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3 <title>pcrepartial specification</title>
5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 <h1>pcrepartial man page</h1>
8 Return to the <a href="index.html">PCRE index page</a>.
11 This page is part of the PCRE HTML documentation. It was generated automatically
12 from the original man page. If there is any nonsense in it, please consult the
13 man page, in case the conversion went wrong.
16 <li><a name="TOC1" href="#SEC1">PARTIAL MATCHING IN PCRE</a>
17 <li><a name="TOC2" href="#SEC2">PARTIAL MATCHING USING pcre_exec() OR pcre[16|32]_exec()</a>
18 <li><a name="TOC3" href="#SEC3">PARTIAL MATCHING USING pcre_dfa_exec() OR pcre[16|32]_dfa_exec()</a>
19 <li><a name="TOC4" href="#SEC4">PARTIAL MATCHING AND WORD BOUNDARIES</a>
20 <li><a name="TOC5" href="#SEC5">FORMERLY RESTRICTED PATTERNS</a>
21 <li><a name="TOC6" href="#SEC6">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a>
22 <li><a name="TOC7" href="#SEC7">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre[16|32]_dfa_exec()</a>
23 <li><a name="TOC8" href="#SEC8">MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre[16|32]_exec()</a>
24 <li><a name="TOC9" href="#SEC9">ISSUES WITH MULTI-SEGMENT MATCHING</a>
25 <li><a name="TOC10" href="#SEC10">AUTHOR</a>
26 <li><a name="TOC11" href="#SEC11">REVISION</a>
28 <br><a name="SEC1" href="#TOC1">PARTIAL MATCHING IN PCRE</a><br>
30 In normal use of PCRE, if the subject string that is passed to a matching
31 function matches as far as it goes, but is too short to match the entire
32 pattern, PCRE_ERROR_NOMATCH is returned. There are circumstances where it might
33 be helpful to distinguish this case from other cases in which there is no
37 Consider, for example, an application where a human is required to type in data
38 for a field with specific formatting requirements. An example might be a date
39 in the form <i>ddmmmyy</i>, defined by this pattern:
43 If the application sees the user's keystrokes one by one, and can check that
44 what has been typed so far is potentially valid, it is able to raise an error
45 as soon as a mistake is made, by beeping and not reflecting the character that
46 has been typed, for example. This immediate feedback is likely to be a better
47 user interface than a check that is delayed until the entire string has been
48 entered. Partial matching can also be useful when the subject string is very
49 long and is not all available at once.
52 PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and
53 PCRE_PARTIAL_HARD options, which can be set when calling any of the matching
54 functions. For backwards compatibility, PCRE_PARTIAL is a synonym for
55 PCRE_PARTIAL_SOFT. The essential difference between the two options is whether
56 or not a partial match is preferred to an alternative complete match, though
57 the details differ between the two types of matching function. If both options
58 are set, PCRE_PARTIAL_HARD takes precedence.
61 If you want to use partial matching with just-in-time optimized code, you must
62 call <b>pcre_study()</b>, <b>pcre16_study()</b> or <b>pcre32_study()</b> with one
63 or both of these options:
68 PCRE_STUDY_JIT_COMPILE should also be set if you are going to run non-partial
69 matches on the same pattern. If the appropriate JIT study mode has not been set
70 for a match, the interpretive matching code is used.
73 Setting a partial matching option disables two of PCRE's standard
74 optimizations. PCRE remembers the last literal data unit in a pattern, and
75 abandons matching immediately if it is not present in the subject string. This
76 optimization cannot be used for a subject string that might match only
77 partially. If the pattern was studied, PCRE knows the minimum length of a
78 matching string, and does not bother to run the matching function on shorter
79 strings. This optimization is also disabled for partial matching.
81 <br><a name="SEC2" href="#TOC1">PARTIAL MATCHING USING pcre_exec() OR pcre[16|32]_exec()</a><br>
83 A partial match occurs during a call to <b>pcre_exec()</b> or
84 <b>pcre[16|32]_exec()</b> when the end of the subject string is reached
85 successfully, but matching cannot continue because more characters are needed.
86 However, at least one character in the subject must have been inspected. This
87 character need not form part of the final matched string; lookbehind assertions
88 and the \K escape sequence provide ways of inspecting characters before the
89 start of a matched substring. The requirement for inspecting at least one
90 character exists because an empty string can always be matched; without such a
91 restriction there would always be a partial match of an empty string at the end
92 of the subject.
95 If there are at least two slots in the offsets vector when a partial match is
96 returned, the first slot is set to the offset of the earliest character that
97 was inspected. For convenience, the second offset points to the end of the
98 subject so that a substring can easily be identified. If there are at least
99 three slots in the offsets vector, the third slot is set to the offset of the
100 character where matching started.
103 For the majority of patterns, the contents of the first and third slots will be
104 the same. However, for patterns that contain lookbehind assertions, or begin
105 with \b or \B, characters before the one where matching started may have been
106 inspected while carrying out the match. For example, consider this pattern:
110 This pattern matches "123", but only if it is preceded by "abc". If the subject
111 string is "xyzabc12", the first two offsets after a partial match are for the
112 substring "abc12", because all these characters were inspected. However, the
113 third offset is set to 6, because that is the offset where matching began.
116 What happens when a partial match is identified depends on which of the two
117 partial matching options are set.
120 PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre[16|32]_exec()
123 If PCRE_PARTIAL_SOFT is set when <b>pcre_exec()</b> or <b>pcre[16|32]_exec()</b>
124 identifies a partial match, the partial match is remembered, but matching
125 continues as normal, and other alternatives in the pattern are tried. If no
126 complete match can be found, PCRE_ERROR_PARTIAL is returned instead of
130 This option is "soft" because it prefers a complete match over a partial match.
131 All the various matching items in a pattern behave as if the subject string is
132 potentially complete. For example, \z, \Z, and $ match at the end of the
133 subject, as normal, and for \b and \B the end of the subject is treated as a
137 If there is more than one partial match, the first one that was found provides
138 the data that is returned. Consider this pattern:
142 If this is matched against the subject string "abc123dog", both
143 alternatives fail to match, but the end of the subject is reached during
144 matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set to 3 and 9,
145 identifying "123dog" as the first partial match that was found. (In this
146 example, there are two partial matches, because "dog" on its own partially
147 matches the second alternative.)
150 PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre[16|32]_exec()
153 If PCRE_PARTIAL_HARD is set for <b>pcre_exec()</b> or <b>pcre[16|32]_exec()</b>,
154 PCRE_ERROR_PARTIAL is returned as soon as a partial match is found, without
155 continuing to search for possible complete matches. This option is "hard"
156 because it prefers an earlier partial match over a later complete match. For
157 this reason, the assumption is made that the end of the supplied subject string
158 may not be the true end of the available data, and so, if \z, \Z, \b, \B,
159 or $ are encountered at the end of the subject, the result is
160 PCRE_ERROR_PARTIAL, provided that at least one character in the subject has
161 been inspected.
164 Setting PCRE_PARTIAL_HARD also affects the way UTF-8 and UTF-16
165 subject strings are checked for validity. Normally, an invalid sequence
166 causes the error PCRE_ERROR_BADUTF8 or PCRE_ERROR_BADUTF16. However, in the
167 special case of a truncated character at the end of the subject,
168 PCRE_ERROR_SHORTUTF8 or PCRE_ERROR_SHORTUTF16 is returned when
169 PCRE_PARTIAL_HARD is set.
172 Comparing hard and soft partial matching
175 The difference between the two partial matching options can be illustrated by a
176 pattern such as:
180 This matches either "dog" or "dogsbody", greedily (that is, it prefers the
181 longer string if possible). If it is matched against the string "dog" with
182 PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if
183 PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand,
184 if the pattern is made ungreedy the result is different:
188 In this case the result is always a complete match because that is found first,
189 and matching never continues after finding a complete match. It might be easier
190 to follow this explanation by thinking of the two patterns like this:
192 /dog(sbody)?/ is the same as /dogsbody|dog/
193 /dog(sbody)??/ is the same as /dog|dogsbody/
195 The second pattern will never match "dogsbody", because it will always find the
196 shorter match first.
198 <br><a name="SEC3" href="#TOC1">PARTIAL MATCHING USING pcre_dfa_exec() OR pcre[16|32]_dfa_exec()</a><br>
200 The DFA functions move along the subject string character by character, without
201 backtracking, searching for all possible matches simultaneously. If the end of
202 the subject is reached before the end of the pattern, there is the possibility
203 of a partial match, again provided that at least one character has been
207 When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there
208 have been no complete matches. Otherwise, the complete matches are returned.
209 However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any
210 complete matches. The portion of the string that was inspected when the longest
211 partial match was found is set as the first matching string, provided there are
212 at least two slots in the offsets vector.
215 Because the DFA functions always search for all possible matches, and there is
216 no difference between greedy and ungreedy repetition, their behaviour is
217 different from the standard functions when PCRE_PARTIAL_HARD is set. Consider
218 the string "dog" matched against the ungreedy pattern shown above:
222 Whereas the standard functions stop as soon as they find the complete match for
223 "dog", the DFA functions also find the partial match for "dogsbody", and so
224 return that when PCRE_PARTIAL_HARD is set.
226 <br><a name="SEC4" href="#TOC1">PARTIAL MATCHING AND WORD BOUNDARIES</a><br>
228 If a pattern ends with one of sequences \b or \B, which test for word
229 boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
230 results. Consider this pattern:
234 This matches "cat", provided there is a word boundary at either end. If the
235 subject string is "the cat", the comparison of the final "t" with a following
236 character cannot take place, so a partial match is found. However, normal
237 matching carries on, and \b matches at the end of the subject when the last
238 character is a letter, so a complete match is found. The result, therefore, is
239 <i>not</i> PCRE_ERROR_PARTIAL. Using PCRE_PARTIAL_HARD in this case does yield
240 PCRE_ERROR_PARTIAL, because then the partial match takes precedence.
242 <br><a name="SEC5" href="#TOC1">FORMERLY RESTRICTED PATTERNS</a><br>
244 For releases of PCRE prior to 8.00, because of the way certain internal
245 optimizations were implemented in the <b>pcre_exec()</b> function, the
246 PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with
247 all patterns. From release 8.00 onwards, the restrictions no longer apply, and
248 partial matching with can be requested for any pattern.
251 Items that were formerly restricted were repeated single characters and
252 repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not
253 conform to the restrictions, <b>pcre_exec()</b> returned the error code
254 PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
255 PCRE_INFO_OKPARTIAL call to <b>pcre_fullinfo()</b> to find out if a compiled
256 pattern can be used for partial matching now always returns 1.
258 <br><a name="SEC6" href="#TOC1">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a><br>
260 If the escape sequence \P is present in a <b>pcretest</b> data line, the
261 PCRE_PARTIAL_SOFT option is used for the match. Here is a run of <b>pcretest</b>
262 that uses the date example quoted above:
264 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
265 data> 25jun04\P
266 0: 25jun04
267 1: jun
268 data> 25dec3\P
269 Partial match: 23dec3
270 data> 3ju\P
271 Partial match: 3ju
272 data> 3juj\P
273 No match
274 data> j\P
275 No match
277 The first data string is matched completely, so <b>pcretest</b> shows the
278 matched substrings. The remaining four strings do not match the complete
279 pattern, but the first two are partial matches. Similar output is obtained
280 if DFA matching is used.
283 If the escape sequence \P is present more than once in a <b>pcretest</b> data
284 line, the PCRE_PARTIAL_HARD option is set for the match.
286 <br><a name="SEC7" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre[16|32]_dfa_exec()</a><br>
288 When a partial match has been found using a DFA matching function, it is
289 possible to continue the match by providing additional subject data and calling
290 the function again with the same compiled regular expression, this time setting
291 the PCRE_DFA_RESTART option. You must pass the same working space as before,
292 because this is where details of the previous partial match are stored. Here is
293 an example using <b>pcretest</b>, using the \R escape sequence to set the
294 PCRE_DFA_RESTART option (\D specifies the use of the DFA matching function):
296 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
297 data> 23ja\P\D
298 Partial match: 23ja
299 data> n05\R\D
300 0: n05
302 The first call has "23ja" as the subject, and requests partial matching; the
303 second call has "n05" as the subject for the continued (restarted) match.
304 Notice that when the match is complete, only the last part is shown; PCRE does
305 not retain the previously partially-matched string. It is up to the calling
306 program to do that if it needs to.
309 That means that, for an unanchored pattern, if a continued match fails, it is
310 not possible to try again at a new starting point. All this facility is capable
311 of doing is continuing with the previous match attempt. In the previous
312 example, if the second set of data is "ug23" the result is no match, even
313 though there would be a match for "aug23" if the entire string were given at
314 once. Depending on the application, this may or may not be what you want.
315 The only way to allow for starting again at the next character is to retain the
316 matched part of the subject and try a new complete match.
319 You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
320 PCRE_DFA_RESTART to continue partial matching over multiple segments. This
321 facility can be used to pass very long subject strings to the DFA matching
324 <br><a name="SEC8" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre[16|32]_exec()</a><br>
326 From release 8.00, the standard matching functions can also be used to do
327 multi-segment matching. Unlike the DFA functions, it is not possible to
328 restart the previous match with a new segment of data. Instead, new data must
329 be added to the previous subject string, and the entire match re-run, starting
330 from the point where the partial match occurred. Earlier data can be discarded.
333 It is best to use PCRE_PARTIAL_HARD in this situation, because it does not
334 treat the end of a segment as the end of the subject when matching \z, \Z,
335 \b, \B, and $. Consider an unanchored pattern that matches dates:
337 re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
338 data> The date is 23ja\P\P
339 Partial match: 23ja
341 At this stage, an application could discard the text preceding "23ja", add on
342 text from the next segment, and call the matching function again. Unlike the
343 DFA matching functions, the entire matching string must always be available,
344 and the complete matching process occurs for each call, so more memory and more
345 processing time is needed.
348 <b>Note:</b> If the pattern contains lookbehind assertions, or \K, or starts
349 with \b or \B, the string that is returned for a partial match includes
350 characters that precede the start of what would be returned for a complete
351 match, because it contains all the characters that were inspected during the
352 partial match.
354 <br><a name="SEC9" href="#TOC1">ISSUES WITH MULTI-SEGMENT MATCHING</a><br>
356 Certain types of pattern may give problems with multi-segment matching,
357 whichever matching function is used.
360 1. If the pattern contains a test for the beginning of a line, you need to pass
361 the PCRE_NOTBOL option when the subject string for any call does start at the
362 beginning of a line. There is also a PCRE_NOTEOL option, but in practice when
363 doing multi-segment matching you should be using PCRE_PARTIAL_HARD, which
364 includes the effect of PCRE_NOTEOL.
367 2. Lookbehind assertions that have already been obeyed are catered for in the
368 offsets that are returned for a partial match. However a lookbehind assertion
369 later in the pattern could require even earlier characters to be inspected. You
370 can handle this case by using the PCRE_INFO_MAXLOOKBEHIND option of the
371 <b>pcre_fullinfo()</b> or <b>pcre[16|32]_fullinfo()</b> functions to obtain the
372 length of the longest lookbehind in the pattern. This length is given in
373 characters, not bytes. If you always retain at least that many characters
374 before the partially matched string, all should be well. (Of course, near the
375 start of the subject, fewer characters may be present; in that case all
376 characters should be retained.)
379 From release 8.33, there is a more accurate way of deciding which characters to
380 retain. Instead of subtracting the length of the longest lookbehind from the
381 earliest inspected character (<i>offsets</i>), the match start position
382 (<i>offsets</i>) should be used, and the next match attempt started at the
383 <i>offsets</i> character by setting the <i>startoffset</i> argument of
384 <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>.
387 For example, if the pattern "(?<=123)abc" is partially
388 matched against the string "xx123a", the three offset values returned are 2, 6,
389 and 5. This indicates that the matching process that gave a partial match
390 started at offset 5, but the characters "123a" were all inspected. The maximum
391 lookbehind for that pattern is 3, so taking that away from 5 shows that we need
392 only keep "123a", and the next match attempt can be started at offset 3 (that
393 is, at "a") when further characters have been added. When the match start is
394 not the earliest inspected character, <b>pcretest</b> shows it explicitly:
396 re> "(?<=123)abc"
397 data> xx123a\P\P
398 Partial match at offset 5: 123a
402 3. Because a partial match must always contain at least one character, what
403 might be considered a partial match of an empty string actually gives a "no
404 match" result. For example:
406 re> /c(?<=abc)x/
407 data> ab\P
408 No match
410 If the next segment begins "cx", a match should be found, but this will only
411 happen if characters from the previous segment are retained. For this reason, a
412 "no match" result should be interpreted as "partial match of an empty string"
413 when the pattern contains lookbehinds.
416 4. Matching a subject string that is split into multiple segments may not
417 always produce exactly the same result as matching over one single long string,
418 especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
419 Word Boundaries" above describes an issue that arises if the pattern ends with
420 \b or \B. Another kind of difference may occur when there are multiple
421 matching possibilities, because (for PCRE_PARTIAL_SOFT) a partial match result
422 is given only when there are no completed matches. This means that as soon as
423 the shortest match has been found, continuation to a new subject segment is no
424 longer possible. Consider again this <b>pcretest</b> example:
426 re> /dog(sbody)?/
427 data> dogsb\P
428 0: dog
429 data> do\P\D
430 Partial match: do
431 data> gsb\R\P\D
432 0: g
433 data> dogsbody\D
434 0: dogsbody
435 1: dog
437 The first data line passes the string "dogsb" to a standard matching function,
438 setting the PCRE_PARTIAL_SOFT option. Although the string is a partial match
439 for "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter
440 string "dog" is a complete match. Similarly, when the subject is presented to
441 a DFA matching function in several parts ("do" and "gsb" being the first two)
442 the match stops when "dog" has been found, and it is not possible to continue.
443 On the other hand, if "dogsbody" is presented as a single string, a DFA
444 matching function finds both matches.
447 Because of these problems, it is best to use PCRE_PARTIAL_HARD when matching
448 multi-segment data. The example above then behaves differently:
450 re> /dog(sbody)?/
451 data> dogsb\P\P
452 Partial match: dogsb
453 data> do\P\D
454 Partial match: do
455 data> gsb\R\P\P\D
456 Partial match: gsb
458 5. Patterns that contain alternatives at the top level which do not all start
459 with the same pattern item may not work as expected when PCRE_DFA_RESTART is
460 used. For example, consider this pattern:
464 If the first part of the subject is "ABC123", a partial match of the first
465 alternative is found at offset 3. There is no partial match for the second
466 alternative, because such a match does not start at the same point in the
467 subject string. Attempting to continue with the string "7890" does not yield a
468 match because only those alternatives that match at one point in the subject
469 are remembered. The problem arises because the start of the second alternative
470 matches within the first alternative. There is no problem with anchored
471 patterns or patterns such as:
475 where no string can be a partial match for both alternatives. This is not a
476 problem if a standard matching function is used, because the entire match has
477 to be rerun each time:
479 re> /1234|3789/
480 data> ABC123\P\P
481 Partial match: 123
482 data> 1237890
483 0: 3789
485 Of course, instead of using PCRE_DFA_RESTART, the same technique of re-running
486 the entire match can also be used with the DFA matching functions. Another
487 possibility is to work with two buffers. If a partial match at offset <i>n</i>
488 in the first buffer is followed by "no match" when PCRE_DFA_RESTART is used on
489 the second buffer, you can then try a new match starting at offset <i>n+1</i> in
490 the first buffer.
492 <br><a name="SEC10" href="#TOC1">AUTHOR</a><br>
494 Philip Hazel
496 University Computing Service
498 Cambridge CB2 3QH, England.
501 <br><a name="SEC11" href="#TOC1">REVISION</a><br>
503 Last updated: 02 July 2013
505 Copyright © 1997-2013 University of Cambridge.
508 Return to the <a href="index.html">PCRE index page</a>.