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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()</a>
18 <li><a name="TOC3" href="#SEC3">PARTIAL MATCHING USING pcre_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()</a>
23 <li><a name="TOC8" href="#SEC8">MULTI-SEGMENT MATCHING WITH pcre_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
31 <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b> matches as far as it goes, but is
32 too short to match the entire pattern, PCRE_ERROR_NOMATCH is returned. There
33 are circumstances where it might be helpful to distinguish this case from other
34 cases in which there is no 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 sometimes be useful when the subject string
49 is very 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 <b>pcre_exec()</b> or
54 <b>pcre_dfa_exec()</b>. For backwards compatibility, PCRE_PARTIAL is a synonym
55 for PCRE_PARTIAL_SOFT. The essential difference between the two options is
56 whether or not a partial match is preferred to an alternative complete match,
57 though the details differ between the two matching functions. If both options
58 are set, PCRE_PARTIAL_HARD takes precedence.
59 </P>
60 <P>
61 Setting a partial matching option disables two of PCRE's optimizations. PCRE
62 remembers the last literal byte in a pattern, and abandons matching immediately
63 if such a byte is not present in the subject string. This optimization cannot
64 be used for a subject string that might match only partially. If the pattern
65 was studied, PCRE knows the minimum length of a matching string, and does not
66 bother to run the matching function on shorter strings. This optimization is
67 also disabled for partial matching.
68 </P>
69 <br><a name="SEC2" href="#TOC1">PARTIAL MATCHING USING pcre_exec()</a><br>
70 <P>
71 A partial match occurs during a call to <b>pcre_exec()</b> whenever the end of
72 the subject string is reached successfully, but matching cannot continue
73 because more characters are needed. However, at least one character must have
74 been matched. (In other words, a partial match can never be an empty string.)
75 </P>
76 <P>
77 If PCRE_PARTIAL_SOFT is set, the partial match is remembered, but matching
78 continues as normal, and other alternatives in the pattern are tried. If no
79 complete match can be found, <b>pcre_exec()</b> returns PCRE_ERROR_PARTIAL
80 instead of PCRE_ERROR_NOMATCH. If there are at least two slots in the offsets
81 vector, the first of them is set to the offset of the earliest character that
82 was inspected when the partial match was found. For convenience, the second
83 offset points to the end of the string so that a substring can easily be
84 identified.
85 </P>
86 <P>
87 For the majority of patterns, the first offset identifies the start of the
88 partially matched string. However, for patterns that contain lookbehind
89 assertions, or \K, or begin with \b or \B, earlier characters have been
90 inspected while carrying out the match. For example:
91 <pre>
92 /(?&#60;=abc)123/
93 </pre>
94 This pattern matches "123", but only if it is preceded by "abc". If the subject
95 string is "xyzabc12", the offsets after a partial match are for the substring
96 "abc12", because all these characters are needed if another match is tried
97 with extra characters added.
98 </P>
99 <P>
100 If there is more than one partial match, the first one that was found provides
101 the data that is returned. Consider this pattern:
102 <pre>
103 /123\w+X|dogY/
104 </pre>
105 If this is matched against the subject string "abc123dog", both
106 alternatives fail to match, but the end of the subject is reached during
107 matching, so PCRE_ERROR_PARTIAL is returned instead of PCRE_ERROR_NOMATCH. The
108 offsets are set to 3 and 9, identifying "123dog" as the first partial match
109 that was found. (In this example, there are two partial matches, because "dog"
110 on its own partially matches the second alternative.)
111 </P>
112 <P>
113 If PCRE_PARTIAL_HARD is set for <b>pcre_exec()</b>, it returns
114 PCRE_ERROR_PARTIAL as soon as a partial match is found, without continuing to
115 search for possible complete matches. The difference between the two options
116 can be illustrated by a pattern such as:
117 <pre>
118 /dog(sbody)?/
119 </pre>
120 This matches either "dog" or "dogsbody", greedily (that is, it prefers the
121 longer string if possible). If it is matched against the string "dog" with
122 PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if
123 PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand,
124 if the pattern is made ungreedy the result is different:
125 <pre>
126 /dog(sbody)??/
127 </pre>
128 In this case the result is always a complete match because <b>pcre_exec()</b>
129 finds that first, and it never continues after finding a match. It might be
130 easier to follow this explanation by thinking of the two patterns like this:
131 <pre>
132 /dog(sbody)?/ is the same as /dogsbody|dog/
133 /dog(sbody)??/ is the same as /dog|dogsbody/
134 </pre>
135 The second pattern will never match "dogsbody" when <b>pcre_exec()</b> is
136 used, because it will always find the shorter match first.
137 </P>
138 <br><a name="SEC3" href="#TOC1">PARTIAL MATCHING USING pcre_dfa_exec()</a><br>
139 <P>
140 The <b>pcre_dfa_exec()</b> function moves along the subject string character by
141 character, without backtracking, searching for all possible matches
142 simultaneously. If the end of the subject is reached before the end of the
143 pattern, there is the possibility of a partial match, again provided that at
144 least one character has matched.
145 </P>
146 <P>
147 When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there
148 have been no complete matches. Otherwise, the complete matches are returned.
149 However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any
150 complete matches. The portion of the string that was inspected when the longest
151 partial match was found is set as the first matching string, provided there are
152 at least two slots in the offsets vector.
153 </P>
154 <P>
155 Because <b>pcre_dfa_exec()</b> always searches for all possible matches, and
156 there is no difference between greedy and ungreedy repetition, its behaviour is
157 different from <b>pcre_exec</b> when PCRE_PARTIAL_HARD is set. Consider the
158 string "dog" matched against the ungreedy pattern shown above:
159 <pre>
160 /dog(sbody)??/
161 </pre>
162 Whereas <b>pcre_exec()</b> stops as soon as it finds the complete match for
163 "dog", <b>pcre_dfa_exec()</b> also finds the partial match for "dogsbody", and
164 so returns that when PCRE_PARTIAL_HARD is set.
165 </P>
166 <br><a name="SEC4" href="#TOC1">PARTIAL MATCHING AND WORD BOUNDARIES</a><br>
167 <P>
168 If a pattern ends with one of sequences \b or \B, which test for word
169 boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
170 results. Consider this pattern:
171 <pre>
172 /\bcat\b/
173 </pre>
174 This matches "cat", provided there is a word boundary at either end. If the
175 subject string is "the cat", the comparison of the final "t" with a following
176 character cannot take place, so a partial match is found. However,
177 <b>pcre_exec()</b> carries on with normal matching, which matches \b at the end
178 of the subject when the last character is a letter, thus finding a complete
179 match. The result, therefore, is <i>not</i> PCRE_ERROR_PARTIAL. The same thing
180 happens with <b>pcre_dfa_exec()</b>, because it also finds the complete match.
181 </P>
182 <P>
183 Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because
184 then the partial match takes precedence.
185 </P>
186 <br><a name="SEC5" href="#TOC1">FORMERLY RESTRICTED PATTERNS</a><br>
187 <P>
188 For releases of PCRE prior to 8.00, because of the way certain internal
189 optimizations were implemented in the <b>pcre_exec()</b> function, the
190 PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with
191 all patterns. From release 8.00 onwards, the restrictions no longer apply, and
192 partial matching with <b>pcre_exec()</b> can be requested for any pattern.
193 </P>
194 <P>
195 Items that were formerly restricted were repeated single characters and
196 repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not
197 conform to the restrictions, <b>pcre_exec()</b> returned the error code
198 PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
199 PCRE_INFO_OKPARTIAL call to <b>pcre_fullinfo()</b> to find out if a compiled
200 pattern can be used for partial matching now always returns 1.
201 </P>
202 <br><a name="SEC6" href="#TOC1">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a><br>
203 <P>
204 If the escape sequence \P is present in a <b>pcretest</b> data line, the
205 PCRE_PARTIAL_SOFT option is used for the match. Here is a run of <b>pcretest</b>
206 that uses the date example quoted above:
207 <pre>
208 re&#62; /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
209 data&#62; 25jun04\P
210 0: 25jun04
211 1: jun
212 data&#62; 25dec3\P
213 Partial match: 23dec3
214 data&#62; 3ju\P
215 Partial match: 3ju
216 data&#62; 3juj\P
217 No match
218 data&#62; j\P
219 No match
220 </pre>
221 The first data string is matched completely, so <b>pcretest</b> shows the
222 matched substrings. The remaining four strings do not match the complete
223 pattern, but the first two are partial matches. Similar output is obtained
224 when <b>pcre_dfa_exec()</b> is used.
225 </P>
226 <P>
227 If the escape sequence \P is present more than once in a <b>pcretest</b> data
228 line, the PCRE_PARTIAL_HARD option is set for the match.
229 </P>
230 <br><a name="SEC7" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()</a><br>
231 <P>
232 When a partial match has been found using <b>pcre_dfa_exec()</b>, it is possible
233 to continue the match by providing additional subject data and calling
234 <b>pcre_dfa_exec()</b> again with the same compiled regular expression, this
235 time setting the PCRE_DFA_RESTART option. You must pass the same working
236 space as before, because this is where details of the previous partial match
237 are stored. Here is an example using <b>pcretest</b>, using the \R escape
238 sequence to set the PCRE_DFA_RESTART option (\D specifies the use of
239 <b>pcre_dfa_exec()</b>):
240 <pre>
241 re&#62; /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
242 data&#62; 23ja\P\D
243 Partial match: 23ja
244 data&#62; n05\R\D
245 0: n05
246 </pre>
247 The first call has "23ja" as the subject, and requests partial matching; the
248 second call has "n05" as the subject for the continued (restarted) match.
249 Notice that when the match is complete, only the last part is shown; PCRE does
250 not retain the previously partially-matched string. It is up to the calling
251 program to do that if it needs to.
252 </P>
253 <P>
254 You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
255 PCRE_DFA_RESTART to continue partial matching over multiple segments. This
256 facility can be used to pass very long subject strings to
257 <b>pcre_dfa_exec()</b>.
258 </P>
259 <br><a name="SEC8" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_exec()</a><br>
260 <P>
261 From release 8.00, <b>pcre_exec()</b> can also be used to do multi-segment
262 matching. Unlike <b>pcre_dfa_exec()</b>, it is not possible to restart the
263 previous match with a new segment of data. Instead, new data must be added to
264 the previous subject string, and the entire match re-run, starting from the
265 point where the partial match occurred. Earlier data can be discarded.
266 Consider an unanchored pattern that matches dates:
267 <pre>
268 re&#62; /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
269 data&#62; The date is 23ja\P
270 Partial match: 23ja
271 </pre>
272 At this stage, an application could discard the text preceding "23ja", add on
273 text from the next segment, and call <b>pcre_exec()</b> again. Unlike
274 <b>pcre_dfa_exec()</b>, the entire matching string must always be available, and
275 the complete matching process occurs for each call, so more memory and more
276 processing time is needed.
277 </P>
278 <P>
279 <b>Note:</b> If the pattern contains lookbehind assertions, or \K, or starts
280 with \b or \B, the string that is returned for a partial match will include
281 characters that precede the partially matched string itself, because these must
282 be retained when adding on more characters for a subsequent matching attempt.
283 </P>
284 <br><a name="SEC9" href="#TOC1">ISSUES WITH MULTI-SEGMENT MATCHING</a><br>
285 <P>
286 Certain types of pattern may give problems with multi-segment matching,
287 whichever matching function is used.
288 </P>
289 <P>
290 1. If the pattern contains tests for the beginning or end of a line, you need
291 to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropriate, when the
292 subject string for any call does not contain the beginning or end of a line.
293 </P>
294 <P>
295 2. Lookbehind assertions at the start of a pattern are catered for in the
296 offsets that are returned for a partial match. However, in theory, a lookbehind
297 assertion later in the pattern could require even earlier characters to be
298 inspected, and it might not have been reached when a partial match occurs. This
299 is probably an extremely unlikely case; you could guard against it to a certain
300 extent by always including extra characters at the start.
301 </P>
302 <P>
303 3. Matching a subject string that is split into multiple segments may not
304 always produce exactly the same result as matching over one single long string,
305 especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
306 Word Boundaries" above describes an issue that arises if the pattern ends with
307 \b or \B. Another kind of difference may occur when there are multiple
308 matching possibilities, because a partial match result is given only when there
309 are no completed matches. This means that as soon as the shortest match has
310 been found, continuation to a new subject segment is no longer possible.
311 Consider again this <b>pcretest</b> example:
312 <pre>
313 re&#62; /dog(sbody)?/
314 data&#62; dogsb\P
315 0: dog
316 data&#62; do\P\D
317 Partial match: do
318 data&#62; gsb\R\P\D
319 0: g
320 data&#62; dogsbody\D
321 0: dogsbody
322 1: dog
323 </pre>
324 The first data line passes the string "dogsb" to <b>pcre_exec()</b>, setting the
325 PCRE_PARTIAL_SOFT option. Although the string is a partial match for
326 "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter string
327 "dog" is a complete match. Similarly, when the subject is presented to
328 <b>pcre_dfa_exec()</b> in several parts ("do" and "gsb" being the first two) the
329 match stops when "dog" has been found, and it is not possible to continue. On
330 the other hand, if "dogsbody" is presented as a single string,
331 <b>pcre_dfa_exec()</b> finds both matches.
332 </P>
333 <P>
334 Because of these problems, it is probably best to use PCRE_PARTIAL_HARD when
335 matching multi-segment data. The example above then behaves differently:
336 <pre>
337 re&#62; /dog(sbody)?/
338 data&#62; dogsb\P\P
339 Partial match: dogsb
340 data&#62; do\P\D
341 Partial match: do
342 data&#62; gsb\R\P\P\D
343 Partial match: gsb
345 </PRE>
346 </P>
347 <P>
348 4. Patterns that contain alternatives at the top level which do not all
349 start with the same pattern item may not work as expected when
350 PCRE_DFA_RESTART is used with <b>pcre_dfa_exec()</b>. For example, consider this
351 pattern:
352 <pre>
353 1234|3789
354 </pre>
355 If the first part of the subject is "ABC123", a partial match of the first
356 alternative is found at offset 3. There is no partial match for the second
357 alternative, because such a match does not start at the same point in the
358 subject string. Attempting to continue with the string "7890" does not yield a
359 match because only those alternatives that match at one point in the subject
360 are remembered. The problem arises because the start of the second alternative
361 matches within the first alternative. There is no problem with anchored
362 patterns or patterns such as:
363 <pre>
364 1234|ABCD
365 </pre>
366 where no string can be a partial match for both alternatives. This is not a
367 problem if <b>pcre_exec()</b> is used, because the entire match has to be rerun
368 each time:
369 <pre>
370 re&#62; /1234|3789/
371 data&#62; ABC123\P
372 Partial match: 123
373 data&#62; 1237890
374 0: 3789
375 </pre>
376 Of course, instead of using PCRE_DFA_PARTIAL, the same technique of re-running
377 the entire match can also be used with <b>pcre_dfa_exec()</b>. Another
378 possibility is to work with two buffers. If a partial match at offset <i>n</i>
379 in the first buffer is followed by "no match" when PCRE_DFA_RESTART is used on
380 the second buffer, you can then try a new match starting at offset <i>n+1</i> in
381 the first buffer.
382 </P>
383 <br><a name="SEC10" href="#TOC1">AUTHOR</a><br>
384 <P>
385 Philip Hazel
386 <br>
387 University Computing Service
388 <br>
389 Cambridge CB2 3QH, England.
390 <br>
391 </P>
392 <br><a name="SEC11" href="#TOC1">REVISION</a><br>
393 <P>
394 Last updated: 19 October 2009
395 <br>
396 Copyright &copy; 1997-2009 University of Cambridge.
397 <br>
398 <p>
399 Return to the <a href="index.html">PCRE index page</a>.
400 </p>


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