpattern.
1.1 misho 240: </P>
241: <P>
242: The <i>pattern_position</i> and <i>next_item_length</i> fields are intended to
243: help in distinguishing between different automatic callouts, which all have the
244: same callout number. However, they are set for all callouts.
245: </P>
246: <P>
1.1.1.2 misho 247: The <i>mark</i> field is present from version 2 of the callout structure. In
1.1.1.3 misho 248: callouts from <b>pcre_exec()</b> or <b>pcre[16|32]_exec()</b> it contains a
249: pointer to the zero-terminated name of the most recently passed (*MARK),
250: (*PRUNE), or (*THEN) item in the match, or NULL if no such items have been
251: passed. Instances of (*PRUNE) or (*THEN) without a name do not obliterate a
252: previous (*MARK). In callouts from the DFA matching functions this field always
253: contains NULL.
1.1 misho 254: </P>
1.1.1.3 misho 255: <br><a name="SEC5" href="#TOC1">RETURN VALUES</a><br>
1.1 misho 256: <P>
257: The external callout function returns an integer to PCRE. If the value is zero,
258: matching proceeds as normal. If the value is greater than zero, matching fails
259: at the current point, but the testing of other matching possibilities goes
260: ahead, just as if a lookahead assertion had failed. If the value is less than
1.1.1.2 misho 261: zero, the match is abandoned, the matching function returns the negative value.
1.1 misho 262: </P>
263: <P>
264: Negative values should normally be chosen from the set of PCRE_ERROR_xxx
265: values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
266: The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
267: it will never be used by PCRE itself.
268: </P>
1.1.1.3 misho 269: <br><a name="SEC6" href="#TOC1">AUTHOR</a><br>
1.1 misho 270: <P>
271: Philip Hazel
272: <br>
273: University Computing Service
274: <br>
275: Cambridge CB2 3QH, England.
276: <br>
277: </P>
1.1.1.3 misho 278: <br><a name="SEC7" href="#TOC1">REVISION</a><br>
1.1 misho 279: <P>
1.1.1.4 ! misho 280: Last updated: 12 November 2013
1.1 misho 281: <br>
1.1.1.3 misho 282: Copyright © 1997-2013 University of Cambridge.
1.1 misho 283: <br>
284: <p>
285: Return to the <a href="index.html">PCRE index page</a>.
286: </p>
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