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Documentation for JIT support.

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 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 \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 for \fBpcre_exec()\fP disables the use of any
36 just-in-time code that was set up by calling \fBpcre_study()\fP with the
37 PCRE_STUDY_JIT_COMPILE option. It also disables two of PCRE's standard
38 optimizations. PCRE remembers the last literal byte in a pattern, and abandons
39 matching immediately if such a byte 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()"
47 .rs
48 .sp
49 A partial match occurs during a call to \fBpcre_exec()\fP when the end of the
50 subject string is reached successfully, but matching cannot continue because
51 more characters are needed. However, at least one character in the subject must
52 have been inspected. This character need not form part of the final matched
53 string; lookbehind assertions and the \eK escape sequence provide ways of
54 inspecting characters before the start of a matched substring. The requirement
55 for inspecting at least one character exists because an empty string can always
56 be matched; without such a restriction there would always be a partial match of
57 an empty string at the end of the subject.
58 .P
59 If there are at least two slots in the offsets vector when \fBpcre_exec()\fP
60 returns with a partial match, the first slot is set to the offset of the
61 earliest character that was inspected when the partial match was found. For
62 convenience, the second offset points to the end of the subject so that a
63 substring can easily be identified.
64 .P
65 For the majority of patterns, the first offset identifies the start of the
66 partially matched string. However, for patterns that contain lookbehind
67 assertions, or \eK, or begin with \eb or \eB, earlier characters have been
68 inspected while carrying out the match. For example:
69 .sp
70 /(?<=abc)123/
71 .sp
72 This pattern matches "123", but only if it is preceded by "abc". If the subject
73 string is "xyzabc12", the offsets after a partial match are for the substring
74 "abc12", because all these characters are needed if another match is tried
75 with extra characters added to the subject.
76 .P
77 What happens when a partial match is identified depends on which of the two
78 partial matching options are set.
79 .
80 .
81 .SS "PCRE_PARTIAL_SOFT with pcre_exec()"
82 .rs
83 .sp
84 If PCRE_PARTIAL_SOFT is set when \fBpcre_exec()\fP identifies a partial match,
85 the partial match is remembered, but matching continues as normal, and other
86 alternatives in the pattern are tried. If no complete match can be found,
87 \fBpcre_exec()\fP returns PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH.
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()"
109 .rs
110 .sp
111 If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP, it returns
112 PCRE_ERROR_PARTIAL as soon as a partial match is found, without continuing to
113 search for possible complete matches. This option is "hard" because it prefers
114 an earlier partial match over a later complete match. For this reason, the
115 assumption is made that the end of the supplied subject string may not be the
116 true end of the available data, and so, if \ez, \eZ, \eb, \eB, or $ are
117 encountered at the end of the subject, the result is PCRE_ERROR_PARTIAL.
118 .P
119 Setting PCRE_PARTIAL_HARD also affects the way \fBpcre_exec()\fP checks UTF-8
120 subject strings for validity. Normally, an invalid UTF-8 sequence causes the
121 error PCRE_ERROR_BADUTF8. However, in the special case of a truncated UTF-8
122 character at the end of the subject, PCRE_ERROR_SHORTUTF8 is returned when
123 PCRE_PARTIAL_HARD is set.
124 .
125 .
126 .SS "Comparing hard and soft partial matching"
127 .rs
128 .sp
129 The difference between the two partial matching options can be illustrated by a
130 pattern such as:
131 .sp
132 /dog(sbody)?/
133 .sp
134 This matches either "dog" or "dogsbody", greedily (that is, it prefers the
135 longer string if possible). If it is matched against the string "dog" with
136 PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if
137 PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand,
138 if the pattern is made ungreedy the result is different:
139 .sp
140 /dog(sbody)??/
141 .sp
142 In this case the result is always a complete match because \fBpcre_exec()\fP
143 finds that first, and it never continues after finding a match. It might be
144 easier to follow this explanation by thinking of the two patterns like this:
145 .sp
146 /dog(sbody)?/ is the same as /dogsbody|dog/
147 /dog(sbody)??/ is the same as /dog|dogsbody/
148 .sp
149 The second pattern will never match "dogsbody" when \fBpcre_exec()\fP is
150 used, because it will always find the shorter match first.
151 .
152 .
153 .SH "PARTIAL MATCHING USING pcre_dfa_exec()"
154 .rs
155 .sp
156 The \fBpcre_dfa_exec()\fP function moves along the subject string character by
157 character, without backtracking, searching for all possible matches
158 simultaneously. If the end of the subject is reached before the end of the
159 pattern, there is the possibility of a partial match, again provided that at
160 least one character has been inspected.
161 .P
162 When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there
163 have been no complete matches. Otherwise, the complete matches are returned.
164 However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any
165 complete matches. The portion of the string that was inspected when the longest
166 partial match was found is set as the first matching string, provided there are
167 at least two slots in the offsets vector.
168 .P
169 Because \fBpcre_dfa_exec()\fP always searches for all possible matches, and
170 there is no difference between greedy and ungreedy repetition, its behaviour is
171 different from \fBpcre_exec\fP when PCRE_PARTIAL_HARD is set. Consider the
172 string "dog" matched against the ungreedy pattern shown above:
173 .sp
174 /dog(sbody)??/
175 .sp
176 Whereas \fBpcre_exec()\fP stops as soon as it finds the complete match for
177 "dog", \fBpcre_dfa_exec()\fP also finds the partial match for "dogsbody", and
178 so returns that when PCRE_PARTIAL_HARD is set.
179 .
180 .
181 .SH "PARTIAL MATCHING AND WORD BOUNDARIES"
182 .rs
183 .sp
184 If a pattern ends with one of sequences \eb or \eB, which test for word
185 boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
186 results. Consider this pattern:
187 .sp
188 /\ebcat\eb/
189 .sp
190 This matches "cat", provided there is a word boundary at either end. If the
191 subject string is "the cat", the comparison of the final "t" with a following
192 character cannot take place, so a partial match is found. However,
193 \fBpcre_exec()\fP carries on with normal matching, which matches \eb at the end
194 of the subject when the last character is a letter, thus finding a complete
195 match. The result, therefore, is \fInot\fP PCRE_ERROR_PARTIAL. The same thing
196 happens with \fBpcre_dfa_exec()\fP, because it also finds the complete match.
197 .P
198 Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because
199 then the partial match takes precedence.
200 .
201 .
202 .SH "FORMERLY RESTRICTED PATTERNS"
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 \fBpcre_exec()\fP 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 .
219 .SH "EXAMPLE OF PARTIAL MATCHING USING PCRETEST"
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$/
227 data> 25jun04\eP
228 0: 25jun04
229 1: jun
230 data> 25dec3\eP
231 Partial match: 23dec3
232 data> 3ju\eP
233 Partial match: 3ju
234 data> 3juj\eP
235 No match
236 data> j\eP
237 No match
238 .sp
239 The first data string is matched completely, so \fBpcretest\fP shows the
240 matched substrings. The remaining four strings do not match the complete
241 pattern, but the first two are partial matches. Similar output is obtained
242 when \fBpcre_dfa_exec()\fP is used.
243 .P
244 If the escape sequence \eP is present more than once in a \fBpcretest\fP data
245 line, the PCRE_PARTIAL_HARD option is set for the match.
246 .
247 .
248 .SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()"
249 .rs
250 .sp
251 When a partial match has been found using \fBpcre_dfa_exec()\fP, it is possible
252 to continue the match by providing additional subject data and calling
253 \fBpcre_dfa_exec()\fP again with the same compiled regular expression, this
254 time setting the PCRE_DFA_RESTART option. You must pass the same working
255 space as before, because this is where details of the previous partial match
256 are stored. Here is an example using \fBpcretest\fP, using the \eR escape
257 sequence to set the PCRE_DFA_RESTART option (\eD specifies the use of
258 \fBpcre_dfa_exec()\fP):
259 .sp
260 re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
261 data> 23ja\eP\eD
262 Partial match: 23ja
263 data> n05\eR\eD
264 0: n05
265 .sp
266 The first call has "23ja" as the subject, and requests partial matching; the
267 second call has "n05" as the subject for the continued (restarted) match.
268 Notice that when the match is complete, only the last part is shown; PCRE does
269 not retain the previously partially-matched string. It is up to the calling
270 program to do that if it needs to.
271 .P
272 You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
273 PCRE_DFA_RESTART to continue partial matching over multiple segments. This
274 facility can be used to pass very long subject strings to
275 \fBpcre_dfa_exec()\fP.
276 .
277 .
278 .SH "MULTI-SEGMENT MATCHING WITH pcre_exec()"
279 .rs
280 .sp
281 From release 8.00, \fBpcre_exec()\fP can also be used to do multi-segment
282 matching. Unlike \fBpcre_dfa_exec()\fP, it is not possible to restart the
283 previous match with a new segment of data. Instead, new data must be added to
284 the previous subject string, and the entire match re-run, starting from the
285 point where the partial match occurred. Earlier data can be discarded. It is
286 best to use PCRE_PARTIAL_HARD in this situation, because it does not treat the
287 end of a segment as the end of the subject when matching \ez, \eZ, \eb, \eB,
288 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 \fBpcre_exec()\fP again. Unlike
296 \fBpcre_dfa_exec()\fP, 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 will include
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 .
306 .SH "ISSUES WITH MULTI-SEGMENT MATCHING"
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 \fBpcre_exec()\fP, setting the
347 PCRE_PARTIAL_SOFT option. Although the string is a partial match for
348 "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter string
349 "dog" is a complete match. Similarly, when the subject is presented to
350 \fBpcre_dfa_exec()\fP in several parts ("do" and "gsb" being the first two) the
351 match stops when "dog" has been found, and it is not possible to continue. On
352 the other hand, if "dogsbody" is presented as a single string,
353 \fBpcre_dfa_exec()\fP 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
367 start with the same pattern item may not work as expected when
368 PCRE_DFA_RESTART is used with \fBpcre_dfa_exec()\fP. For example, consider this
369 pattern:
370 .sp
371 1234|3789
372 .sp
373 If the first part of the subject is "ABC123", a partial match of the first
374 alternative is found at offset 3. There is no partial match for the second
375 alternative, because such a match does not start at the same point in the
376 subject string. Attempting to continue with the string "7890" does not yield a
377 match because only those alternatives that match at one point in the subject
378 are remembered. The problem arises because the start of the second alternative
379 matches within the first alternative. There is no problem with anchored
380 patterns or patterns such as:
381 .sp
382 1234|ABCD
383 .sp
384 where no string can be a partial match for both alternatives. This is not a
385 problem if \fBpcre_exec()\fP is used, because the entire match has to be rerun
386 each time:
387 .sp
388 re> /1234|3789/
389 data> ABC123\eP\eP
390 Partial match: 123
391 data> 1237890
392 0: 3789
393 .sp
394 Of course, instead of using PCRE_DFA_RESTART, the same technique of re-running
395 the entire match can also be used with \fBpcre_dfa_exec()\fP. Another
396 possibility is to work with two buffers. If a partial match at offset \fIn\fP
397 in the first buffer is followed by "no match" when PCRE_DFA_RESTART is used on
398 the second buffer, you can then try a new match starting at offset \fIn+1\fP in
399 the first buffer.
400 .
401 .
402 .SH AUTHOR
403 .rs
404 .sp
405 .nf
406 Philip Hazel
407 University Computing Service
408 Cambridge CB2 3QH, England.
409 .fi
410 .
411 .
412 .SH REVISION
413 .rs
414 .sp
415 .nf
416 Last updated: 26 August 2011
417 Copyright (c) 1997-2011 University of Cambridge.
418 .fi

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