Annotation of embedaddon/pcre/doc/pcrecallout.3, revision 1.1.1.3
1.1.1.3 ! misho 1: .TH PCRECALLOUT 3 "08 January 2012" "PCRE 8.30"
1.1 misho 2: .SH NAME
3: PCRE - Perl-compatible regular expressions
4: .SH "PCRE CALLOUTS"
5: .rs
6: .sp
7: .B int (*pcre_callout)(pcre_callout_block *);
8: .PP
1.1.1.2 misho 9: .B int (*pcre16_callout)(pcre16_callout_block *);
10: .PP
1.1 misho 11: PCRE provides a feature called "callout", which is a means of temporarily
12: passing control to the caller of PCRE in the middle of pattern matching. The
13: caller of PCRE provides an external function by putting its entry point in the
1.1.1.2 misho 14: global variable \fIpcre_callout\fP (\fIpcre16_callout\fP for the 16-bit
15: library). By default, this variable contains NULL, which disables all calling
16: out.
1.1 misho 17: .P
18: Within a regular expression, (?C) indicates the points at which the external
19: function is to be called. Different callout points can be identified by putting
20: a number less than 256 after the letter C. The default value is zero.
21: For example, this pattern has two callout points:
22: .sp
23: (?C1)abc(?C2)def
24: .sp
1.1.1.2 misho 25: If the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled, PCRE
26: automatically inserts callouts, all with number 255, before each item in the
27: pattern. For example, if PCRE_AUTO_CALLOUT is used with the pattern
1.1 misho 28: .sp
29: A(\ed{2}|--)
30: .sp
31: it is processed as if it were
32: .sp
33: (?C255)A(?C255)((?C255)\ed{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
34: .sp
35: Notice that there is a callout before and after each parenthesis and
36: alternation bar. Automatic callouts can be used for tracking the progress of
37: pattern matching. The
38: .\" HREF
39: \fBpcretest\fP
40: .\"
41: command has an option that sets automatic callouts; when it is used, the output
42: indicates how the pattern is matched. This is useful information when you are
43: trying to optimize the performance of a particular pattern.
44: .P
45: The use of callouts in a pattern makes it ineligible for optimization by the
46: just-in-time compiler. Studying such a pattern with the PCRE_STUDY_JIT_COMPILE
47: option always fails.
48: .
49: .
50: .SH "MISSING CALLOUTS"
51: .rs
52: .sp
53: You should be aware that, because of optimizations in the way PCRE matches
54: patterns by default, callouts sometimes do not happen. For example, if the
55: pattern is
56: .sp
57: ab(?C4)cd
58: .sp
59: PCRE knows that any matching string must contain the letter "d". If the subject
60: string is "abyz", the lack of "d" means that matching doesn't ever start, and
61: the callout is never reached. However, with "abyd", though the result is still
62: no match, the callout is obeyed.
63: .P
64: If the pattern is studied, PCRE knows the minimum length of a matching string,
65: and will immediately give a "no match" return without actually running a match
66: if the subject is not long enough, or, for unanchored patterns, if it has
67: been scanned far enough.
68: .P
69: You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
1.1.1.2 misho 70: option to the matching function, or by starting the pattern with
71: (*NO_START_OPT). This slows down the matching process, but does ensure that
72: callouts such as the example above are obeyed.
1.1 misho 73: .
74: .
75: .SH "THE CALLOUT INTERFACE"
76: .rs
77: .sp
78: During matching, when PCRE reaches a callout point, the external function
1.1.1.2 misho 79: defined by \fIpcre_callout\fP or \fIpcre16_callout\fP is called (if it is set).
80: This applies to both normal and DFA matching. The only argument to the callout
81: function is a pointer to a \fBpcre_callout\fP or \fBpcre16_callout\fP block.
82: These structures contains the following fields:
83: .sp
84: int \fIversion\fP;
85: int \fIcallout_number\fP;
86: int *\fIoffset_vector\fP;
87: const char *\fIsubject\fP; (8-bit version)
88: PCRE_SPTR16 \fIsubject\fP; (16-bit version)
89: int \fIsubject_length\fP;
90: int \fIstart_match\fP;
91: int \fIcurrent_position\fP;
92: int \fIcapture_top\fP;
93: int \fIcapture_last\fP;
94: void *\fIcallout_data\fP;
95: int \fIpattern_position\fP;
96: int \fInext_item_length\fP;
97: const unsigned char *\fImark\fP; (8-bit version)
98: const PCRE_UCHAR16 *\fImark\fP; (16-bit version)
1.1 misho 99: .sp
100: The \fIversion\fP field is an integer containing the version number of the
101: block format. The initial version was 0; the current version is 2. The version
102: number will change again in future if additional fields are added, but the
103: intention is never to remove any of the existing fields.
104: .P
105: The \fIcallout_number\fP field contains the number of the callout, as compiled
106: into the pattern (that is, the number after ?C for manual callouts, and 255 for
107: automatically generated callouts).
108: .P
109: The \fIoffset_vector\fP field is a pointer to the vector of offsets that was
1.1.1.2 misho 110: passed by the caller to the matching function. When \fBpcre_exec()\fP or
111: \fBpcre16_exec()\fP is used, the contents can be inspected, in order to extract
1.1 misho 112: substrings that have been matched so far, in the same way as for extracting
1.1.1.2 misho 113: substrings after a match has completed. For the DFA matching functions, this
114: field is not useful.
1.1 misho 115: .P
116: The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values
1.1.1.2 misho 117: that were passed to the matching function.
1.1 misho 118: .P
119: The \fIstart_match\fP field normally contains the offset within the subject at
120: which the current match attempt started. However, if the escape sequence \eK
121: has been encountered, this value is changed to reflect the modified starting
122: point. If the pattern is not anchored, the callout function may be called
123: several times from the same point in the pattern for different starting points
124: in the subject.
125: .P
126: The \fIcurrent_position\fP field contains the offset within the subject of the
127: current match pointer.
128: .P
1.1.1.2 misho 129: When the \fBpcre_exec()\fP or \fBpcre16_exec()\fP is used, the
130: \fIcapture_top\fP field contains one more than the number of the highest
131: numbered captured substring so far. If no substrings have been captured, the
132: value of \fIcapture_top\fP is one. This is always the case when the DFA
133: functions are used, because they do not support captured substrings.
1.1 misho 134: .P
135: The \fIcapture_last\fP field contains the number of the most recently captured
136: substring. If no substrings have been captured, its value is -1. This is always
1.1.1.2 misho 137: the case for the DFA matching functions.
1.1 misho 138: .P
1.1.1.2 misho 139: The \fIcallout_data\fP field contains a value that is passed to a matching
140: function specifically so that it can be passed back in callouts. It is passed
141: in the \fIcallout_data\fP field of a \fBpcre_extra\fP or \fBpcre16_extra\fP
142: data structure. If no such data was passed, the value of \fIcallout_data\fP in
143: a callout block is NULL. There is a description of the \fBpcre_extra\fP
144: structure in the
1.1 misho 145: .\" HREF
146: \fBpcreapi\fP
147: .\"
148: documentation.
149: .P
1.1.1.2 misho 150: The \fIpattern_position\fP field is present from version 1 of the callout
151: structure. It contains the offset to the next item to be matched in the pattern
152: string.
153: .P
154: The \fInext_item_length\fP field is present from version 1 of the callout
155: structure. It contains the length of the next item to be matched in the pattern
156: string. When the callout immediately precedes an alternation bar, a closing
157: parenthesis, or the end of the pattern, the length is zero. When the callout
158: precedes an opening parenthesis, the length is that of the entire subpattern.
1.1 misho 159: .P
160: The \fIpattern_position\fP and \fInext_item_length\fP fields are intended to
161: help in distinguishing between different automatic callouts, which all have the
162: same callout number. However, they are set for all callouts.
163: .P
1.1.1.2 misho 164: The \fImark\fP field is present from version 2 of the callout structure. In
165: callouts from \fBpcre_exec()\fP or \fBpcre16_exec()\fP it contains a pointer to
166: the zero-terminated name of the most recently passed (*MARK), (*PRUNE), or
167: (*THEN) item in the match, or NULL if no such items have been passed. Instances
168: of (*PRUNE) or (*THEN) without a name do not obliterate a previous (*MARK). In
169: callouts from the DFA matching functions this field always contains NULL.
1.1 misho 170: .
171: .
172: .SH "RETURN VALUES"
173: .rs
174: .sp
175: The external callout function returns an integer to PCRE. If the value is zero,
176: matching proceeds as normal. If the value is greater than zero, matching fails
177: at the current point, but the testing of other matching possibilities goes
178: ahead, just as if a lookahead assertion had failed. If the value is less than
1.1.1.2 misho 179: zero, the match is abandoned, the matching function returns the negative value.
1.1 misho 180: .P
181: Negative values should normally be chosen from the set of PCRE_ERROR_xxx
182: values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
183: The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
184: it will never be used by PCRE itself.
185: .
186: .
187: .SH AUTHOR
188: .rs
189: .sp
190: .nf
191: Philip Hazel
192: University Computing Service
193: Cambridge CB2 3QH, England.
194: .fi
195: .
196: .
197: .SH REVISION
198: .rs
199: .sp
200: .nf
1.1.1.2 misho 201: Last updated: 08 Janurary 2012
202: Copyright (c) 1997-2012 University of Cambridge.
1.1 misho 203: .fi
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