ViewVC logotype

Contents of /code/trunk/doc/pcrecallout.3

Parent Directory Parent Directory | Revision Log Revision Log

Revision 1398 - (show annotations)
Tue Nov 12 15:20:26 2013 UTC (7 years, 5 months ago) by ph10
File size: 10374 byte(s)
Rename the (as yet unreleased) PCRE_NO_AUTO_POSSESSIFY option to 
PCRE_NO_AUTO_POSSESS - no need to be so long - and to match (*NO_AUTO_POSSESS).
1 .TH PCRECALLOUT 3 "12 November 2013" "PCRE 8.34"
3 PCRE - Perl-compatible regular expressions
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 .
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 .
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 .
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 .
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 .
240 .rs
241 .sp
242 .nf
243 Philip Hazel
244 University Computing Service
245 Cambridge CB2 3QH, England.
246 .fi
247 .
248 .
250 .rs
251 .sp
252 .nf
253 Last updated: 12 November 2013
254 Copyright (c) 1997-2013 University of Cambridge.
255 .fi


Name Value
svn:eol-style native
svn:keywords "Author Date Id Revision Url"

  ViewVC Help
Powered by ViewVC 1.1.5