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3 PCRE - Perl-compatible regular expressions
5 .rs
6 .sp
7 In normal use of PCRE, if the subject string that is passed to a matching
8 function matches as far as it goes, but is too short to match the entire
9 pattern, PCRE_ERROR_NOMATCH is returned. There are circumstances where it might
10 be helpful to distinguish this case from other cases in which there is no
11 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 be useful when the subject string is very
25 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 any of the matching
29 functions. For backwards compatibility, PCRE_PARTIAL is a synonym for
30 PCRE_PARTIAL_SOFT. The essential difference between the two options is whether
31 or not a partial match is preferred to an alternative complete match, though
32 the details differ between the two types of matching function. If both options
33 are set, PCRE_PARTIAL_HARD takes precedence.
34 .P
35 Setting a partial matching option disables the use of any just-in-time code
36 that was set up by studying the compiled pattern with the
37 PCRE_STUDY_JIT_COMPILE option. It also disables two of PCRE's standard
38 optimizations. PCRE remembers the last literal data unit in a pattern, and
39 abandons matching immediately if it is not present in the subject string. This
40 optimization cannot be used for a subject string that might match only
41 partially. If the pattern was studied, PCRE knows the minimum length of a
42 matching string, and does not bother to run the matching function on shorter
43 strings. This optimization is also disabled for partial matching.
44 .
45 .
46 .SH "PARTIAL MATCHING USING pcre_exec() OR pcre16_exec()"
47 .rs
48 .sp
49 A partial match occurs during a call to \fBpcre_exec()\fP or
50 \fBpcre16_exec()\fP when the end of the subject string is reached successfully,
51 but matching cannot continue because more characters are needed. However, at
52 least one character in the subject must have been inspected. This character
53 need not form part of the final matched string; lookbehind assertions and the
54 \eK escape sequence provide ways of inspecting characters before the start of a
55 matched substring. The requirement for inspecting at least one character exists
56 because an empty string can always be matched; without such a restriction there
57 would always be a partial match of an empty string at the end of the subject.
58 .P
59 If there are at least two slots in the offsets vector when a partial match is
60 returned, the first slot is set to the offset of the earliest character that
61 was inspected. For convenience, the second offset points to the end of the
62 subject so that a substring can easily be identified.
63 .P
64 For the majority of patterns, the first offset identifies the start of the
65 partially matched string. However, for patterns that contain lookbehind
66 assertions, or \eK, or begin with \eb or \eB, earlier characters have been
67 inspected while carrying out the match. For example:
68 .sp
69 /(?<=abc)123/
70 .sp
71 This pattern matches "123", but only if it is preceded by "abc". If the subject
72 string is "xyzabc12", the offsets after a partial match are for the substring
73 "abc12", because all these characters are needed if another match is tried
74 with extra characters added to the subject.
75 .P
76 What happens when a partial match is identified depends on which of the two
77 partial matching options are set.
78 .
79 .
80 .SS "PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre16_exec()"
81 .rs
82 .sp
83 If PCRE_PARTIAL_SOFT is set when \fBpcre_exec()\fP or \fBpcre16_exec()\fP
84 identifies a partial match, the partial match is remembered, but matching
85 continues as normal, and other alternatives in the pattern are tried. If no
86 complete match can be found, PCRE_ERROR_PARTIAL is returned instead of
88 .P
89 This option is "soft" because it prefers a complete match over a partial match.
90 All the various matching items in a pattern behave as if the subject string is
91 potentially complete. For example, \ez, \eZ, and $ match at the end of the
92 subject, as normal, and for \eb and \eB the end of the subject is treated as a
93 non-alphanumeric.
94 .P
95 If there is more than one partial match, the first one that was found provides
96 the data that is returned. Consider this pattern:
97 .sp
98 /123\ew+X|dogY/
99 .sp
100 If this is matched against the subject string "abc123dog", both
101 alternatives fail to match, but the end of the subject is reached during
102 matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set to 3 and 9,
103 identifying "123dog" as the first partial match that was found. (In this
104 example, there are two partial matches, because "dog" on its own partially
105 matches the second alternative.)
106 .
107 .
108 .SS "PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre16_exec()"
109 .rs
110 .sp
111 If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP or \fBpcre16_exec()\fP,
112 PCRE_ERROR_PARTIAL is returned as soon as a partial match is found, without
113 continuing to search for possible complete matches. This option is "hard"
114 because it prefers an earlier partial match over a later complete match. For
115 this reason, the assumption is made that the end of the supplied subject string
116 may not be the true end of the available data, and so, if \ez, \eZ, \eb, \eB,
117 or $ are encountered at the end of the subject, the result is
118 PCRE_ERROR_PARTIAL, provided that at least one character in the subject has
119 been inspected.
120 .P
121 Setting PCRE_PARTIAL_HARD also affects the way UTF-8 and UTF-16
122 subject strings are checked for validity. Normally, an invalid sequence
123 causes the error PCRE_ERROR_BADUTF8 or PCRE_ERROR_BADUTF16. However, in the
124 special case of a truncated character at the end of the subject,
127 .
128 .
129 .SS "Comparing hard and soft partial matching"
130 .rs
131 .sp
132 The difference between the two partial matching options can be illustrated by a
133 pattern such as:
134 .sp
135 /dog(sbody)?/
136 .sp
137 This matches either "dog" or "dogsbody", greedily (that is, it prefers the
138 longer string if possible). If it is matched against the string "dog" with
139 PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if
140 PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand,
141 if the pattern is made ungreedy the result is different:
142 .sp
143 /dog(sbody)??/
144 .sp
145 In this case the result is always a complete match because that is found first,
146 and matching never continues after finding a complete match. It might be easier
147 to follow this explanation by thinking of the two patterns like this:
148 .sp
149 /dog(sbody)?/ is the same as /dogsbody|dog/
150 /dog(sbody)??/ is the same as /dog|dogsbody/
151 .sp
152 The second pattern will never match "dogsbody", because it will always find the
153 shorter match first.
154 .
155 .
156 .SH "PARTIAL MATCHING USING pcre_dfa_exec() OR pcre16_dfa_exec()"
157 .rs
158 .sp
159 The DFA functions move along the subject string character by character, without
160 backtracking, searching for all possible matches simultaneously. If the end of
161 the subject is reached before the end of the pattern, there is the possibility
162 of a partial match, again provided that at least one character has been
163 inspected.
164 .P
165 When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there
166 have been no complete matches. Otherwise, the complete matches are returned.
167 However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any
168 complete matches. The portion of the string that was inspected when the longest
169 partial match was found is set as the first matching string, provided there are
170 at least two slots in the offsets vector.
171 .P
172 Because the DFA functions always search for all possible matches, and there is
173 no difference between greedy and ungreedy repetition, their behaviour is
174 different from the standard functions when PCRE_PARTIAL_HARD is set. Consider
175 the string "dog" matched against the ungreedy pattern shown above:
176 .sp
177 /dog(sbody)??/
178 .sp
179 Whereas the standard functions stop as soon as they find the complete match for
180 "dog", the DFA functions also find the partial match for "dogsbody", and so
181 return that when PCRE_PARTIAL_HARD is set.
182 .
183 .
185 .rs
186 .sp
187 If a pattern ends with one of sequences \eb or \eB, which test for word
188 boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
189 results. Consider this pattern:
190 .sp
191 /\ebcat\eb/
192 .sp
193 This matches "cat", provided there is a word boundary at either end. If the
194 subject string is "the cat", the comparison of the final "t" with a following
195 character cannot take place, so a partial match is found. However, normal
196 matching carries on, and \eb matches at the end of the subject when the last
197 character is a letter, so a complete match is found. The result, therefore, is
198 \fInot\fP PCRE_ERROR_PARTIAL. Using PCRE_PARTIAL_HARD in this case does yield
199 PCRE_ERROR_PARTIAL, because then the partial match takes precedence.
200 .
201 .
203 .rs
204 .sp
205 For releases of PCRE prior to 8.00, because of the way certain internal
206 optimizations were implemented in the \fBpcre_exec()\fP function, the
207 PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with
208 all patterns. From release 8.00 onwards, the restrictions no longer apply, and
209 partial matching with can be requested for any pattern.
210 .P
211 Items that were formerly restricted were repeated single characters and
212 repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not
213 conform to the restrictions, \fBpcre_exec()\fP returned the error code
214 PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
215 PCRE_INFO_OKPARTIAL call to \fBpcre_fullinfo()\fP to find out if a compiled
216 pattern can be used for partial matching now always returns 1.
217 .
218 .
220 .rs
221 .sp
222 If the escape sequence \eP is present in a \fBpcretest\fP data line, the
223 PCRE_PARTIAL_SOFT option is used for the match. Here is a run of \fBpcretest\fP
224 that uses the date example quoted above:
225 .sp
226 re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
227 data> 25jun04\eP
228 0: 25jun04
229 1: jun
230 data> 25dec3\eP
231 Partial match: 23dec3
232 data> 3ju\eP
233 Partial match: 3ju
234 data> 3juj\eP
235 No match
236 data> j\eP
237 No match
238 .sp
239 The first data string is matched completely, so \fBpcretest\fP shows the
240 matched substrings. The remaining four strings do not match the complete
241 pattern, but the first two are partial matches. Similar output is obtained
242 if DFA matching is used.
243 .P
244 If the escape sequence \eP is present more than once in a \fBpcretest\fP data
245 line, the PCRE_PARTIAL_HARD option is set for the match.
246 .
247 .
248 .SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre16_dfa_exec()"
249 .rs
250 .sp
251 When a partial match has been found using a DFA matching function, it is
252 possible to continue the match by providing additional subject data and calling
253 the function again with the same compiled regular expression, this time setting
254 the PCRE_DFA_RESTART option. You must pass the same working space as before,
255 because this is where details of the previous partial match are stored. Here is
256 an example using \fBpcretest\fP, using the \eR escape sequence to set the
257 PCRE_DFA_RESTART option (\eD specifies the use of the DFA matching function):
258 .sp
259 re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
260 data> 23ja\eP\eD
261 Partial match: 23ja
262 data> n05\eR\eD
263 0: n05
264 .sp
265 The first call has "23ja" as the subject, and requests partial matching; the
266 second call has "n05" as the subject for the continued (restarted) match.
267 Notice that when the match is complete, only the last part is shown; PCRE does
268 not retain the previously partially-matched string. It is up to the calling
269 program to do that if it needs to.
270 .P
271 You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
272 PCRE_DFA_RESTART to continue partial matching over multiple segments. This
273 facility can be used to pass very long subject strings to the DFA matching
274 functions.
275 .
276 .
277 .SH "MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre16_exec()"
278 .rs
279 .sp
280 From release 8.00, the standard matching functions can also be used to do
281 multi-segment matching. Unlike the DFA functions, it is not possible to
282 restart the previous match with a new segment of data. Instead, new data must
283 be added to the previous subject string, and the entire match re-run, starting
284 from the point where the partial match occurred. Earlier data can be discarded.
285 .P
286 It is best to use PCRE_PARTIAL_HARD in this situation, because it does not
287 treat the end of a segment as the end of the subject when matching \ez, \eZ,
288 \eb, \eB, and $. Consider an unanchored pattern that matches dates:
289 .sp
290 re> /\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed/
291 data> The date is 23ja\eP\eP
292 Partial match: 23ja
293 .sp
294 At this stage, an application could discard the text preceding "23ja", add on
295 text from the next segment, and call the matching function again. Unlike the
296 DFA matching functions the entire matching string must always be available, and
297 the complete matching process occurs for each call, so more memory and more
298 processing time is needed.
299 .P
300 \fBNote:\fP If the pattern contains lookbehind assertions, or \eK, or starts
301 with \eb or \eB, the string that is returned for a partial match includes
302 characters that precede the partially matched string itself, because these must
303 be retained when adding on more characters for a subsequent matching attempt.
304 .
305 .
307 .rs
308 .sp
309 Certain types of pattern may give problems with multi-segment matching,
310 whichever matching function is used.
311 .P
312 1. If the pattern contains a test for the beginning of a line, you need to pass
313 the PCRE_NOTBOL option when the subject string for any call does start at the
314 beginning of a line. There is also a PCRE_NOTEOL option, but in practice when
315 doing multi-segment matching you should be using PCRE_PARTIAL_HARD, which
316 includes the effect of PCRE_NOTEOL.
317 .P
318 2. Lookbehind assertions at the start of a pattern are catered for in the
319 offsets that are returned for a partial match. However, in theory, a lookbehind
320 assertion later in the pattern could require even earlier characters to be
321 inspected, and it might not have been reached when a partial match occurs. This
322 is probably an extremely unlikely case; you could guard against it to a certain
323 extent by always including extra characters at the start.
324 .P
325 3. Matching a subject string that is split into multiple segments may not
326 always produce exactly the same result as matching over one single long string,
327 especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
328 Word Boundaries" above describes an issue that arises if the pattern ends with
329 \eb or \eB. Another kind of difference may occur when there are multiple
330 matching possibilities, because (for PCRE_PARTIAL_SOFT) a partial match result
331 is given only when there are no completed matches. This means that as soon as
332 the shortest match has been found, continuation to a new subject segment is no
333 longer possible. Consider again this \fBpcretest\fP example:
334 .sp
335 re> /dog(sbody)?/
336 data> dogsb\eP
337 0: dog
338 data> do\eP\eD
339 Partial match: do
340 data> gsb\eR\eP\eD
341 0: g
342 data> dogsbody\eD
343 0: dogsbody
344 1: dog
345 .sp
346 The first data line passes the string "dogsb" to a standard matching function,
347 setting the PCRE_PARTIAL_SOFT option. Although the string is a partial match
348 for "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter
349 string "dog" is a complete match. Similarly, when the subject is presented to
350 a DFA matching function in several parts ("do" and "gsb" being the first two)
351 the match stops when "dog" has been found, and it is not possible to continue.
352 On the other hand, if "dogsbody" is presented as a single string, a DFA
353 matching function finds both matches.
354 .P
355 Because of these problems, it is best to use PCRE_PARTIAL_HARD when matching
356 multi-segment data. The example above then behaves differently:
357 .sp
358 re> /dog(sbody)?/
359 data> dogsb\eP\eP
360 Partial match: dogsb
361 data> do\eP\eD
362 Partial match: do
363 data> gsb\eR\eP\eP\eD
364 Partial match: gsb
365 .sp
366 4. Patterns that contain alternatives at the top level which do not all start
367 with the same pattern item may not work as expected when PCRE_DFA_RESTART is
368 used. For example, consider this pattern:
369 .sp
370 1234|3789
371 .sp
372 If the first part of the subject is "ABC123", a partial match of the first
373 alternative is found at offset 3. There is no partial match for the second
374 alternative, because such a match does not start at the same point in the
375 subject string. Attempting to continue with the string "7890" does not yield a
376 match because only those alternatives that match at one point in the subject
377 are remembered. The problem arises because the start of the second alternative
378 matches within the first alternative. There is no problem with anchored
379 patterns or patterns such as:
380 .sp
381 1234|ABCD
382 .sp
383 where no string can be a partial match for both alternatives. This is not a
384 problem if a standard matching function is used, because the entire match has
385 to be rerun each time:
386 .sp
387 re> /1234|3789/
388 data> ABC123\eP\eP
389 Partial match: 123
390 data> 1237890
391 0: 3789
392 .sp
393 Of course, instead of using PCRE_DFA_RESTART, the same technique of re-running
394 the entire match can also be used with the DFA matching functions. Another
395 possibility is to work with two buffers. If a partial match at offset \fIn\fP
396 in the first buffer is followed by "no match" when PCRE_DFA_RESTART is used on
397 the second buffer, you can then try a new match starting at offset \fIn+1\fP in
398 the first buffer.
399 .
400 .
402 .rs
403 .sp
404 .nf
405 Philip Hazel
406 University Computing Service
407 Cambridge CB2 3QH, England.
408 .fi
409 .
410 .
412 .rs
413 .sp
414 .nf
415 Last updated: 21 January 2012
416 Copyright (c) 1997-2012 University of Cambridge.
417 .fi


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