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Revision 172 - (show annotations) (download)
Tue Jun 5 10:40:13 2007 UTC (6 years, 10 months ago) by ph10
File size: 8657 byte(s)
Drastically reduce workspace used for alternatives in groups; also some 
trailing space removals for a test release.

3 PCRE - Perl-compatible regular expressions
5 .rs
6 .sp
7 In normal use of PCRE, if the subject string that is passed to
8 \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP matches as far as it goes, but is
9 too short to match the entire pattern, PCRE_ERROR_NOMATCH is returned. There
10 are circumstances where it might be helpful to distinguish this case from other
11 cases in which there is no match.
12 .P
13 Consider, for example, an application where a human is required to type in data
14 for a field with specific formatting requirements. An example might be a date
15 in the form \fIddmmmyy\fP, defined by this pattern:
16 .sp
17 ^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$
18 .sp
19 If the application sees the user's keystrokes one by one, and can check that
20 what has been typed so far is potentially valid, it is able to raise an error
21 as soon as a mistake is made, possibly beeping and not reflecting the
22 character that has been typed. This immediate feedback is likely to be a better
23 user interface than a check that is delayed until the entire string has been
24 entered.
25 .P
26 PCRE supports the concept of partial matching by means of the PCRE_PARTIAL
27 option, which can be set when calling \fBpcre_exec()\fP or
28 \fBpcre_dfa_exec()\fP. When this flag is set for \fBpcre_exec()\fP, the return
29 code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any time
30 during the matching process the last part of the subject string matched part of
31 the pattern. Unfortunately, for non-anchored matching, it is not possible to
32 obtain the position of the start of the partial match. No captured data is set
33 when PCRE_ERROR_PARTIAL is returned.
34 .P
35 When PCRE_PARTIAL is set for \fBpcre_dfa_exec()\fP, the return code
36 PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end of the
37 subject is reached, there have been no complete matches, but there is still at
38 least one matching possibility. The portion of the string that provided the
39 partial match is set as the first matching string.
40 .P
41 Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers the
42 last literal byte in a pattern, and abandons matching immediately if such a
43 byte is not present in the subject string. This optimization cannot be used
44 for a subject string that might match only partially.
45 .
46 .
48 .rs
49 .sp
50 Because of the way certain internal optimizations are implemented in the
51 \fBpcre_exec()\fP function, the PCRE_PARTIAL option cannot be used with all
52 patterns. These restrictions do not apply when \fBpcre_dfa_exec()\fP is used.
53 For \fBpcre_exec()\fP, repeated single characters such as
54 .sp
55 a{2,4}
56 .sp
57 and repeated single metasequences such as
58 .sp
59 \ed+
60 .sp
61 are not permitted if the maximum number of occurrences is greater than one.
62 Optional items such as \ed? (where the maximum is one) are permitted.
63 Quantifiers with any values are permitted after parentheses, so the invalid
64 examples above can be coded thus:
65 .sp
66 (a){2,4}
67 (\ed)+
68 .sp
69 These constructions run more slowly, but for the kinds of application that are
70 envisaged for this facility, this is not felt to be a major restriction.
71 .P
72 If PCRE_PARTIAL is set for a pattern that does not conform to the restrictions,
73 \fBpcre_exec()\fP returns the error code PCRE_ERROR_BADPARTIAL (-13).
74 You can use the PCRE_INFO_OKPARTIAL call to \fBpcre_fullinfo()\fP to find out
75 if a compiled pattern can be used for partial matching.
76 .
77 .
79 .rs
80 .sp
81 If the escape sequence \eP is present in a \fBpcretest\fP data line, the
82 PCRE_PARTIAL flag is used for the match. Here is a run of \fBpcretest\fP that
83 uses the date example quoted above:
84 .sp
85 re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
86 data> 25jun04\eP
87 0: 25jun04
88 1: jun
89 data> 25dec3\eP
90 Partial match
91 data> 3ju\eP
92 Partial match
93 data> 3juj\eP
94 No match
95 data> j\eP
96 No match
97 .sp
98 The first data string is matched completely, so \fBpcretest\fP shows the
99 matched substrings. The remaining four strings do not match the complete
100 pattern, but the first two are partial matches. The same test, using
101 \fBpcre_dfa_exec()\fP matching (by means of the \eD escape sequence), produces
102 the following output:
103 .sp
104 re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
105 data> 25jun04\eP\eD
106 0: 25jun04
107 data> 23dec3\eP\eD
108 Partial match: 23dec3
109 data> 3ju\eP\eD
110 Partial match: 3ju
111 data> 3juj\eP\eD
112 No match
113 data> j\eP\eD
114 No match
115 .sp
116 Notice that in this case the portion of the string that was matched is made
117 available.
118 .
119 .
120 .SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()"
121 .rs
122 .sp
123 When a partial match has been found using \fBpcre_dfa_exec()\fP, it is possible
124 to continue the match by providing additional subject data and calling
125 \fBpcre_dfa_exec()\fP again with the same compiled regular expression, this
126 time setting the PCRE_DFA_RESTART option. You must also pass the same working
127 space as before, because this is where details of the previous partial match
128 are stored. Here is an example using \fBpcretest\fP, using the \eR escape
129 sequence to set the PCRE_DFA_RESTART option (\eP and \eD are as above):
130 .sp
131 re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
132 data> 23ja\eP\eD
133 Partial match: 23ja
134 data> n05\eR\eD
135 0: n05
136 .sp
137 The first call has "23ja" as the subject, and requests partial matching; the
138 second call has "n05" as the subject for the continued (restarted) match.
139 Notice that when the match is complete, only the last part is shown; PCRE does
140 not retain the previously partially-matched string. It is up to the calling
141 program to do that if it needs to.
142 .P
143 You can set PCRE_PARTIAL with PCRE_DFA_RESTART to continue partial matching
144 over multiple segments. This facility can be used to pass very long subject
145 strings to \fBpcre_dfa_exec()\fP. However, some care is needed for certain
146 types of pattern.
147 .P
148 1. If the pattern contains tests for the beginning or end of a line, you need
149 to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropriate, when the
150 subject string for any call does not contain the beginning or end of a line.
151 .P
152 2. If the pattern contains backward assertions (including \eb or \eB), you need
153 to arrange for some overlap in the subject strings to allow for this. For
154 example, you could pass the subject in chunks that are 500 bytes long, but in
155 a buffer of 700 bytes, with the starting offset set to 200 and the previous 200
156 bytes at the start of the buffer.
157 .P
158 3. Matching a subject string that is split into multiple segments does not
159 always produce exactly the same result as matching over one single long string.
160 The difference arises when there are multiple matching possibilities, because a
161 partial match result is given only when there are no completed matches in a
162 call to \fBpcre_dfa_exec()\fP. This means that as soon as the shortest match has
163 been found, continuation to a new subject segment is no longer possible.
164 Consider this \fBpcretest\fP example:
165 .sp
166 re> /dog(sbody)?/
167 data> do\eP\eD
168 Partial match: do
169 data> gsb\eR\eP\eD
170 0: g
171 data> dogsbody\eD
172 0: dogsbody
173 1: dog
174 .sp
175 The pattern matches the words "dog" or "dogsbody". When the subject is
176 presented in several parts ("do" and "gsb" being the first two) the match stops
177 when "dog" has been found, and it is not possible to continue. On the other
178 hand, if "dogsbody" is presented as a single string, both matches are found.
179 .P
180 Because of this phenomenon, it does not usually make sense to end a pattern
181 that is going to be matched in this way with a variable repeat.
182 .P
183 4. Patterns that contain alternatives at the top level which do not all
184 start with the same pattern item may not work as expected. For example,
185 consider this pattern:
186 .sp
187 1234|3789
188 .sp
189 If the first part of the subject is "ABC123", a partial match of the first
190 alternative is found at offset 3. There is no partial match for the second
191 alternative, because such a match does not start at the same point in the
192 subject string. Attempting to continue with the string "789" does not yield a
193 match because only those alternatives that match at one point in the subject
194 are remembered. The problem arises because the start of the second alternative
195 matches within the first alternative. There is no problem with anchored
196 patterns or patterns such as:
197 .sp
198 1234|ABCD
199 .sp
200 where no string can be a partial match for both alternatives.
201 .
202 .
204 .rs
205 .sp
206 .nf
207 Philip Hazel
208 University Computing Service
209 Cambridge CB2 3QH, England.
210 .fi
211 .
212 .
214 .rs
215 .sp
216 .nf
217 Last updated: 04 June 2007
218 Copyright (c) 1997-2007 University of Cambridge.
219 .fi


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