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1 <html>
2 <head>
3 <title>pcrepartial specification</title>
4 </head>
5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 <h1>pcrepartial man page</h1>
7 <p>
8 Return to the <a href="index.html">PCRE index page</a>.
9 </p>
10 <p>
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.
14 <br>
15 <ul>
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 pcre16_exec()</a>
18 <li><a name="TOC3" href="#SEC3">PARTIAL MATCHING USING pcre_dfa_exec() OR pcre16_dfa_exec()</a>
20 <li><a name="TOC5" href="#SEC5">FORMERLY RESTRICTED PATTERNS</a>
22 <li><a name="TOC7" href="#SEC7">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre16_dfa_exec()</a>
23 <li><a name="TOC8" href="#SEC8">MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre16_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>
27 </ul>
28 <br><a name="SEC1" href="#TOC1">PARTIAL MATCHING IN PCRE</a><br>
29 <P>
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
34 match.
35 </P>
36 <P>
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:
40 <pre>
41 ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$
42 </pre>
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.
50 </P>
51 <P>
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.
59 </P>
60 <P>
61 Setting a partial matching option disables the use of any just-in-time code
62 that was set up by studying the compiled pattern with the
63 PCRE_STUDY_JIT_COMPILE option. It also disables two of PCRE's standard
64 optimizations. PCRE remembers the last literal data unit in a pattern, and
65 abandons matching immediately if it is not present in the subject string. This
66 optimization cannot be used for a subject string that might match only
67 partially. If the pattern was studied, PCRE knows the minimum length of a
68 matching string, and does not bother to run the matching function on shorter
69 strings. This optimization is also disabled for partial matching.
70 </P>
71 <br><a name="SEC2" href="#TOC1">PARTIAL MATCHING USING pcre_exec() OR pcre16_exec()</a><br>
72 <P>
73 A partial match occurs during a call to <b>pcre_exec()</b> or
74 <b>pcre16_exec()</b> when the end of the subject string is reached successfully,
75 but matching cannot continue because more characters are needed. However, at
76 least one character in the subject must have been inspected. This character
77 need not form part of the final matched string; lookbehind assertions and the
78 \K escape sequence provide ways of inspecting characters before the start of a
79 matched substring. The requirement for inspecting at least one character exists
80 because an empty string can always be matched; without such a restriction there
81 would always be a partial match of an empty string at the end of the subject.
82 </P>
83 <P>
84 If there are at least two slots in the offsets vector when a partial match is
85 returned, the first slot is set to the offset of the earliest character that
86 was inspected. For convenience, the second offset points to the end of the
87 subject so that a substring can easily be identified.
88 </P>
89 <P>
90 For the majority of patterns, the first offset identifies the start of the
91 partially matched string. However, for patterns that contain lookbehind
92 assertions, or \K, or begin with \b or \B, earlier characters have been
93 inspected while carrying out the match. For example:
94 <pre>
95 /(?&#60;=abc)123/
96 </pre>
97 This pattern matches "123", but only if it is preceded by "abc". If the subject
98 string is "xyzabc12", the offsets after a partial match are for the substring
99 "abc12", because all these characters are needed if another match is tried
100 with extra characters added to the subject.
101 </P>
102 <P>
103 What happens when a partial match is identified depends on which of the two
104 partial matching options are set.
105 </P>
106 <br><b>
107 PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre16_exec()
108 </b><br>
109 <P>
110 If PCRE_PARTIAL_SOFT is set when <b>pcre_exec()</b> or <b>pcre16_exec()</b>
111 identifies a partial match, the partial match is remembered, but matching
112 continues as normal, and other alternatives in the pattern are tried. If no
113 complete match can be found, PCRE_ERROR_PARTIAL is returned instead of
115 </P>
116 <P>
117 This option is "soft" because it prefers a complete match over a partial match.
118 All the various matching items in a pattern behave as if the subject string is
119 potentially complete. For example, \z, \Z, and $ match at the end of the
120 subject, as normal, and for \b and \B the end of the subject is treated as a
121 non-alphanumeric.
122 </P>
123 <P>
124 If there is more than one partial match, the first one that was found provides
125 the data that is returned. Consider this pattern:
126 <pre>
127 /123\w+X|dogY/
128 </pre>
129 If this is matched against the subject string "abc123dog", both
130 alternatives fail to match, but the end of the subject is reached during
131 matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set to 3 and 9,
132 identifying "123dog" as the first partial match that was found. (In this
133 example, there are two partial matches, because "dog" on its own partially
134 matches the second alternative.)
135 </P>
136 <br><b>
137 PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre16_exec()
138 </b><br>
139 <P>
140 If PCRE_PARTIAL_HARD is set for <b>pcre_exec()</b> or <b>pcre16_exec()</b>,
141 PCRE_ERROR_PARTIAL is returned as soon as a partial match is found, without
142 continuing to search for possible complete matches. This option is "hard"
143 because it prefers an earlier partial match over a later complete match. For
144 this reason, the assumption is made that the end of the supplied subject string
145 may not be the true end of the available data, and so, if \z, \Z, \b, \B,
146 or $ are encountered at the end of the subject, the result is
147 PCRE_ERROR_PARTIAL, provided that at least one character in the subject has
148 been inspected.
149 </P>
150 <P>
151 Setting PCRE_PARTIAL_HARD also affects the way UTF-8 and UTF-16
152 subject strings are checked for validity. Normally, an invalid sequence
153 causes the error PCRE_ERROR_BADUTF8 or PCRE_ERROR_BADUTF16. However, in the
154 special case of a truncated character at the end of the subject,
157 </P>
158 <br><b>
159 Comparing hard and soft partial matching
160 </b><br>
161 <P>
162 The difference between the two partial matching options can be illustrated by a
163 pattern such as:
164 <pre>
165 /dog(sbody)?/
166 </pre>
167 This matches either "dog" or "dogsbody", greedily (that is, it prefers the
168 longer string if possible). If it is matched against the string "dog" with
169 PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if
170 PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand,
171 if the pattern is made ungreedy the result is different:
172 <pre>
173 /dog(sbody)??/
174 </pre>
175 In this case the result is always a complete match because that is found first,
176 and matching never continues after finding a complete match. It might be easier
177 to follow this explanation by thinking of the two patterns like this:
178 <pre>
179 /dog(sbody)?/ is the same as /dogsbody|dog/
180 /dog(sbody)??/ is the same as /dog|dogsbody/
181 </pre>
182 The second pattern will never match "dogsbody", because it will always find the
183 shorter match first.
184 </P>
185 <br><a name="SEC3" href="#TOC1">PARTIAL MATCHING USING pcre_dfa_exec() OR pcre16_dfa_exec()</a><br>
186 <P>
187 The DFA functions move along the subject string character by character, without
188 backtracking, searching for all possible matches simultaneously. If the end of
189 the subject is reached before the end of the pattern, there is the possibility
190 of a partial match, again provided that at least one character has been
191 inspected.
192 </P>
193 <P>
194 When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there
195 have been no complete matches. Otherwise, the complete matches are returned.
196 However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any
197 complete matches. The portion of the string that was inspected when the longest
198 partial match was found is set as the first matching string, provided there are
199 at least two slots in the offsets vector.
200 </P>
201 <P>
202 Because the DFA functions always search for all possible matches, and there is
203 no difference between greedy and ungreedy repetition, their behaviour is
204 different from the standard functions when PCRE_PARTIAL_HARD is set. Consider
205 the string "dog" matched against the ungreedy pattern shown above:
206 <pre>
207 /dog(sbody)??/
208 </pre>
209 Whereas the standard functions stop as soon as they find the complete match for
210 "dog", the DFA functions also find the partial match for "dogsbody", and so
211 return that when PCRE_PARTIAL_HARD is set.
212 </P>
213 <br><a name="SEC4" href="#TOC1">PARTIAL MATCHING AND WORD BOUNDARIES</a><br>
214 <P>
215 If a pattern ends with one of sequences \b or \B, which test for word
216 boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
217 results. Consider this pattern:
218 <pre>
219 /\bcat\b/
220 </pre>
221 This matches "cat", provided there is a word boundary at either end. If the
222 subject string is "the cat", the comparison of the final "t" with a following
223 character cannot take place, so a partial match is found. However, normal
224 matching carries on, and \b matches at the end of the subject when the last
225 character is a letter, so a complete match is found. The result, therefore, is
226 <i>not</i> PCRE_ERROR_PARTIAL. Using PCRE_PARTIAL_HARD in this case does yield
227 PCRE_ERROR_PARTIAL, because then the partial match takes precedence.
228 </P>
229 <br><a name="SEC5" href="#TOC1">FORMERLY RESTRICTED PATTERNS</a><br>
230 <P>
231 For releases of PCRE prior to 8.00, because of the way certain internal
232 optimizations were implemented in the <b>pcre_exec()</b> function, the
233 PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with
234 all patterns. From release 8.00 onwards, the restrictions no longer apply, and
235 partial matching with can be requested for any pattern.
236 </P>
237 <P>
238 Items that were formerly restricted were repeated single characters and
239 repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not
240 conform to the restrictions, <b>pcre_exec()</b> returned the error code
241 PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
242 PCRE_INFO_OKPARTIAL call to <b>pcre_fullinfo()</b> to find out if a compiled
243 pattern can be used for partial matching now always returns 1.
244 </P>
245 <br><a name="SEC6" href="#TOC1">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a><br>
246 <P>
247 If the escape sequence \P is present in a <b>pcretest</b> data line, the
248 PCRE_PARTIAL_SOFT option is used for the match. Here is a run of <b>pcretest</b>
249 that uses the date example quoted above:
250 <pre>
251 re&#62; /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
252 data&#62; 25jun04\P
253 0: 25jun04
254 1: jun
255 data&#62; 25dec3\P
256 Partial match: 23dec3
257 data&#62; 3ju\P
258 Partial match: 3ju
259 data&#62; 3juj\P
260 No match
261 data&#62; j\P
262 No match
263 </pre>
264 The first data string is matched completely, so <b>pcretest</b> shows the
265 matched substrings. The remaining four strings do not match the complete
266 pattern, but the first two are partial matches. Similar output is obtained
267 if DFA matching is used.
268 </P>
269 <P>
270 If the escape sequence \P is present more than once in a <b>pcretest</b> data
271 line, the PCRE_PARTIAL_HARD option is set for the match.
272 </P>
273 <br><a name="SEC7" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre16_dfa_exec()</a><br>
274 <P>
275 When a partial match has been found using a DFA matching function, it is
276 possible to continue the match by providing additional subject data and calling
277 the function again with the same compiled regular expression, this time setting
278 the PCRE_DFA_RESTART option. You must pass the same working space as before,
279 because this is where details of the previous partial match are stored. Here is
280 an example using <b>pcretest</b>, using the \R escape sequence to set the
281 PCRE_DFA_RESTART option (\D specifies the use of the DFA matching function):
282 <pre>
283 re&#62; /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
284 data&#62; 23ja\P\D
285 Partial match: 23ja
286 data&#62; n05\R\D
287 0: n05
288 </pre>
289 The first call has "23ja" as the subject, and requests partial matching; the
290 second call has "n05" as the subject for the continued (restarted) match.
291 Notice that when the match is complete, only the last part is shown; PCRE does
292 not retain the previously partially-matched string. It is up to the calling
293 program to do that if it needs to.
294 </P>
295 <P>
296 You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
297 PCRE_DFA_RESTART to continue partial matching over multiple segments. This
298 facility can be used to pass very long subject strings to the DFA matching
299 functions.
300 </P>
301 <br><a name="SEC8" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre16_exec()</a><br>
302 <P>
303 From release 8.00, the standard matching functions can also be used to do
304 multi-segment matching. Unlike the DFA functions, it is not possible to
305 restart the previous match with a new segment of data. Instead, new data must
306 be added to the previous subject string, and the entire match re-run, starting
307 from the point where the partial match occurred. Earlier data can be discarded.
308 </P>
309 <P>
310 It is best to use PCRE_PARTIAL_HARD in this situation, because it does not
311 treat the end of a segment as the end of the subject when matching \z, \Z,
312 \b, \B, and $. Consider an unanchored pattern that matches dates:
313 <pre>
314 re&#62; /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
315 data&#62; The date is 23ja\P\P
316 Partial match: 23ja
317 </pre>
318 At this stage, an application could discard the text preceding "23ja", add on
319 text from the next segment, and call the matching function again. Unlike the
320 DFA matching functions the entire matching string must always be available, and
321 the complete matching process occurs for each call, so more memory and more
322 processing time is needed.
323 </P>
324 <P>
325 <b>Note:</b> If the pattern contains lookbehind assertions, or \K, or starts
326 with \b or \B, the string that is returned for a partial match includes
327 characters that precede the partially matched string itself, because these must
328 be retained when adding on more characters for a subsequent matching attempt.
329 </P>
330 <br><a name="SEC9" href="#TOC1">ISSUES WITH MULTI-SEGMENT MATCHING</a><br>
331 <P>
332 Certain types of pattern may give problems with multi-segment matching,
333 whichever matching function is used.
334 </P>
335 <P>
336 1. If the pattern contains a test for the beginning of a line, you need to pass
337 the PCRE_NOTBOL option when the subject string for any call does start at the
338 beginning of a line. There is also a PCRE_NOTEOL option, but in practice when
339 doing multi-segment matching you should be using PCRE_PARTIAL_HARD, which
340 includes the effect of PCRE_NOTEOL.
341 </P>
342 <P>
343 2. Lookbehind assertions at the start of a pattern are catered for in the
344 offsets that are returned for a partial match. However, in theory, a lookbehind
345 assertion later in the pattern could require even earlier characters to be
346 inspected, and it might not have been reached when a partial match occurs. This
347 is probably an extremely unlikely case; you could guard against it to a certain
348 extent by always including extra characters at the start.
349 </P>
350 <P>
351 3. Matching a subject string that is split into multiple segments may not
352 always produce exactly the same result as matching over one single long string,
353 especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
354 Word Boundaries" above describes an issue that arises if the pattern ends with
355 \b or \B. Another kind of difference may occur when there are multiple
356 matching possibilities, because (for PCRE_PARTIAL_SOFT) a partial match result
357 is given only when there are no completed matches. This means that as soon as
358 the shortest match has been found, continuation to a new subject segment is no
359 longer possible. Consider again this <b>pcretest</b> example:
360 <pre>
361 re&#62; /dog(sbody)?/
362 data&#62; dogsb\P
363 0: dog
364 data&#62; do\P\D
365 Partial match: do
366 data&#62; gsb\R\P\D
367 0: g
368 data&#62; dogsbody\D
369 0: dogsbody
370 1: dog
371 </pre>
372 The first data line passes the string "dogsb" to a standard matching function,
373 setting the PCRE_PARTIAL_SOFT option. Although the string is a partial match
374 for "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter
375 string "dog" is a complete match. Similarly, when the subject is presented to
376 a DFA matching function in several parts ("do" and "gsb" being the first two)
377 the match stops when "dog" has been found, and it is not possible to continue.
378 On the other hand, if "dogsbody" is presented as a single string, a DFA
379 matching function finds both matches.
380 </P>
381 <P>
382 Because of these problems, it is best to use PCRE_PARTIAL_HARD when matching
383 multi-segment data. The example above then behaves differently:
384 <pre>
385 re&#62; /dog(sbody)?/
386 data&#62; dogsb\P\P
387 Partial match: dogsb
388 data&#62; do\P\D
389 Partial match: do
390 data&#62; gsb\R\P\P\D
391 Partial match: gsb
392 </pre>
393 4. Patterns that contain alternatives at the top level which do not all start
394 with the same pattern item may not work as expected when PCRE_DFA_RESTART is
395 used. For example, consider this pattern:
396 <pre>
397 1234|3789
398 </pre>
399 If the first part of the subject is "ABC123", a partial match of the first
400 alternative is found at offset 3. There is no partial match for the second
401 alternative, because such a match does not start at the same point in the
402 subject string. Attempting to continue with the string "7890" does not yield a
403 match because only those alternatives that match at one point in the subject
404 are remembered. The problem arises because the start of the second alternative
405 matches within the first alternative. There is no problem with anchored
406 patterns or patterns such as:
407 <pre>
408 1234|ABCD
409 </pre>
410 where no string can be a partial match for both alternatives. This is not a
411 problem if a standard matching function is used, because the entire match has
412 to be rerun each time:
413 <pre>
414 re&#62; /1234|3789/
415 data&#62; ABC123\P\P
416 Partial match: 123
417 data&#62; 1237890
418 0: 3789
419 </pre>
420 Of course, instead of using PCRE_DFA_RESTART, the same technique of re-running
421 the entire match can also be used with the DFA matching functions. Another
422 possibility is to work with two buffers. If a partial match at offset <i>n</i>
423 in the first buffer is followed by "no match" when PCRE_DFA_RESTART is used on
424 the second buffer, you can then try a new match starting at offset <i>n+1</i> in
425 the first buffer.
426 </P>
427 <br><a name="SEC10" href="#TOC1">AUTHOR</a><br>
428 <P>
429 Philip Hazel
430 <br>
431 University Computing Service
432 <br>
433 Cambridge CB2 3QH, England.
434 <br>
435 </P>
436 <br><a name="SEC11" href="#TOC1">REVISION</a><br>
437 <P>
438 Last updated: 21 January 2012
439 <br>
440 Copyright &copy; 1997-2012 University of Cambridge.
441 <br>
442 <p>
443 Return to the <a href="index.html">PCRE index page</a>.
444 </p>


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