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Further partial match change: add PCRE_PARTIAL_HARD and make more intuitive.
1 .TH PCREPARTIAL 3
2 .SH NAME
3 PCRE - Perl-compatible regular expressions
4 .SH "PARTIAL MATCHING IN PCRE"
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, by beeping and not reflecting the character that
22 has been typed, for example. 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. Partial matching can also sometimes be useful when the subject string
25 is very long and is not all available at once.
26 .P
27 PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and
28 PCRE_PARTIAL_HARD options, which can be set when calling \fBpcre_exec()\fP or
29 \fBpcre_dfa_exec()\fP. For backwards compatibility, PCRE_PARTIAL is a synonym
30 for PCRE_PARTIAL_SOFT. The essential difference between the two options is
31 whether or not a partial match is preferred to an alternative complete match,
32 though the details differ between the two matching functions. If both options
33 are set, PCRE_PARTIAL_HARD takes precedence.
34 .P
35 Setting a partial matching option disables one of PCRE's optimizations. PCRE
36 remembers the last literal byte in a pattern, and abandons matching immediately
37 if such a byte is not present in the subject string. This optimization cannot
38 be used for a subject string that might match only partially.
39 .
40 .
41 .SH "PARTIAL MATCHING USING pcre_exec()"
42 .rs
43 .sp
44 A partial match occurs during a call to \fBpcre_exec()\fP whenever the end of
45 the subject string is reached successfully, but matching cannot continue
46 because more characters are needed. However, at least one character must have
47 been matched. (In other words, a partial match can never be an empty string.)
48 .P
49 If PCRE_PARTIAL_SOFT is set, the partial match is remembered, but matching
50 continues as normal, and other alternatives in the pattern are tried. If no
51 complete match can be found, \fBpcre_exec()\fP returns PCRE_ERROR_PARTIAL
52 instead of PCRE_ERROR_NOMATCH, and if there are at least two slots in the
53 offsets vector, they are filled in with the offsets of the longest string that
54 partially matched. Consider this pattern:
55 .sp
56 /123\ew+X|dogY/
57 .sp
58 If this is matched against the subject string "abc123dog", both
59 alternatives fail to match, but the end of the subject is reached during
60 matching, so PCRE_ERROR_PARTIAL is returned instead of PCRE_ERROR_NOMATCH. The
61 offsets are set to 3 and 9, identifying "123dog" as the longest partial match
62 that was found. (In this example, there are two partial matches, because "dog"
63 on its own partially matches the second alternative.)
64 .P
65 If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP, it returns
66 PCRE_ERROR_PARTIAL as soon as a partial match is found, without continuing to
67 search for possible complete matches. The difference between the two options
68 can be illustrated by a pattern such as:
69 .sp
70 /dog(sbody)?/
71 .sp
72 This matches either "dog" or "dogsbody", greedily (that is, it prefers the
73 longer string if possible). If it is matched against the string "dog" with
74 PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if
75 PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand,
76 if the pattern is made ungreedy the result is different:
77 .sp
78 /dog(sbody)??/
79 .sp
80 In this case the result is always a complete match because \fBpcre_exec()\fP
81 finds that first, and it never continues after finding a match. It might be
82 easier to follow this explanation by thinking of the two patterns like this:
83 .sp
84 /dog(sbody)?/ is the same as /dogsbody|dog/
85 /dog(sbody)??/ is the same as /dog|dogsbody/
86 .sp
87 The second pattern will never match "dogsbody" when \fBpcre_exec()\fP is
88 used, because it will always find the shorter match first.
89 .
90 .
91 .SH "PARTIAL MATCHING USING pcre_dfa_exec()"
92 .rs
93 .sp
94 The \fBpcre_dfa_exec()\fP function moves along the subject string character by
95 character, without backtracking, searching for all possible matches
96 simultaneously. If the end of the subject is reached before the end of the
97 pattern, there is the possibility of a partial match, again provided that at
98 least one character has matched.
99 .P
100 When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there
101 have been no complete matches. Otherwise, the complete matches are returned.
102 However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any
103 complete matches. The portion of the string that provided the longest partial
104 match is set as the first matching string, provided there are at least two
105 slots in the offsets vector.
106 .P
107 Because \fBpcre_dfa_exec()\fP always searches for all possible matches, and
108 there is no difference between greedy and ungreedy repetition, its behaviour is
109 different from \fBpcre_exec\fP when PCRE_PARTIAL_HARD is set. Consider the
110 string "dog" matched against the ungreedy pattern shown above:
111 .sp
112 /dog(sbody)??/
113 .sp
114 Whereas \fBpcre_exec()\fP stops as soon as it finds the complete match for
115 "dog", \fBpcre_dfa_exec()\fP also finds the partial match for "dogsbody", and
116 so returns that when PCRE_PARTIAL_HARD is set.
117 .
118 .
119 .SH "PARTIAL MATCHING AND WORD BOUNDARIES"
120 .rs
121 .sp
122 If a pattern ends with one of sequences \ew or \eW, which test for word
123 boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
124 results. Consider this pattern:
125 .sp
126 /\ebcat\eb/
127 .sp
128 This matches "cat", provided there is a word boundary at either end. If the
129 subject string is "the cat", the comparison of the final "t" with a following
130 character cannot take place, so a partial match is found. However,
131 \fBpcre_exec()\fP carries on with normal matching, which matches \eb at the end
132 of the subject when the last character is a letter, thus finding a complete
133 match. The result, therefore, is \fInot\fP PCRE_ERROR_PARTIAL. The same thing
134 happens with \fBpcre_dfa_exec()\fP, because it also finds the complete match.
135 .P
136 Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because
137 then the partial match takes precedence.
138 .
139 .
140 .SH "FORMERLY RESTRICTED PATTERNS"
141 .rs
142 .sp
143 For releases of PCRE prior to 8.00, because of the way certain internal
144 optimizations were implemented in the \fBpcre_exec()\fP function, the
145 PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with
146 all patterns. From release 8.00 onwards, the restrictions no longer apply, and
147 partial matching with \fBpcre_exec()\fP can be requested for any pattern.
148 .P
149 Items that were formerly restricted were repeated single characters and
150 repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not
151 conform to the restrictions, \fBpcre_exec()\fP returned the error code
152 PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
153 PCRE_INFO_OKPARTIAL call to \fBpcre_fullinfo()\fP to find out if a compiled
154 pattern can be used for partial matching now always returns 1.
155 .
156 .
157 .SH "EXAMPLE OF PARTIAL MATCHING USING PCRETEST"
158 .rs
159 .sp
160 If the escape sequence \eP is present in a \fBpcretest\fP data line, the
161 PCRE_PARTIAL_SOFT option is used for the match. Here is a run of \fBpcretest\fP
162 that uses the date example quoted above:
163 .sp
164 re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
165 data> 25jun04\eP
166 0: 25jun04
167 1: jun
168 data> 25dec3\eP
169 Partial match: 23dec3
170 data> 3ju\eP
171 Partial match: 3ju
172 data> 3juj\eP
173 No match
174 data> j\eP
175 No match
176 .sp
177 The first data string is matched completely, so \fBpcretest\fP shows the
178 matched substrings. The remaining four strings do not match the complete
179 pattern, but the first two are partial matches. Similar output is obtained
180 when \fBpcre_dfa_exec()\fP is used.
181 .P
182 If the escape sequence \eP is present more than once in a \fBpcretest\fP data
183 line, the PCRE_PARTIAL_HARD option is set for the match.
184 .
185 .
186 .SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()"
187 .rs
188 .sp
189 When a partial match has been found using \fBpcre_dfa_exec()\fP, it is possible
190 to continue the match by providing additional subject data and calling
191 \fBpcre_dfa_exec()\fP again with the same compiled regular expression, this
192 time setting the PCRE_DFA_RESTART option. You must pass the same working
193 space as before, because this is where details of the previous partial match
194 are stored. Here is an example using \fBpcretest\fP, using the \eR escape
195 sequence to set the PCRE_DFA_RESTART option (\eD specifies the use of
196 \fBpcre_dfa_exec()\fP):
197 .sp
198 re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
199 data> 23ja\eP\eD
200 Partial match: 23ja
201 data> n05\eR\eD
202 0: n05
203 .sp
204 The first call has "23ja" as the subject, and requests partial matching; the
205 second call has "n05" as the subject for the continued (restarted) match.
206 Notice that when the match is complete, only the last part is shown; PCRE does
207 not retain the previously partially-matched string. It is up to the calling
208 program to do that if it needs to.
209 .P
210 You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
211 PCRE_DFA_RESTART to continue partial matching over multiple segments. This
212 facility can be used to pass very long subject strings to
213 \fBpcre_dfa_exec()\fP.
214 .
215 .
216 .SH "MULTI-SEGMENT MATCHING WITH pcre_exec()"
217 .rs
218 .sp
219 From release 8.00, \fBpcre_exec()\fP can also be used to do multi-segment
220 matching. Unlike \fBpcre_dfa_exec()\fP, it is not possible to restart the
221 previous match with a new segment of data. Instead, new data must be added to
222 the previous subject string, and the entire match re-run, starting from the
223 point where the partial match occurred. Earlier data can be discarded.
224 Consider an unanchored pattern that matches dates:
225 .sp
226 re> /\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed/
227 data> The date is 23ja\eP
228 Partial match: 23ja
229 .sp
230 The this stage, an application could discard the text preceding "23ja", add on
231 text from the next segment, and call \fBpcre_exec()\fP again. Unlike
232 \fBpcre_dfa_exec()\fP, the entire matching string must always be available, and
233 the complete matching process occurs for each call, so more memory and more
234 processing time is needed.
235 .
236 .
237 .SH "ISSUES WITH MULTI-SEGMENT MATCHING"
238 .rs
239 .sp
240 Certain types of pattern may give problems with multi-segment matching,
241 whichever matching function is used.
242 .P
243 1. If the pattern contains tests for the beginning or end of a line, you need
244 to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropriate, when the
245 subject string for any call does not contain the beginning or end of a line.
246 .P
247 2. If the pattern contains backward assertions (including \eb or \eB), you need
248 to arrange for some overlap in the subject strings to allow for them to be
249 correctly tested at the start of each substring. For example, using
250 \fBpcre_dfa_exec()\fP, you could pass the subject in chunks that are 500 bytes
251 long, but in a buffer of 700 bytes, with the starting offset set to 200 and the
252 previous 200 bytes at the start of the buffer.
253 .P
254 3. Matching a subject string that is split into multiple segments may not
255 always produce exactly the same result as matching over one single long string,
256 especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
257 Word Boundaries" above describes an issue that arises if the pattern ends with
258 \eb or \eB. Another kind of difference may occur when there are multiple
259 matching possibilities, because a partial match result is given only when there
260 are no completed matches. This means that as soon as the shortest match has
261 been found, continuation to a new subject segment is no longer possible.
262 Consider again this \fBpcretest\fP example:
263 .sp
264 re> /dog(sbody)?/
265 data> dogsb\eP
266 0: dog
267 data> do\eP\eD
268 Partial match: do
269 data> gsb\eR\eP\eD
270 0: g
271 data> dogsbody\eD
272 0: dogsbody
273 1: dog
274 .sp
275 The first data line passes the string "dogsb" to \fBpcre_exec()\fP, setting the
276 PCRE_PARTIAL_SOFT option. Although the string is a partial match for
277 "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter string
278 "dog" is a complete match. Similarly, when the subject is presented to
279 \fBpcre_dfa_exec()\fP in several parts ("do" and "gsb" being the first two) the
280 match stops when "dog" has been found, and it is not possible to continue. On
281 the other hand, if "dogsbody" is presented as a single string,
282 \fBpcre_dfa_exec()\fP finds both matches.
283 .P
284 Because of these problems, it is probably best to use PCRE_PARTIAL_HARD when
285 matching multi-segment data. The example above then behaves differently:
286 .sp
287 re> /dog(sbody)?/
288 data> dogsb\eP\eP
289 Partial match: dogsb
290 data> do\eP\eD
291 Partial match: do
292 data> gsb\eR\eP\eP\eD
293 Partial match: gsb
294 .sp
295 .P
296 4. Patterns that contain alternatives at the top level which do not all
297 start with the same pattern item may not work as expected when
298 \fBpcre_dfa_exec()\fP is used. For example, consider this pattern:
299 .sp
300 1234|3789
301 .sp
302 If the first part of the subject is "ABC123", a partial match of the first
303 alternative is found at offset 3. There is no partial match for the second
304 alternative, because such a match does not start at the same point in the
305 subject string. Attempting to continue with the string "7890" does not yield a
306 match because only those alternatives that match at one point in the subject
307 are remembered. The problem arises because the start of the second alternative
308 matches within the first alternative. There is no problem with anchored
309 patterns or patterns such as:
310 .sp
311 1234|ABCD
312 .sp
313 where no string can be a partial match for both alternatives. This is not a
314 problem if \fPpcre_exec()\fP is used, because the entire match has to be rerun
315 each time:
316 .sp
317 re> /1234|3789/
318 data> ABC123\eP
319 Partial match: 123
320 data> 1237890
321 0: 3789
322 .sp
323 .
324 .
325 .SH AUTHOR
326 .rs
327 .sp
328 .nf
329 Philip Hazel
330 University Computing Service
331 Cambridge CB2 3QH, England.
332 .fi
333 .
334 .
335 .SH REVISION
336 .rs
337 .sp
338 .nf
339 Last updated: 31 August 2009
340 Copyright (c) 1997-2009 University of Cambridge.
341 .fi

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