/[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 1404 - (hide annotations) (download) (as text)
Tue Nov 19 15:36:57 2013 UTC (9 months, 1 week ago) by ph10
File MIME type: text/html
File size: 23592 byte(s)
Source tidies for 8.34-RC1.

1 nigel 75 <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 ph10 111 <p>
11 nigel 75 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 ph10 111 <br>
15 nigel 75 <ul>
16     <li><a name="TOC1" href="#SEC1">PARTIAL MATCHING IN PCRE</a>
17 ph10 1194 <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 ph10 429 <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 ph10 1194 <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 ph10 429 <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 nigel 75 </ul>
28     <br><a name="SEC1" href="#TOC1">PARTIAL MATCHING IN PCRE</a><br>
29     <P>
30 ph10 869 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 nigel 75 </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 ph10 429 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 nigel 75 user interface than a check that is delayed until the entire string has been
48 ph10 567 entered. Partial matching can also be useful when the subject string is very
49     long and is not all available at once.
50 nigel 75 </P>
51     <P>
52 ph10 429 PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and
53 ph10 903 PCRE_PARTIAL_HARD options, which can be set when calling any of the matching
54 ph10 869 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 ph10 429 are set, PCRE_PARTIAL_HARD takes precedence.
59 nigel 75 </P>
60     <P>
61 ph10 975 If you want to use partial matching with just-in-time optimized code, you must
62 ph10 1194 call <b>pcre_study()</b>, <b>pcre16_study()</b> or <b>pcre32_study()</b> with one
63     or both of these options:
64 ph10 930 <pre>
65     PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE
66     PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
67     </pre>
68 ph10 975 PCRE_STUDY_JIT_COMPILE should also be set if you are going to run non-partial
69 ph10 930 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 ph10 869 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 ph10 691 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 nigel 77 </P>
81 ph10 1194 <br><a name="SEC2" href="#TOC1">PARTIAL MATCHING USING pcre_exec() OR pcre[16|32]_exec()</a><br>
82 nigel 77 <P>
83 ph10 903 A partial match occurs during a call to <b>pcre_exec()</b> or
84 ph10 1298 <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.
93 nigel 75 </P>
94     <P>
95 ph10 869 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 ph10 1335 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 ph10 1298 character where matching started.
101 ph10 453 </P>
102     <P>
103 ph10 1298 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:
107 nigel 75 <pre>
108 ph10 453 /(?&#60;=abc)123/
109     </pre>
110     This pattern matches "123", but only if it is preceded by "abc". If the subject
111 ph10 1298 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 ph10 1335 third offset is set to 6, because that is the offset where matching began.
114 ph10 453 </P>
115     <P>
116 ph10 567 What happens when a partial match is identified depends on which of the two
117 ph10 579 partial matching options are set.
118 ph10 567 </P>
119     <br><b>
120 ph10 1194 PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre[16|32]_exec()
121 ph10 567 </b><br>
122     <P>
123 ph10 1194 If PCRE_PARTIAL_SOFT is set when <b>pcre_exec()</b> or <b>pcre[16|32]_exec()</b>
124 ph10 869 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
127     PCRE_ERROR_NOMATCH.
128 ph10 567 </P>
129     <P>
130 ph10 579 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 ph10 567 potentially complete. For example, \z, \Z, and $ match at the end of the
133 ph10 579 subject, as normal, and for \b and \B the end of the subject is treated as a
134 ph10 567 non-alphanumeric.
135     </P>
136     <P>
137 ph10 453 If there is more than one partial match, the first one that was found provides
138     the data that is returned. Consider this pattern:
139     <pre>
140 ph10 429 /123\w+X|dogY/
141 nigel 75 </pre>
142 ph10 429 If this is matched against the subject string "abc123dog", both
143 ph10 453 alternatives fail to match, but the end of the subject is reached during
144 ph10 567 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.)
148 ph10 429 </P>
149 ph10 567 <br><b>
150 ph10 1194 PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre[16|32]_exec()
151 ph10 567 </b><br>
152 ph10 429 <P>
153 ph10 1194 If PCRE_PARTIAL_HARD is set for <b>pcre_exec()</b> or <b>pcre[16|32]_exec()</b>,
154 ph10 869 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 ph10 903 PCRE_ERROR_PARTIAL, provided that at least one character in the subject has
161     been inspected.
162 ph10 567 </P>
163 ph10 572 <P>
164 ph10 869 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 ph10 572 PCRE_PARTIAL_HARD is set.
170     </P>
171 ph10 567 <br><b>
172     Comparing hard and soft partial matching
173     </b><br>
174     <P>
175     The difference between the two partial matching options can be illustrated by a
176     pattern such as:
177 nigel 75 <pre>
178 ph10 429 /dog(sbody)?/
179 nigel 75 </pre>
180 ph10 453 This matches either "dog" or "dogsbody", greedily (that is, it prefers the
181 ph10 429 longer string if possible). If it is matched against the string "dog" with
182 ph10 453 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 ph10 429 if the pattern is made ungreedy the result is different:
185 nigel 75 <pre>
186 ph10 429 /dog(sbody)??/
187 nigel 75 </pre>
188 ph10 869 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:
191 ph10 429 <pre>
192     /dog(sbody)?/ is the same as /dogsbody|dog/
193     /dog(sbody)??/ is the same as /dog|dogsbody/
194     </pre>
195 ph10 869 The second pattern will never match "dogsbody", because it will always find the
196     shorter match first.
197 nigel 75 </P>
198 ph10 1194 <br><a name="SEC3" href="#TOC1">PARTIAL MATCHING USING pcre_dfa_exec() OR pcre[16|32]_dfa_exec()</a><br>
199 nigel 75 <P>
200 ph10 869 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
204     inspected.
205 nigel 75 </P>
206     <P>
207 ph10 429 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 ph10 453 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.
213 ph10 429 </P>
214     <P>
215 ph10 869 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:
219 ph10 429 <pre>
220     /dog(sbody)??/
221     </pre>
222 ph10 869 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.
225 ph10 429 </P>
226     <br><a name="SEC4" href="#TOC1">PARTIAL MATCHING AND WORD BOUNDARIES</a><br>
227     <P>
228 ph10 469 If a pattern ends with one of sequences \b or \B, which test for word
229 ph10 453 boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
230 ph10 429 results. Consider this pattern:
231     <pre>
232     /\bcat\b/
233     </pre>
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 ph10 869 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.
241 ph10 429 </P>
242     <br><a name="SEC5" href="#TOC1">FORMERLY RESTRICTED PATTERNS</a><br>
243     <P>
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 ph10 869 partial matching with can be requested for any pattern.
249 ph10 429 </P>
250     <P>
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.
257     </P>
258     <br><a name="SEC6" href="#TOC1">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a><br>
259     <P>
260 nigel 75 If the escape sequence \P is present in a <b>pcretest</b> data line, the
261 ph10 429 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:
263 nigel 75 <pre>
264     re&#62; /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
265     data&#62; 25jun04\P
266     0: 25jun04
267     1: jun
268     data&#62; 25dec3\P
269 ph10 429 Partial match: 23dec3
270 nigel 75 data&#62; 3ju\P
271 ph10 429 Partial match: 3ju
272 nigel 75 data&#62; 3juj\P
273     No match
274     data&#62; j\P
275     No match
276     </pre>
277     The first data string is matched completely, so <b>pcretest</b> shows the
278     matched substrings. The remaining four strings do not match the complete
279 ph10 429 pattern, but the first two are partial matches. Similar output is obtained
280 ph10 869 if DFA matching is used.
281 nigel 75 </P>
282     <P>
283 ph10 429 If the escape sequence \P is present more than once in a <b>pcretest</b> data
284     line, the PCRE_PARTIAL_HARD option is set for the match.
285     </P>
286 ph10 1194 <br><a name="SEC7" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre[16|32]_dfa_exec()</a><br>
287 ph10 429 <P>
288 ph10 869 When a partial match has been found using a DFA matching function, it is
289     possible to continue the match by providing additional subject data and calling
290     the function again with the same compiled regular expression, this time setting
291     the PCRE_DFA_RESTART option. You must pass the same working space as before,
292     because this is where details of the previous partial match are stored. Here is
293     an example using <b>pcretest</b>, using the \R escape sequence to set the
294     PCRE_DFA_RESTART option (\D specifies the use of the DFA matching function):
295 nigel 77 <pre>
296     re&#62; /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
297     data&#62; 23ja\P\D
298     Partial match: 23ja
299     data&#62; n05\R\D
300     0: n05
301     </pre>
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.
307     </P>
308     <P>
309 ph10 1404 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.
317     </P>
318     <P>
319 ph10 429 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 ph10 903 facility can be used to pass very long subject strings to the DFA matching
322 ph10 869 functions.
323 nigel 77 </P>
324 ph10 1194 <br><a name="SEC8" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre[16|32]_exec()</a><br>
325 nigel 77 <P>
326 ph10 869 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.
331     </P>
332     <P>
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:
336 ph10 429 <pre>
337     re&#62; /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
338 ph10 567 data&#62; The date is 23ja\P\P
339 ph10 429 Partial match: 23ja
340     </pre>
341 ph10 469 At this stage, an application could discard the text preceding "23ja", add on
342 ph10 869 text from the next segment, and call the matching function again. Unlike the
343 ph10 954 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 ph10 429 processing time is needed.
346     </P>
347 ph10 453 <P>
348     <b>Note:</b> If the pattern contains lookbehind assertions, or \K, or starts
349 ph10 869 with \b or \B, the string that is returned for a partial match includes
350 ph10 1335 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 ph10 1298 partial match.
353 ph10 453 </P>
354 ph10 429 <br><a name="SEC9" href="#TOC1">ISSUES WITH MULTI-SEGMENT MATCHING</a><br>
355     <P>
356 ph10 453 Certain types of pattern may give problems with multi-segment matching,
357 ph10 429 whichever matching function is used.
358     </P>
359     <P>
360 ph10 567 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 ph10 579 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 ph10 567 includes the effect of PCRE_NOTEOL.
365 nigel 77 </P>
366     <P>
367 ph10 954 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 ph10 975 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 ph10 1298 <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.)
377 nigel 77 </P>
378     <P>
379 ph10 1298 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[0]</i>), the match start position
382 ph10 1335 (<i>offsets[2]</i>) should be used, and the next match attempt started at the
383     <i>offsets[2]</i> character by setting the <i>startoffset</i> argument of
384 ph10 1298 <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>.
385     </P>
386     <P>
387     For example, if the pattern "(?&#60;=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 ph10 1335 is, at "a") when further characters have been added. When the match start is
394 ph10 1298 not the earliest inspected character, <b>pcretest</b> shows it explicitly:
395     <pre>
396     re&#62; "(?&#60;=123)abc"
397     data&#62; xx123a\P\P
398     Partial match at offset 5: 123a
399     </PRE>
400     </P>
401     <P>
402 ph10 954 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:
405     <pre>
406     re&#62; /c(?&#60;=abc)x/
407     data&#62; ab\P
408     No match
409     </pre>
410 ph10 975 If the next segment begins "cx", a match should be found, but this will only
411 ph10 954 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.
414     </P>
415     <P>
416     4. Matching a subject string that is split into multiple segments may not
417 ph10 429 always produce exactly the same result as matching over one single long string,
418 ph10 453 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 ph10 429 \b or \B. Another kind of difference may occur when there are multiple
421 ph10 567 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:
425 nigel 77 <pre>
426     re&#62; /dog(sbody)?/
427 ph10 429 data&#62; dogsb\P
428 ph10 453 0: dog
429 nigel 77 data&#62; do\P\D
430     Partial match: do
431     data&#62; gsb\R\P\D
432     0: g
433     data&#62; dogsbody\D
434     0: dogsbody
435     1: dog
436     </pre>
437 ph10 869 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.
445 nigel 77 </P>
446     <P>
447 ph10 567 Because of these problems, it is best to use PCRE_PARTIAL_HARD when matching
448     multi-segment data. The example above then behaves differently:
449 ph10 429 <pre>
450     re&#62; /dog(sbody)?/
451     data&#62; dogsb\P\P
452 ph10 453 Partial match: dogsb
453 ph10 429 data&#62; do\P\D
454     Partial match: do
455     data&#62; gsb\R\P\P\D
456 ph10 453 Partial match: gsb
457 ph10 572 </pre>
458 ph10 954 5. Patterns that contain alternatives at the top level which do not all start
459 ph10 869 with the same pattern item may not work as expected when PCRE_DFA_RESTART is
460     used. For example, consider this pattern:
461 nigel 87 <pre>
462     1234|3789
463     </pre>
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 ph10 429 subject string. Attempting to continue with the string "7890" does not yield a
468 nigel 87 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:
472     <pre>
473     1234|ABCD
474     </pre>
475 ph10 429 where no string can be a partial match for both alternatives. This is not a
476 ph10 869 problem if a standard matching function is used, because the entire match has
477     to be rerun each time:
478 ph10 429 <pre>
479     re&#62; /1234|3789/
480 ph10 567 data&#62; ABC123\P\P
481 ph10 429 Partial match: 123
482     data&#62; 1237890
483     0: 3789
484 ph10 469 </pre>
485 ph10 567 Of course, instead of using PCRE_DFA_RESTART, the same technique of re-running
486 ph10 869 the entire match can also be used with the DFA matching functions. Another
487 ph10 469 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.
491 nigel 87 </P>
492 ph10 429 <br><a name="SEC10" href="#TOC1">AUTHOR</a><br>
493 nigel 87 <P>
494 ph10 99 Philip Hazel
495 nigel 75 <br>
496 ph10 99 University Computing Service
497     <br>
498     Cambridge CB2 3QH, England.
499     <br>
500     </P>
501 ph10 429 <br><a name="SEC11" href="#TOC1">REVISION</a><br>
502 ph10 99 <P>
503 ph10 1404 Last updated: 02 July 2013
504 ph10 99 <br>
505 ph10 1298 Copyright &copy; 1997-2013 University of Cambridge.
506 ph10 99 <br>
507 nigel 75 <p>
508     Return to the <a href="index.html">PCRE index page</a>.
509     </p>

Properties

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

webmaster@exim.org
ViewVC Help
Powered by ViewVC 1.1.12