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Revision 1347 - (show annotations)
Tue Jul 2 18:52:42 2013 UTC (7 years, 11 months ago) by ph10
File size: 21608 byte(s)
Clarify the action of PCRE_DFA_RESTART.
1 .TH PCREPARTIAL 3 "02 July 2013" "PCRE 8.34"
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 If you want to use partial matching with just-in-time optimized code, you must
36 call \fBpcre_study()\fP, \fBpcre16_study()\fP or \fBpcre32_study()\fP with one
37 or both of these options:
38 .sp
41 .sp
42 PCRE_STUDY_JIT_COMPILE should also be set if you are going to run non-partial
43 matches on the same pattern. If the appropriate JIT study mode has not been set
44 for a match, the interpretive matching code is used.
45 .P
46 Setting a partial matching option disables two of PCRE's standard
47 optimizations. PCRE remembers the last literal data unit in a pattern, and
48 abandons matching immediately if it is not present in the subject string. This
49 optimization cannot be used for a subject string that might match only
50 partially. If the pattern was studied, PCRE knows the minimum length of a
51 matching string, and does not bother to run the matching function on shorter
52 strings. This optimization is also disabled for partial matching.
53 .
54 .
55 .SH "PARTIAL MATCHING USING pcre_exec() OR pcre[16|32]_exec()"
56 .rs
57 .sp
58 A partial match occurs during a call to \fBpcre_exec()\fP or
59 \fBpcre[16|32]_exec()\fP when the end of the subject string is reached
60 successfully, but matching cannot continue because more characters are needed.
61 However, at least one character in the subject must have been inspected. This
62 character need not form part of the final matched string; lookbehind assertions
63 and the \eK escape sequence provide ways of inspecting characters before the
64 start of a matched substring. The requirement for inspecting at least one
65 character exists because an empty string can always be matched; without such a
66 restriction there would always be a partial match of an empty string at the end
67 of the subject.
68 .P
69 If there are at least two slots in the offsets vector when a partial match is
70 returned, the first slot is set to the offset of the earliest character that
71 was inspected. For convenience, the second offset points to the end of the
72 subject so that a substring can easily be identified. If there are at least
73 three slots in the offsets vector, the third slot is set to the offset of the
74 character where matching started.
75 .P
76 For the majority of patterns, the contents of the first and third slots will be
77 the same. However, for patterns that contain lookbehind assertions, or begin
78 with \eb or \eB, characters before the one where matching started may have been
79 inspected while carrying out the match. For example, consider this pattern:
80 .sp
81 /(?<=abc)123/
82 .sp
83 This pattern matches "123", but only if it is preceded by "abc". If the subject
84 string is "xyzabc12", the first two offsets after a partial match are for the
85 substring "abc12", because all these characters were inspected. However, the
86 third offset is set to 6, because that is the offset where matching began.
87 .P
88 What happens when a partial match is identified depends on which of the two
89 partial matching options are set.
90 .
91 .
92 .SS "PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre[16|32]_exec()"
93 .rs
94 .sp
95 If PCRE_PARTIAL_SOFT is set when \fBpcre_exec()\fP or \fBpcre[16|32]_exec()\fP
96 identifies a partial match, the partial match is remembered, but matching
97 continues as normal, and other alternatives in the pattern are tried. If no
98 complete match can be found, PCRE_ERROR_PARTIAL is returned instead of
100 .P
101 This option is "soft" because it prefers a complete match over a partial match.
102 All the various matching items in a pattern behave as if the subject string is
103 potentially complete. For example, \ez, \eZ, and $ match at the end of the
104 subject, as normal, and for \eb and \eB the end of the subject is treated as a
105 non-alphanumeric.
106 .P
107 If there is more than one partial match, the first one that was found provides
108 the data that is returned. Consider this pattern:
109 .sp
110 /123\ew+X|dogY/
111 .sp
112 If this is matched against the subject string "abc123dog", both
113 alternatives fail to match, but the end of the subject is reached during
114 matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set to 3 and 9,
115 identifying "123dog" as the first partial match that was found. (In this
116 example, there are two partial matches, because "dog" on its own partially
117 matches the second alternative.)
118 .
119 .
120 .SS "PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre[16|32]_exec()"
121 .rs
122 .sp
123 If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP or \fBpcre[16|32]_exec()\fP,
124 PCRE_ERROR_PARTIAL is returned as soon as a partial match is found, without
125 continuing to search for possible complete matches. This option is "hard"
126 because it prefers an earlier partial match over a later complete match. For
127 this reason, the assumption is made that the end of the supplied subject string
128 may not be the true end of the available data, and so, if \ez, \eZ, \eb, \eB,
129 or $ are encountered at the end of the subject, the result is
130 PCRE_ERROR_PARTIAL, provided that at least one character in the subject has
131 been inspected.
132 .P
133 Setting PCRE_PARTIAL_HARD also affects the way UTF-8 and UTF-16
134 subject strings are checked for validity. Normally, an invalid sequence
135 causes the error PCRE_ERROR_BADUTF8 or PCRE_ERROR_BADUTF16. However, in the
136 special case of a truncated character at the end of the subject,
139 .
140 .
141 .SS "Comparing hard and soft partial matching"
142 .rs
143 .sp
144 The difference between the two partial matching options can be illustrated by a
145 pattern such as:
146 .sp
147 /dog(sbody)?/
148 .sp
149 This matches either "dog" or "dogsbody", greedily (that is, it prefers the
150 longer string if possible). If it is matched against the string "dog" with
151 PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if
152 PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand,
153 if the pattern is made ungreedy the result is different:
154 .sp
155 /dog(sbody)??/
156 .sp
157 In this case the result is always a complete match because that is found first,
158 and matching never continues after finding a complete match. It might be easier
159 to follow this explanation by thinking of the two patterns like this:
160 .sp
161 /dog(sbody)?/ is the same as /dogsbody|dog/
162 /dog(sbody)??/ is the same as /dog|dogsbody/
163 .sp
164 The second pattern will never match "dogsbody", because it will always find the
165 shorter match first.
166 .
167 .
168 .SH "PARTIAL MATCHING USING pcre_dfa_exec() OR pcre[16|32]_dfa_exec()"
169 .rs
170 .sp
171 The DFA functions move along the subject string character by character, without
172 backtracking, searching for all possible matches simultaneously. If the end of
173 the subject is reached before the end of the pattern, there is the possibility
174 of a partial match, again provided that at least one character has been
175 inspected.
176 .P
177 When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there
178 have been no complete matches. Otherwise, the complete matches are returned.
179 However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any
180 complete matches. The portion of the string that was inspected when the longest
181 partial match was found is set as the first matching string, provided there are
182 at least two slots in the offsets vector.
183 .P
184 Because the DFA functions always search for all possible matches, and there is
185 no difference between greedy and ungreedy repetition, their behaviour is
186 different from the standard functions when PCRE_PARTIAL_HARD is set. Consider
187 the string "dog" matched against the ungreedy pattern shown above:
188 .sp
189 /dog(sbody)??/
190 .sp
191 Whereas the standard functions stop as soon as they find the complete match for
192 "dog", the DFA functions also find the partial match for "dogsbody", and so
193 return that when PCRE_PARTIAL_HARD is set.
194 .
195 .
197 .rs
198 .sp
199 If a pattern ends with one of sequences \eb or \eB, which test for word
200 boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
201 results. Consider this pattern:
202 .sp
203 /\ebcat\eb/
204 .sp
205 This matches "cat", provided there is a word boundary at either end. If the
206 subject string is "the cat", the comparison of the final "t" with a following
207 character cannot take place, so a partial match is found. However, normal
208 matching carries on, and \eb matches at the end of the subject when the last
209 character is a letter, so a complete match is found. The result, therefore, is
210 \fInot\fP PCRE_ERROR_PARTIAL. Using PCRE_PARTIAL_HARD in this case does yield
211 PCRE_ERROR_PARTIAL, because then the partial match takes precedence.
212 .
213 .
215 .rs
216 .sp
217 For releases of PCRE prior to 8.00, because of the way certain internal
218 optimizations were implemented in the \fBpcre_exec()\fP function, the
219 PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with
220 all patterns. From release 8.00 onwards, the restrictions no longer apply, and
221 partial matching with can be requested for any pattern.
222 .P
223 Items that were formerly restricted were repeated single characters and
224 repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not
225 conform to the restrictions, \fBpcre_exec()\fP returned the error code
226 PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
227 PCRE_INFO_OKPARTIAL call to \fBpcre_fullinfo()\fP to find out if a compiled
228 pattern can be used for partial matching now always returns 1.
229 .
230 .
232 .rs
233 .sp
234 If the escape sequence \eP is present in a \fBpcretest\fP data line, the
235 PCRE_PARTIAL_SOFT option is used for the match. Here is a run of \fBpcretest\fP
236 that uses the date example quoted above:
237 .sp
238 re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
239 data> 25jun04\eP
240 0: 25jun04
241 1: jun
242 data> 25dec3\eP
243 Partial match: 23dec3
244 data> 3ju\eP
245 Partial match: 3ju
246 data> 3juj\eP
247 No match
248 data> j\eP
249 No match
250 .sp
251 The first data string is matched completely, so \fBpcretest\fP shows the
252 matched substrings. The remaining four strings do not match the complete
253 pattern, but the first two are partial matches. Similar output is obtained
254 if DFA matching is used.
255 .P
256 If the escape sequence \eP is present more than once in a \fBpcretest\fP data
257 line, the PCRE_PARTIAL_HARD option is set for the match.
258 .
259 .
260 .SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre[16|32]_dfa_exec()"
261 .rs
262 .sp
263 When a partial match has been found using a DFA matching function, it is
264 possible to continue the match by providing additional subject data and calling
265 the function again with the same compiled regular expression, this time setting
266 the PCRE_DFA_RESTART option. You must pass the same working space as before,
267 because this is where details of the previous partial match are stored. Here is
268 an example using \fBpcretest\fP, using the \eR escape sequence to set the
269 PCRE_DFA_RESTART option (\eD specifies the use of the DFA matching function):
270 .sp
271 re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
272 data> 23ja\eP\eD
273 Partial match: 23ja
274 data> n05\eR\eD
275 0: n05
276 .sp
277 The first call has "23ja" as the subject, and requests partial matching; the
278 second call has "n05" as the subject for the continued (restarted) match.
279 Notice that when the match is complete, only the last part is shown; PCRE does
280 not retain the previously partially-matched string. It is up to the calling
281 program to do that if it needs to.
282 .P
283 That means that, for an unanchored pattern, if a continued match fails, it is
284 not possible to try again at a new starting point. All this facility is capable
285 of doing is continuing with the previous match attempt. In the previous
286 example, if the second set of data is "ug23" the result is no match, even
287 though there would be a match for "aug23" if the entire string were given at
288 once. Depending on the application, this may or may not be what you want.
289 The only way to allow for starting again at the next character is to retain the
290 matched part of the subject and try a new complete match.
291 .P
292 You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
293 PCRE_DFA_RESTART to continue partial matching over multiple segments. This
294 facility can be used to pass very long subject strings to the DFA matching
295 functions.
296 .
297 .
298 .SH "MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre[16|32]_exec()"
299 .rs
300 .sp
301 From release 8.00, the standard matching functions can also be used to do
302 multi-segment matching. Unlike the DFA functions, it is not possible to
303 restart the previous match with a new segment of data. Instead, new data must
304 be added to the previous subject string, and the entire match re-run, starting
305 from the point where the partial match occurred. Earlier data can be discarded.
306 .P
307 It is best to use PCRE_PARTIAL_HARD in this situation, because it does not
308 treat the end of a segment as the end of the subject when matching \ez, \eZ,
309 \eb, \eB, and $. Consider an unanchored pattern that matches dates:
310 .sp
311 re> /\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed/
312 data> The date is 23ja\eP\eP
313 Partial match: 23ja
314 .sp
315 At this stage, an application could discard the text preceding "23ja", add on
316 text from the next segment, and call the matching function again. Unlike the
317 DFA matching functions, the entire matching string must always be available,
318 and the complete matching process occurs for each call, so more memory and more
319 processing time is needed.
320 .P
321 \fBNote:\fP If the pattern contains lookbehind assertions, or \eK, or starts
322 with \eb or \eB, the string that is returned for a partial match includes
323 characters that precede the start of what would be returned for a complete
324 match, because it contains all the characters that were inspected during the
325 partial match.
326 .
327 .
329 .rs
330 .sp
331 Certain types of pattern may give problems with multi-segment matching,
332 whichever matching function is used.
333 .P
334 1. If the pattern contains a test for the beginning of a line, you need to pass
335 the PCRE_NOTBOL option when the subject string for any call does start at the
336 beginning of a line. There is also a PCRE_NOTEOL option, but in practice when
337 doing multi-segment matching you should be using PCRE_PARTIAL_HARD, which
338 includes the effect of PCRE_NOTEOL.
339 .P
340 2. Lookbehind assertions that have already been obeyed are catered for in the
341 offsets that are returned for a partial match. However a lookbehind assertion
342 later in the pattern could require even earlier characters to be inspected. You
343 can handle this case by using the PCRE_INFO_MAXLOOKBEHIND option of the
344 \fBpcre_fullinfo()\fP or \fBpcre[16|32]_fullinfo()\fP functions to obtain the
345 length of the longest lookbehind in the pattern. This length is given in
346 characters, not bytes. If you always retain at least that many characters
347 before the partially matched string, all should be well. (Of course, near the
348 start of the subject, fewer characters may be present; in that case all
349 characters should be retained.)
350 .P
351 From release 8.33, there is a more accurate way of deciding which characters to
352 retain. Instead of subtracting the length of the longest lookbehind from the
353 earliest inspected character (\fIoffsets[0]\fP), the match start position
354 (\fIoffsets[2]\fP) should be used, and the next match attempt started at the
355 \fIoffsets[2]\fP character by setting the \fIstartoffset\fP argument of
356 \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP.
357 .P
358 For example, if the pattern "(?<=123)abc" is partially
359 matched against the string "xx123a", the three offset values returned are 2, 6,
360 and 5. This indicates that the matching process that gave a partial match
361 started at offset 5, but the characters "123a" were all inspected. The maximum
362 lookbehind for that pattern is 3, so taking that away from 5 shows that we need
363 only keep "123a", and the next match attempt can be started at offset 3 (that
364 is, at "a") when further characters have been added. When the match start is
365 not the earliest inspected character, \fBpcretest\fP shows it explicitly:
366 .sp
367 re> "(?<=123)abc"
368 data> xx123a\eP\eP
369 Partial match at offset 5: 123a
370 .P
371 3. Because a partial match must always contain at least one character, what
372 might be considered a partial match of an empty string actually gives a "no
373 match" result. For example:
374 .sp
375 re> /c(?<=abc)x/
376 data> ab\eP
377 No match
378 .sp
379 If the next segment begins "cx", a match should be found, but this will only
380 happen if characters from the previous segment are retained. For this reason, a
381 "no match" result should be interpreted as "partial match of an empty string"
382 when the pattern contains lookbehinds.
383 .P
384 4. Matching a subject string that is split into multiple segments may not
385 always produce exactly the same result as matching over one single long string,
386 especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
387 Word Boundaries" above describes an issue that arises if the pattern ends with
388 \eb or \eB. Another kind of difference may occur when there are multiple
389 matching possibilities, because (for PCRE_PARTIAL_SOFT) a partial match result
390 is given only when there are no completed matches. This means that as soon as
391 the shortest match has been found, continuation to a new subject segment is no
392 longer possible. Consider again this \fBpcretest\fP example:
393 .sp
394 re> /dog(sbody)?/
395 data> dogsb\eP
396 0: dog
397 data> do\eP\eD
398 Partial match: do
399 data> gsb\eR\eP\eD
400 0: g
401 data> dogsbody\eD
402 0: dogsbody
403 1: dog
404 .sp
405 The first data line passes the string "dogsb" to a standard matching function,
406 setting the PCRE_PARTIAL_SOFT option. Although the string is a partial match
407 for "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter
408 string "dog" is a complete match. Similarly, when the subject is presented to
409 a DFA matching function in several parts ("do" and "gsb" being the first two)
410 the match stops when "dog" has been found, and it is not possible to continue.
411 On the other hand, if "dogsbody" is presented as a single string, a DFA
412 matching function finds both matches.
413 .P
414 Because of these problems, it is best to use PCRE_PARTIAL_HARD when matching
415 multi-segment data. The example above then behaves differently:
416 .sp
417 re> /dog(sbody)?/
418 data> dogsb\eP\eP
419 Partial match: dogsb
420 data> do\eP\eD
421 Partial match: do
422 data> gsb\eR\eP\eP\eD
423 Partial match: gsb
424 .sp
425 5. Patterns that contain alternatives at the top level which do not all start
426 with the same pattern item may not work as expected when PCRE_DFA_RESTART is
427 used. For example, consider this pattern:
428 .sp
429 1234|3789
430 .sp
431 If the first part of the subject is "ABC123", a partial match of the first
432 alternative is found at offset 3. There is no partial match for the second
433 alternative, because such a match does not start at the same point in the
434 subject string. Attempting to continue with the string "7890" does not yield a
435 match because only those alternatives that match at one point in the subject
436 are remembered. The problem arises because the start of the second alternative
437 matches within the first alternative. There is no problem with anchored
438 patterns or patterns such as:
439 .sp
440 1234|ABCD
441 .sp
442 where no string can be a partial match for both alternatives. This is not a
443 problem if a standard matching function is used, because the entire match has
444 to be rerun each time:
445 .sp
446 re> /1234|3789/
447 data> ABC123\eP\eP
448 Partial match: 123
449 data> 1237890
450 0: 3789
451 .sp
452 Of course, instead of using PCRE_DFA_RESTART, the same technique of re-running
453 the entire match can also be used with the DFA matching functions. Another
454 possibility is to work with two buffers. If a partial match at offset \fIn\fP
455 in the first buffer is followed by "no match" when PCRE_DFA_RESTART is used on
456 the second buffer, you can then try a new match starting at offset \fIn+1\fP in
457 the first buffer.
458 .
459 .
461 .rs
462 .sp
463 .nf
464 Philip Hazel
465 University Computing Service
466 Cambridge CB2 3QH, England.
467 .fi
468 .
469 .
471 .rs
472 .sp
473 .nf
474 Last updated: 02 July 2013
475 Copyright (c) 1997-2013 University of Cambridge.
476 .fi


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