1: .TH PCRECALLOUT 3 "12 November 2013" "PCRE 8.34"
2: .SH NAME
3: PCRE - Perl-compatible regular expressions
4: .SH SYNOPSIS
5: .rs
6: .sp
7: .B #include <pcre.h>
8: .PP
9: .SM
10: .B int (*pcre_callout)(pcre_callout_block *);
11: .PP
12: .B int (*pcre16_callout)(pcre16_callout_block *);
13: .PP
14: .B int (*pcre32_callout)(pcre32_callout_block *);
15: .
16: .SH DESCRIPTION
17: .rs
18: .sp
19: PCRE provides a feature called "callout", which is a means of temporarily
20: passing control to the caller of PCRE in the middle of pattern matching. The
21: caller of PCRE provides an external function by putting its entry point in the
22: global variable \fIpcre_callout\fP (\fIpcre16_callout\fP for the 16-bit
23: library, \fIpcre32_callout\fP for the 32-bit library). By default, this
24: variable contains NULL, which disables all calling out.
25: .P
26: Within a regular expression, (?C) indicates the points at which the external
27: function is to be called. Different callout points can be identified by putting
28: a number less than 256 after the letter C. The default value is zero.
29: For example, this pattern has two callout points:
30: .sp
31: (?C1)abc(?C2)def
32: .sp
33: If the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled, PCRE
34: automatically inserts callouts, all with number 255, before each item in the
35: pattern. For example, if PCRE_AUTO_CALLOUT is used with the pattern
36: .sp
37: A(\ed{2}|--)
38: .sp
39: it is processed as if it were
40: .sp
41: (?C255)A(?C255)((?C255)\ed{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
42: .sp
43: Notice that there is a callout before and after each parenthesis and
44: alternation bar. If the pattern contains a conditional group whose condition is
45: an assertion, an automatic callout is inserted immediately before the
46: condition. Such a callout may also be inserted explicitly, for example:
47: .sp
48: (?(?C9)(?=a)ab|de)
49: .sp
50: This applies only to assertion conditions (because they are themselves
51: independent groups).
52: .P
53: Automatic callouts can be used for tracking the progress of pattern matching.
54: The
55: .\" HREF
56: \fBpcretest\fP
57: .\"
58: program has a pattern qualifier (/C) that sets automatic callouts; when it is
59: used, the output indicates how the pattern is being matched. This is useful
60: information when you are trying to optimize the performance of a particular
61: pattern.
62: .
63: .
64: .SH "MISSING CALLOUTS"
65: .rs
66: .sp
67: You should be aware that, because of optimizations in the way PCRE compiles and
68: matches patterns, callouts sometimes do not happen exactly as you might expect.
69: .P
70: At compile time, PCRE "auto-possessifies" repeated items when it knows that
71: what follows cannot be part of the repeat. For example, a+[bc] is compiled as
72: if it were a++[bc]. The \fBpcretest\fP output when this pattern is anchored and
73: then applied with automatic callouts to the string "aaaa" is:
74: .sp
75: --->aaaa
76: +0 ^ ^
77: +1 ^ a+
78: +3 ^ ^ [bc]
79: No match
80: .sp
81: This indicates that when matching [bc] fails, there is no backtracking into a+
82: and therefore the callouts that would be taken for the backtracks do not occur.
83: You can disable the auto-possessify feature by passing PCRE_NO_AUTO_POSSESS
84: to \fBpcre_compile()\fP, or starting the pattern with (*NO_AUTO_POSSESS). If
85: this is done in \fBpcretest\fP (using the /O qualifier), the output changes to
86: this:
87: .sp
88: --->aaaa
89: +0 ^ ^
90: +1 ^ a+
91: +3 ^ ^ [bc]
92: +3 ^ ^ [bc]
93: +3 ^ ^ [bc]
94: +3 ^^ [bc]
95: No match
96: .sp
97: This time, when matching [bc] fails, the matcher backtracks into a+ and tries
98: again, repeatedly, until a+ itself fails.
99: .P
100: Other optimizations that provide fast "no match" results also affect callouts.
101: For example, if the pattern is
102: .sp
103: ab(?C4)cd
104: .sp
105: PCRE knows that any matching string must contain the letter "d". If the subject
106: string is "abyz", the lack of "d" means that matching doesn't ever start, and
107: the callout is never reached. However, with "abyd", though the result is still
108: no match, the callout is obeyed.
109: .P
110: If the pattern is studied, PCRE knows the minimum length of a matching string,
111: and will immediately give a "no match" return without actually running a match
112: if the subject is not long enough, or, for unanchored patterns, if it has
113: been scanned far enough.
114: .P
115: You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
116: option to the matching function, or by starting the pattern with
117: (*NO_START_OPT). This slows down the matching process, but does ensure that
118: callouts such as the example above are obeyed.
119: .
120: .
121: .SH "THE CALLOUT INTERFACE"
122: .rs
123: .sp
124: During matching, when PCRE reaches a callout point, the external function
125: defined by \fIpcre_callout\fP or \fIpcre[16|32]_callout\fP is called (if it is
126: set). This applies to both normal and DFA matching. The only argument to the
127: callout function is a pointer to a \fBpcre_callout\fP or
128: \fBpcre[16|32]_callout\fP block. These structures contains the following
129: fields:
130: .sp
131: int \fIversion\fP;
132: int \fIcallout_number\fP;
133: int *\fIoffset_vector\fP;
134: const char *\fIsubject\fP; (8-bit version)
135: PCRE_SPTR16 \fIsubject\fP; (16-bit version)
136: PCRE_SPTR32 \fIsubject\fP; (32-bit version)
137: int \fIsubject_length\fP;
138: int \fIstart_match\fP;
139: int \fIcurrent_position\fP;
140: int \fIcapture_top\fP;
141: int \fIcapture_last\fP;
142: void *\fIcallout_data\fP;
143: int \fIpattern_position\fP;
144: int \fInext_item_length\fP;
145: const unsigned char *\fImark\fP; (8-bit version)
146: const PCRE_UCHAR16 *\fImark\fP; (16-bit version)
147: const PCRE_UCHAR32 *\fImark\fP; (32-bit version)
148: .sp
149: The \fIversion\fP field is an integer containing the version number of the
150: block format. The initial version was 0; the current version is 2. The version
151: number will change again in future if additional fields are added, but the
152: intention is never to remove any of the existing fields.
153: .P
154: The \fIcallout_number\fP field contains the number of the callout, as compiled
155: into the pattern (that is, the number after ?C for manual callouts, and 255 for
156: automatically generated callouts).
157: .P
158: The \fIoffset_vector\fP field is a pointer to the vector of offsets that was
159: passed by the caller to the matching function. When \fBpcre_exec()\fP or
160: \fBpcre[16|32]_exec()\fP is used, the contents can be inspected, in order to
161: extract substrings that have been matched so far, in the same way as for
162: extracting substrings after a match has completed. For the DFA matching
163: functions, this field is not useful.
164: .P
165: The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values
166: that were passed to the matching function.
167: .P
168: The \fIstart_match\fP field normally contains the offset within the subject at
169: which the current match attempt started. However, if the escape sequence \eK
170: has been encountered, this value is changed to reflect the modified starting
171: point. If the pattern is not anchored, the callout function may be called
172: several times from the same point in the pattern for different starting points
173: in the subject.
174: .P
175: The \fIcurrent_position\fP field contains the offset within the subject of the
176: current match pointer.
177: .P
178: When the \fBpcre_exec()\fP or \fBpcre[16|32]_exec()\fP is used, the
179: \fIcapture_top\fP field contains one more than the number of the highest
180: numbered captured substring so far. If no substrings have been captured, the
181: value of \fIcapture_top\fP is one. This is always the case when the DFA
182: functions are used, because they do not support captured substrings.
183: .P
184: The \fIcapture_last\fP field contains the number of the most recently captured
185: substring. However, when a recursion exits, the value reverts to what it was
186: outside the recursion, as do the values of all captured substrings. If no
187: substrings have been captured, the value of \fIcapture_last\fP is -1. This is
188: always the case for the DFA matching functions.
189: .P
190: The \fIcallout_data\fP field contains a value that is passed to a matching
191: function specifically so that it can be passed back in callouts. It is passed
192: in the \fIcallout_data\fP field of a \fBpcre_extra\fP or \fBpcre[16|32]_extra\fP
193: data structure. If no such data was passed, the value of \fIcallout_data\fP in
194: a callout block is NULL. There is a description of the \fBpcre_extra\fP
195: structure in the
196: .\" HREF
197: \fBpcreapi\fP
198: .\"
199: documentation.
200: .P
201: The \fIpattern_position\fP field is present from version 1 of the callout
202: structure. It contains the offset to the next item to be matched in the pattern
203: string.
204: .P
205: The \fInext_item_length\fP field is present from version 1 of the callout
206: structure. It contains the length of the next item to be matched in the pattern
207: string. When the callout immediately precedes an alternation bar, a closing
208: parenthesis, or the end of the pattern, the length is zero. When the callout
209: precedes an opening parenthesis, the length is that of the entire subpattern.
210: .P
211: The \fIpattern_position\fP and \fInext_item_length\fP fields are intended to
212: help in distinguishing between different automatic callouts, which all have the
213: same callout number. However, they are set for all callouts.
214: .P
215: The \fImark\fP field is present from version 2 of the callout structure. In
216: callouts from \fBpcre_exec()\fP or \fBpcre[16|32]_exec()\fP it contains a
217: pointer to the zero-terminated name of the most recently passed (*MARK),
218: (*PRUNE), or (*THEN) item in the match, or NULL if no such items have been
219: passed. Instances of (*PRUNE) or (*THEN) without a name do not obliterate a
220: previous (*MARK). In callouts from the DFA matching functions this field always
221: contains NULL.
222: .
223: .
224: .SH "RETURN VALUES"
225: .rs
226: .sp
227: The external callout function returns an integer to PCRE. If the value is zero,
228: matching proceeds as normal. If the value is greater than zero, matching fails
229: at the current point, but the testing of other matching possibilities goes
230: ahead, just as if a lookahead assertion had failed. If the value is less than
231: zero, the match is abandoned, the matching function returns the negative value.
232: .P
233: Negative values should normally be chosen from the set of PCRE_ERROR_xxx
234: values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
235: The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
236: it will never be used by PCRE itself.
237: .
238: .
239: .SH AUTHOR
240: .rs
241: .sp
242: .nf
243: Philip Hazel
244: University Computing Service
245: Cambridge CB2 3QH, England.
246: .fi
247: .
248: .
249: .SH REVISION
250: .rs
251: .sp
252: .nf
253: Last updated: 12 November 2013
254: Copyright (c) 1997-2013 University of Cambridge.
255: .fi
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