/[pcre]/code/trunk/doc/html/pcrepartial.html
ViewVC logotype

Contents of /code/trunk/doc/html/pcrepartial.html

Parent Directory Parent Directory | Revision Log Revision Log


Revision 975 - (show annotations)
Sat Jun 2 11:03:06 2012 UTC (2 years, 11 months ago) by ph10
File MIME type: text/html
File size: 21640 byte(s)
Error occurred while calculating annotation data.
Document update for 8.31-RC1 test release.
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>
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 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 If you want to use partial matching with just-in-time optimized code, you must
62 call <b>pcre_study()</b> or <b>pcre16_study()</b> with one or both of these
63 options:
64 <pre>
65 PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE
66 PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
67 </pre>
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.
71 </P>
72 <P>
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.
80 </P>
81 <br><a name="SEC2" href="#TOC1">PARTIAL MATCHING USING pcre_exec() OR pcre16_exec()</a><br>
82 <P>
83 A partial match occurs during a call to <b>pcre_exec()</b> or
84 <b>pcre16_exec()</b> when the end of the subject string is reached successfully,
85 but matching cannot continue because more characters are needed. However, at
86 least one character in the subject must have been inspected. This character
87 need not form part of the final matched string; lookbehind assertions and the
88 \K escape sequence provide ways of inspecting characters before the start of a
89 matched substring. The requirement for inspecting at least one character exists
90 because an empty string can always be matched; without such a restriction there
91 would always be a partial match of an empty string at the end of the subject.
92 </P>
93 <P>
94 If there are at least two slots in the offsets vector when a partial match is
95 returned, the first slot is set to the offset of the earliest character that
96 was inspected. For convenience, the second offset points to the end of the
97 subject so that a substring can easily be identified.
98 </P>
99 <P>
100 For the majority of patterns, the first offset identifies the start of the
101 partially matched string. However, for patterns that contain lookbehind
102 assertions, or \K, or begin with \b or \B, earlier characters have been
103 inspected while carrying out the match. For example:
104 <pre>
105 /(?&#60;=abc)123/
106 </pre>
107 This pattern matches "123", but only if it is preceded by "abc". If the subject
108 string is "xyzabc12", the offsets after a partial match are for the substring
109 "abc12", because all these characters are needed if another match is tried
110 with extra characters added to the subject.
111 </P>
112 <P>
113 What happens when a partial match is identified depends on which of the two
114 partial matching options are set.
115 </P>
116 <br><b>
117 PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre16_exec()
118 </b><br>
119 <P>
120 If PCRE_PARTIAL_SOFT is set when <b>pcre_exec()</b> or <b>pcre16_exec()</b>
121 identifies a partial match, the partial match is remembered, but matching
122 continues as normal, and other alternatives in the pattern are tried. If no
123 complete match can be found, PCRE_ERROR_PARTIAL is returned instead of
124 PCRE_ERROR_NOMATCH.
125 </P>
126 <P>
127 This option is "soft" because it prefers a complete match over a partial match.
128 All the various matching items in a pattern behave as if the subject string is
129 potentially complete. For example, \z, \Z, and $ match at the end of the
130 subject, as normal, and for \b and \B the end of the subject is treated as a
131 non-alphanumeric.
132 </P>
133 <P>
134 If there is more than one partial match, the first one that was found provides
135 the data that is returned. Consider this pattern:
136 <pre>
137 /123\w+X|dogY/
138 </pre>
139 If this is matched against the subject string "abc123dog", both
140 alternatives fail to match, but the end of the subject is reached during
141 matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set to 3 and 9,
142 identifying "123dog" as the first partial match that was found. (In this
143 example, there are two partial matches, because "dog" on its own partially
144 matches the second alternative.)
145 </P>
146 <br><b>
147 PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre16_exec()
148 </b><br>
149 <P>
150 If PCRE_PARTIAL_HARD is set for <b>pcre_exec()</b> or <b>pcre16_exec()</b>,
151 PCRE_ERROR_PARTIAL is returned as soon as a partial match is found, without
152 continuing to search for possible complete matches. This option is "hard"
153 because it prefers an earlier partial match over a later complete match. For
154 this reason, the assumption is made that the end of the supplied subject string
155 may not be the true end of the available data, and so, if \z, \Z, \b, \B,
156 or $ are encountered at the end of the subject, the result is
157 PCRE_ERROR_PARTIAL, provided that at least one character in the subject has
158 been inspected.
159 </P>
160 <P>
161 Setting PCRE_PARTIAL_HARD also affects the way UTF-8 and UTF-16
162 subject strings are checked for validity. Normally, an invalid sequence
163 causes the error PCRE_ERROR_BADUTF8 or PCRE_ERROR_BADUTF16. However, in the
164 special case of a truncated character at the end of the subject,
165 PCRE_ERROR_SHORTUTF8 or PCRE_ERROR_SHORTUTF16 is returned when
166 PCRE_PARTIAL_HARD is set.
167 </P>
168 <br><b>
169 Comparing hard and soft partial matching
170 </b><br>
171 <P>
172 The difference between the two partial matching options can be illustrated by a
173 pattern such as:
174 <pre>
175 /dog(sbody)?/
176 </pre>
177 This matches either "dog" or "dogsbody", greedily (that is, it prefers the
178 longer string if possible). If it is matched against the string "dog" with
179 PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if
180 PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand,
181 if the pattern is made ungreedy the result is different:
182 <pre>
183 /dog(sbody)??/
184 </pre>
185 In this case the result is always a complete match because that is found first,
186 and matching never continues after finding a complete match. It might be easier
187 to follow this explanation by thinking of the two patterns like this:
188 <pre>
189 /dog(sbody)?/ is the same as /dogsbody|dog/
190 /dog(sbody)??/ is the same as /dog|dogsbody/
191 </pre>
192 The second pattern will never match "dogsbody", because it will always find the
193 shorter match first.
194 </P>
195 <br><a name="SEC3" href="#TOC1">PARTIAL MATCHING USING pcre_dfa_exec() OR pcre16_dfa_exec()</a><br>
196 <P>
197 The DFA functions move along the subject string character by character, without
198 backtracking, searching for all possible matches simultaneously. If the end of
199 the subject is reached before the end of the pattern, there is the possibility
200 of a partial match, again provided that at least one character has been
201 inspected.
202 </P>
203 <P>
204 When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there
205 have been no complete matches. Otherwise, the complete matches are returned.
206 However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any
207 complete matches. The portion of the string that was inspected when the longest
208 partial match was found is set as the first matching string, provided there are
209 at least two slots in the offsets vector.
210 </P>
211 <P>
212 Because the DFA functions always search for all possible matches, and there is
213 no difference between greedy and ungreedy repetition, their behaviour is
214 different from the standard functions when PCRE_PARTIAL_HARD is set. Consider
215 the string "dog" matched against the ungreedy pattern shown above:
216 <pre>
217 /dog(sbody)??/
218 </pre>
219 Whereas the standard functions stop as soon as they find the complete match for
220 "dog", the DFA functions also find the partial match for "dogsbody", and so
221 return that when PCRE_PARTIAL_HARD is set.
222 </P>
223 <br><a name="SEC4" href="#TOC1">PARTIAL MATCHING AND WORD BOUNDARIES</a><br>
224 <P>
225 If a pattern ends with one of sequences \b or \B, which test for word
226 boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
227 results. Consider this pattern:
228 <pre>
229 /\bcat\b/
230 </pre>
231 This matches "cat", provided there is a word boundary at either end. If the
232 subject string is "the cat", the comparison of the final "t" with a following
233 character cannot take place, so a partial match is found. However, normal
234 matching carries on, and \b matches at the end of the subject when the last
235 character is a letter, so a complete match is found. The result, therefore, is
236 <i>not</i> PCRE_ERROR_PARTIAL. Using PCRE_PARTIAL_HARD in this case does yield
237 PCRE_ERROR_PARTIAL, because then the partial match takes precedence.
238 </P>
239 <br><a name="SEC5" href="#TOC1">FORMERLY RESTRICTED PATTERNS</a><br>
240 <P>
241 For releases of PCRE prior to 8.00, because of the way certain internal
242 optimizations were implemented in the <b>pcre_exec()</b> function, the
243 PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with
244 all patterns. From release 8.00 onwards, the restrictions no longer apply, and
245 partial matching with can be requested for any pattern.
246 </P>
247 <P>
248 Items that were formerly restricted were repeated single characters and
249 repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not
250 conform to the restrictions, <b>pcre_exec()</b> returned the error code
251 PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
252 PCRE_INFO_OKPARTIAL call to <b>pcre_fullinfo()</b> to find out if a compiled
253 pattern can be used for partial matching now always returns 1.
254 </P>
255 <br><a name="SEC6" href="#TOC1">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a><br>
256 <P>
257 If the escape sequence \P is present in a <b>pcretest</b> data line, the
258 PCRE_PARTIAL_SOFT option is used for the match. Here is a run of <b>pcretest</b>
259 that uses the date example quoted above:
260 <pre>
261 re&#62; /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
262 data&#62; 25jun04\P
263 0: 25jun04
264 1: jun
265 data&#62; 25dec3\P
266 Partial match: 23dec3
267 data&#62; 3ju\P
268 Partial match: 3ju
269 data&#62; 3juj\P
270 No match
271 data&#62; j\P
272 No match
273 </pre>
274 The first data string is matched completely, so <b>pcretest</b> shows the
275 matched substrings. The remaining four strings do not match the complete
276 pattern, but the first two are partial matches. Similar output is obtained
277 if DFA matching is used.
278 </P>
279 <P>
280 If the escape sequence \P is present more than once in a <b>pcretest</b> data
281 line, the PCRE_PARTIAL_HARD option is set for the match.
282 </P>
283 <br><a name="SEC7" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre16_dfa_exec()</a><br>
284 <P>
285 When a partial match has been found using a DFA matching function, it is
286 possible to continue the match by providing additional subject data and calling
287 the function again with the same compiled regular expression, this time setting
288 the PCRE_DFA_RESTART option. You must pass the same working space as before,
289 because this is where details of the previous partial match are stored. Here is
290 an example using <b>pcretest</b>, using the \R escape sequence to set the
291 PCRE_DFA_RESTART option (\D specifies the use of the DFA matching function):
292 <pre>
293 re&#62; /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
294 data&#62; 23ja\P\D
295 Partial match: 23ja
296 data&#62; n05\R\D
297 0: n05
298 </pre>
299 The first call has "23ja" as the subject, and requests partial matching; the
300 second call has "n05" as the subject for the continued (restarted) match.
301 Notice that when the match is complete, only the last part is shown; PCRE does
302 not retain the previously partially-matched string. It is up to the calling
303 program to do that if it needs to.
304 </P>
305 <P>
306 You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
307 PCRE_DFA_RESTART to continue partial matching over multiple segments. This
308 facility can be used to pass very long subject strings to the DFA matching
309 functions.
310 </P>
311 <br><a name="SEC8" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre16_exec()</a><br>
312 <P>
313 From release 8.00, the standard matching functions can also be used to do
314 multi-segment matching. Unlike the DFA functions, it is not possible to
315 restart the previous match with a new segment of data. Instead, new data must
316 be added to the previous subject string, and the entire match re-run, starting
317 from the point where the partial match occurred. Earlier data can be discarded.
318 </P>
319 <P>
320 It is best to use PCRE_PARTIAL_HARD in this situation, because it does not
321 treat the end of a segment as the end of the subject when matching \z, \Z,
322 \b, \B, and $. Consider an unanchored pattern that matches dates:
323 <pre>
324 re&#62; /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
325 data&#62; The date is 23ja\P\P
326 Partial match: 23ja
327 </pre>
328 At this stage, an application could discard the text preceding "23ja", add on
329 text from the next segment, and call the matching function again. Unlike the
330 DFA matching functions, the entire matching string must always be available,
331 and the complete matching process occurs for each call, so more memory and more
332 processing time is needed.
333 </P>
334 <P>
335 <b>Note:</b> If the pattern contains lookbehind assertions, or \K, or starts
336 with \b or \B, the string that is returned for a partial match includes
337 characters that precede the partially matched string itself, because these must
338 be retained when adding on more characters for a subsequent matching attempt.
339 However, in some cases you may need to retain even earlier characters, as
340 discussed in the next section.
341 </P>
342 <br><a name="SEC9" href="#TOC1">ISSUES WITH MULTI-SEGMENT MATCHING</a><br>
343 <P>
344 Certain types of pattern may give problems with multi-segment matching,
345 whichever matching function is used.
346 </P>
347 <P>
348 1. If the pattern contains a test for the beginning of a line, you need to pass
349 the PCRE_NOTBOL option when the subject string for any call does start at the
350 beginning of a line. There is also a PCRE_NOTEOL option, but in practice when
351 doing multi-segment matching you should be using PCRE_PARTIAL_HARD, which
352 includes the effect of PCRE_NOTEOL.
353 </P>
354 <P>
355 2. Lookbehind assertions that have already been obeyed are catered for in the
356 offsets that are returned for a partial match. However a lookbehind assertion
357 later in the pattern could require even earlier characters to be inspected. You
358 can handle this case by using the PCRE_INFO_MAXLOOKBEHIND option of the
359 <b>pcre_fullinfo()</b> or <b>pcre16_fullinfo()</b> functions to obtain the length
360 of the largest lookbehind in the pattern. This length is given in characters,
361 not bytes. If you always retain at least that many characters before the
362 partially matched string, all should be well. (Of course, near the start of the
363 subject, fewer characters may be present; in that case all characters should be
364 retained.)
365 </P>
366 <P>
367 3. Because a partial match must always contain at least one character, what
368 might be considered a partial match of an empty string actually gives a "no
369 match" result. For example:
370 <pre>
371 re&#62; /c(?&#60;=abc)x/
372 data&#62; ab\P
373 No match
374 </pre>
375 If the next segment begins "cx", a match should be found, but this will only
376 happen if characters from the previous segment are retained. For this reason, a
377 "no match" result should be interpreted as "partial match of an empty string"
378 when the pattern contains lookbehinds.
379 </P>
380 <P>
381 4. Matching a subject string that is split into multiple segments may not
382 always produce exactly the same result as matching over one single long string,
383 especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
384 Word Boundaries" above describes an issue that arises if the pattern ends with
385 \b or \B. Another kind of difference may occur when there are multiple
386 matching possibilities, because (for PCRE_PARTIAL_SOFT) a partial match result
387 is given only when there are no completed matches. This means that as soon as
388 the shortest match has been found, continuation to a new subject segment is no
389 longer possible. Consider again this <b>pcretest</b> example:
390 <pre>
391 re&#62; /dog(sbody)?/
392 data&#62; dogsb\P
393 0: dog
394 data&#62; do\P\D
395 Partial match: do
396 data&#62; gsb\R\P\D
397 0: g
398 data&#62; dogsbody\D
399 0: dogsbody
400 1: dog
401 </pre>
402 The first data line passes the string "dogsb" to a standard matching function,
403 setting the PCRE_PARTIAL_SOFT option. Although the string is a partial match
404 for "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter
405 string "dog" is a complete match. Similarly, when the subject is presented to
406 a DFA matching function in several parts ("do" and "gsb" being the first two)
407 the match stops when "dog" has been found, and it is not possible to continue.
408 On the other hand, if "dogsbody" is presented as a single string, a DFA
409 matching function finds both matches.
410 </P>
411 <P>
412 Because of these problems, it is best to use PCRE_PARTIAL_HARD when matching
413 multi-segment data. The example above then behaves differently:
414 <pre>
415 re&#62; /dog(sbody)?/
416 data&#62; dogsb\P\P
417 Partial match: dogsb
418 data&#62; do\P\D
419 Partial match: do
420 data&#62; gsb\R\P\P\D
421 Partial match: gsb
422 </pre>
423 5. Patterns that contain alternatives at the top level which do not all start
424 with the same pattern item may not work as expected when PCRE_DFA_RESTART is
425 used. For example, consider this pattern:
426 <pre>
427 1234|3789
428 </pre>
429 If the first part of the subject is "ABC123", a partial match of the first
430 alternative is found at offset 3. There is no partial match for the second
431 alternative, because such a match does not start at the same point in the
432 subject string. Attempting to continue with the string "7890" does not yield a
433 match because only those alternatives that match at one point in the subject
434 are remembered. The problem arises because the start of the second alternative
435 matches within the first alternative. There is no problem with anchored
436 patterns or patterns such as:
437 <pre>
438 1234|ABCD
439 </pre>
440 where no string can be a partial match for both alternatives. This is not a
441 problem if a standard matching function is used, because the entire match has
442 to be rerun each time:
443 <pre>
444 re&#62; /1234|3789/
445 data&#62; ABC123\P\P
446 Partial match: 123
447 data&#62; 1237890
448 0: 3789
449 </pre>
450 Of course, instead of using PCRE_DFA_RESTART, the same technique of re-running
451 the entire match can also be used with the DFA matching functions. Another
452 possibility is to work with two buffers. If a partial match at offset <i>n</i>
453 in the first buffer is followed by "no match" when PCRE_DFA_RESTART is used on
454 the second buffer, you can then try a new match starting at offset <i>n+1</i> in
455 the first buffer.
456 </P>
457 <br><a name="SEC10" href="#TOC1">AUTHOR</a><br>
458 <P>
459 Philip Hazel
460 <br>
461 University Computing Service
462 <br>
463 Cambridge CB2 3QH, England.
464 <br>
465 </P>
466 <br><a name="SEC11" href="#TOC1">REVISION</a><br>
467 <P>
468 Last updated: 24 February 2012
469 <br>
470 Copyright &copy; 1997-2012 University of Cambridge.
471 <br>
472 <p>
473 Return to the <a href="index.html">PCRE index page</a>.
474 </p>

Properties

Name Value
svn:eol-style native
svn:keywords "Author Date Id Revision Url"

  ViewVC Help
Powered by ViewVC 1.1.5