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1 nigel 79 .TH PCREPARTIAL 3
2 nigel 75 .SH NAME
3     PCRE - Perl-compatible regular expressions
4     .SH "PARTIAL MATCHING IN PCRE"
5     .rs
6     .sp
7     In normal use of PCRE, if the subject string that is passed to
8 nigel 77 \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP matches as far as it goes, but is
9     too short to match the entire pattern, PCRE_ERROR_NOMATCH is returned. There
10     are circumstances where it might be helpful to distinguish this case from other
11     cases in which there is no match.
12 nigel 75 .P
13     Consider, for example, an application where a human is required to type in data
14     for a field with specific formatting requirements. An example might be a date
15     in the form \fIddmmmyy\fP, defined by this pattern:
16     .sp
17     ^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$
18     .sp
19     If the application sees the user's keystrokes one by one, and can check that
20     what has been typed so far is potentially valid, it is able to raise an error
21 ph10 428 as soon as a mistake is made, by beeping and not reflecting the character that
22     has been typed, for example. This immediate feedback is likely to be a better
23 nigel 75 user interface than a check that is delayed until the entire string has been
24 ph10 428 entered. Partial matching can also sometimes be useful when the subject string
25     is very long and is not all available at once.
26 nigel 75 .P
27 ph10 428 PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and
28     PCRE_PARTIAL_HARD options, which can be set when calling \fBpcre_exec()\fP or
29 ph10 435 \fBpcre_dfa_exec()\fP. For backwards compatibility, PCRE_PARTIAL is a synonym
30     for PCRE_PARTIAL_SOFT. The essential difference between the two options is
31     whether or not a partial match is preferred to an alternative complete match,
32     though the details differ between the two matching functions. If both options
33 ph10 428 are set, PCRE_PARTIAL_HARD takes precedence.
34 nigel 75 .P
35 ph10 456 Setting a partial matching option disables two of PCRE's optimizations. PCRE
36 ph10 428 remembers the last literal byte in a pattern, and abandons matching immediately
37     if such a byte is not present in the subject string. This optimization cannot
38 ph10 456 be used for a subject string that might match only partially. If the pattern
39     was studied, PCRE knows the minimum length of a matching string, and does not
40     bother to run the matching function on shorter strings. This optimization is
41     also disabled for partial matching.
42 ph10 428 .
43     .
44     .SH "PARTIAL MATCHING USING pcre_exec()"
45     .rs
46 ph10 426 .sp
47 ph10 428 A partial match occurs during a call to \fBpcre_exec()\fP whenever the end of
48     the subject string is reached successfully, but matching cannot continue
49     because more characters are needed. However, at least one character must have
50     been matched. (In other words, a partial match can never be an empty string.)
51     .P
52     If PCRE_PARTIAL_SOFT is set, the partial match is remembered, but matching
53     continues as normal, and other alternatives in the pattern are tried. If no
54     complete match can be found, \fBpcre_exec()\fP returns PCRE_ERROR_PARTIAL
55 ph10 435 instead of PCRE_ERROR_NOMATCH. If there are at least two slots in the offsets
56     vector, the first of them is set to the offset of the earliest character that
57     was inspected when the partial match was found. For convenience, the second
58     offset points to the end of the string so that a substring can easily be
59 ph10 456 identified.
60 ph10 435 .P
61     For the majority of patterns, the first offset identifies the start of the
62     partially matched string. However, for patterns that contain lookbehind
63     assertions, or \eK, or begin with \eb or \eB, earlier characters have been
64     inspected while carrying out the match. For example:
65 ph10 428 .sp
66 ph10 435 /(?<=abc)123/
67     .sp
68     This pattern matches "123", but only if it is preceded by "abc". If the subject
69     string is "xyzabc12", the offsets after a partial match are for the substring
70     "abc12", because all these characters are needed if another match is tried
71     with extra characters added.
72     .P
73     If there is more than one partial match, the first one that was found provides
74     the data that is returned. Consider this pattern:
75     .sp
76 ph10 426 /123\ew+X|dogY/
77     .sp
78     If this is matched against the subject string "abc123dog", both
79 ph10 435 alternatives fail to match, but the end of the subject is reached during
80 ph10 428 matching, so PCRE_ERROR_PARTIAL is returned instead of PCRE_ERROR_NOMATCH. The
81 ph10 435 offsets are set to 3 and 9, identifying "123dog" as the first partial match
82 ph10 428 that was found. (In this example, there are two partial matches, because "dog"
83     on its own partially matches the second alternative.)
84 ph10 426 .P
85 ph10 435 If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP, it returns
86 ph10 428 PCRE_ERROR_PARTIAL as soon as a partial match is found, without continuing to
87     search for possible complete matches. The difference between the two options
88     can be illustrated by a pattern such as:
89     .sp
90     /dog(sbody)?/
91     .sp
92 ph10 435 This matches either "dog" or "dogsbody", greedily (that is, it prefers the
93 ph10 428 longer string if possible). If it is matched against the string "dog" with
94 ph10 435 PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if
95     PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand,
96 ph10 428 if the pattern is made ungreedy the result is different:
97     .sp
98     /dog(sbody)??/
99     .sp
100 ph10 435 In this case the result is always a complete match because \fBpcre_exec()\fP
101     finds that first, and it never continues after finding a match. It might be
102 ph10 428 easier to follow this explanation by thinking of the two patterns like this:
103     .sp
104     /dog(sbody)?/ is the same as /dogsbody|dog/
105     /dog(sbody)??/ is the same as /dog|dogsbody/
106     .sp
107 ph10 435 The second pattern will never match "dogsbody" when \fBpcre_exec()\fP is
108 ph10 428 used, because it will always find the shorter match first.
109     .
110     .
111     .SH "PARTIAL MATCHING USING pcre_dfa_exec()"
112     .rs
113     .sp
114 ph10 435 The \fBpcre_dfa_exec()\fP function moves along the subject string character by
115     character, without backtracking, searching for all possible matches
116     simultaneously. If the end of the subject is reached before the end of the
117 ph10 428 pattern, there is the possibility of a partial match, again provided that at
118     least one character has matched.
119 nigel 77 .P
120 ph10 428 When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there
121     have been no complete matches. Otherwise, the complete matches are returned.
122     However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any
123 ph10 435 complete matches. The portion of the string that was inspected when the longest
124     partial match was found is set as the first matching string, provided there are
125     at least two slots in the offsets vector.
126 ph10 428 .P
127 ph10 435 Because \fBpcre_dfa_exec()\fP always searches for all possible matches, and
128 ph10 428 there is no difference between greedy and ungreedy repetition, its behaviour is
129 ph10 435 different from \fBpcre_exec\fP when PCRE_PARTIAL_HARD is set. Consider the
130 ph10 428 string "dog" matched against the ungreedy pattern shown above:
131     .sp
132     /dog(sbody)??/
133     .sp
134 ph10 435 Whereas \fBpcre_exec()\fP stops as soon as it finds the complete match for
135 ph10 428 "dog", \fBpcre_dfa_exec()\fP also finds the partial match for "dogsbody", and
136     so returns that when PCRE_PARTIAL_HARD is set.
137 nigel 75 .
138     .
139 ph10 428 .SH "PARTIAL MATCHING AND WORD BOUNDARIES"
140 nigel 75 .rs
141     .sp
142 ph10 435 If a pattern ends with one of sequences \ew or \eW, which test for word
143     boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
144 ph10 428 results. Consider this pattern:
145     .sp
146     /\ebcat\eb/
147     .sp
148     This matches "cat", provided there is a word boundary at either end. If the
149     subject string is "the cat", the comparison of the final "t" with a following
150 ph10 435 character cannot take place, so a partial match is found. However,
151     \fBpcre_exec()\fP carries on with normal matching, which matches \eb at the end
152     of the subject when the last character is a letter, thus finding a complete
153     match. The result, therefore, is \fInot\fP PCRE_ERROR_PARTIAL. The same thing
154 ph10 428 happens with \fBpcre_dfa_exec()\fP, because it also finds the complete match.
155     .P
156 ph10 435 Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because
157 ph10 428 then the partial match takes precedence.
158     .
159     .
160     .SH "FORMERLY RESTRICTED PATTERNS"
161     .rs
162     .sp
163 ph10 426 For releases of PCRE prior to 8.00, because of the way certain internal
164     optimizations were implemented in the \fBpcre_exec()\fP function, the
165 ph10 428 PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with
166     all patterns. From release 8.00 onwards, the restrictions no longer apply, and
167     partial matching with \fBpcre_exec()\fP can be requested for any pattern.
168 nigel 75 .P
169 ph10 426 Items that were formerly restricted were repeated single characters and
170     repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not
171     conform to the restrictions, \fBpcre_exec()\fP returned the error code
172     PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
173     PCRE_INFO_OKPARTIAL call to \fBpcre_fullinfo()\fP to find out if a compiled
174     pattern can be used for partial matching now always returns 1.
175 nigel 75 .
176     .
177     .SH "EXAMPLE OF PARTIAL MATCHING USING PCRETEST"
178     .rs
179     .sp
180     If the escape sequence \eP is present in a \fBpcretest\fP data line, the
181 ph10 428 PCRE_PARTIAL_SOFT option is used for the match. Here is a run of \fBpcretest\fP
182     that uses the date example quoted above:
183 nigel 75 .sp
184     re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
185 nigel 87 data> 25jun04\eP
186 nigel 75 0: 25jun04
187     1: jun
188 nigel 87 data> 25dec3\eP
189 ph10 426 Partial match: 23dec3
190 nigel 87 data> 3ju\eP
191 ph10 426 Partial match: 3ju
192 nigel 87 data> 3juj\eP
193 nigel 75 No match
194 nigel 87 data> j\eP
195 nigel 75 No match
196     .sp
197     The first data string is matched completely, so \fBpcretest\fP shows the
198     matched substrings. The remaining four strings do not match the complete
199 ph10 426 pattern, but the first two are partial matches. Similar output is obtained
200     when \fBpcre_dfa_exec()\fP is used.
201 ph10 428 .P
202     If the escape sequence \eP is present more than once in a \fBpcretest\fP data
203     line, the PCRE_PARTIAL_HARD option is set for the match.
204 ph10 426 .
205 ph10 435 .
206 nigel 77 .SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()"
207     .rs
208     .sp
209     When a partial match has been found using \fBpcre_dfa_exec()\fP, it is possible
210     to continue the match by providing additional subject data and calling
211 nigel 93 \fBpcre_dfa_exec()\fP again with the same compiled regular expression, this
212 ph10 428 time setting the PCRE_DFA_RESTART option. You must pass the same working
213 nigel 93 space as before, because this is where details of the previous partial match
214     are stored. Here is an example using \fBpcretest\fP, using the \eR escape
215 ph10 428 sequence to set the PCRE_DFA_RESTART option (\eD specifies the use of
216     \fBpcre_dfa_exec()\fP):
217 nigel 77 .sp
218 ph10 155 re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
219 nigel 77 data> 23ja\eP\eD
220     Partial match: 23ja
221     data> n05\eR\eD
222     0: n05
223     .sp
224     The first call has "23ja" as the subject, and requests partial matching; the
225     second call has "n05" as the subject for the continued (restarted) match.
226     Notice that when the match is complete, only the last part is shown; PCRE does
227     not retain the previously partially-matched string. It is up to the calling
228     program to do that if it needs to.
229 nigel 75 .P
230 ph10 428 You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
231     PCRE_DFA_RESTART to continue partial matching over multiple segments. This
232     facility can be used to pass very long subject strings to
233     \fBpcre_dfa_exec()\fP.
234 ph10 426 .
235     .
236     .SH "MULTI-SEGMENT MATCHING WITH pcre_exec()"
237     .rs
238     .sp
239 ph10 435 From release 8.00, \fBpcre_exec()\fP can also be used to do multi-segment
240     matching. Unlike \fBpcre_dfa_exec()\fP, it is not possible to restart the
241     previous match with a new segment of data. Instead, new data must be added to
242     the previous subject string, and the entire match re-run, starting from the
243 ph10 426 point where the partial match occurred. Earlier data can be discarded.
244     Consider an unanchored pattern that matches dates:
245     .sp
246     re> /\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed/
247     data> The date is 23ja\eP
248     Partial match: 23ja
249     .sp
250 ph10 435 The this stage, an application could discard the text preceding "23ja", add on
251     text from the next segment, and call \fBpcre_exec()\fP again. Unlike
252     \fBpcre_dfa_exec()\fP, the entire matching string must always be available, and
253     the complete matching process occurs for each call, so more memory and more
254 ph10 426 processing time is needed.
255 ph10 435 .P
256     \fBNote:\fP If the pattern contains lookbehind assertions, or \eK, or starts
257     with \eb or \eB, the string that is returned for a partial match will include
258     characters that precede the partially matched string itself, because these must
259     be retained when adding on more characters for a subsequent matching attempt.
260 ph10 426 .
261 ph10 435 .
262 ph10 426 .SH "ISSUES WITH MULTI-SEGMENT MATCHING"
263     .rs
264     .sp
265 ph10 435 Certain types of pattern may give problems with multi-segment matching,
266 ph10 426 whichever matching function is used.
267 nigel 77 .P
268     1. If the pattern contains tests for the beginning or end of a line, you need
269     to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropriate, when the
270     subject string for any call does not contain the beginning or end of a line.
271     .P
272 ph10 435 2. Lookbehind assertions at the start of a pattern are catered for in the
273     offsets that are returned for a partial match. However, in theory, a lookbehind
274     assertion later in the pattern could require even earlier characters to be
275     inspected, and it might not have been reached when a partial match occurs. This
276     is probably an extremely unlikely case; you could guard against it to a certain
277     extent by always including extra characters at the start.
278 nigel 77 .P
279 ph10 428 3. Matching a subject string that is split into multiple segments may not
280     always produce exactly the same result as matching over one single long string,
281 ph10 435 especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
282     Word Boundaries" above describes an issue that arises if the pattern ends with
283 ph10 428 \eb or \eB. Another kind of difference may occur when there are multiple
284     matching possibilities, because a partial match result is given only when there
285     are no completed matches. This means that as soon as the shortest match has
286     been found, continuation to a new subject segment is no longer possible.
287     Consider again this \fBpcretest\fP example:
288 nigel 77 .sp
289     re> /dog(sbody)?/
290 ph10 426 data> dogsb\eP
291 ph10 435 0: dog
292 nigel 77 data> do\eP\eD
293     Partial match: do
294     data> gsb\eR\eP\eD
295     0: g
296     data> dogsbody\eD
297     0: dogsbody
298     1: dog
299     .sp
300 ph10 428 The first data line passes the string "dogsb" to \fBpcre_exec()\fP, setting the
301     PCRE_PARTIAL_SOFT option. Although the string is a partial match for
302     "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter string
303     "dog" is a complete match. Similarly, when the subject is presented to
304     \fBpcre_dfa_exec()\fP in several parts ("do" and "gsb" being the first two) the
305     match stops when "dog" has been found, and it is not possible to continue. On
306     the other hand, if "dogsbody" is presented as a single string,
307     \fBpcre_dfa_exec()\fP finds both matches.
308 nigel 77 .P
309 ph10 428 Because of these problems, it is probably best to use PCRE_PARTIAL_HARD when
310     matching multi-segment data. The example above then behaves differently:
311     .sp
312     re> /dog(sbody)?/
313     data> dogsb\eP\eP
314 ph10 435 Partial match: dogsb
315 ph10 428 data> do\eP\eD
316     Partial match: do
317     data> gsb\eR\eP\eP\eD
318 ph10 435 Partial match: gsb
319 ph10 428 .sp
320 nigel 87 .P
321     4. Patterns that contain alternatives at the top level which do not all
322 ph10 435 start with the same pattern item may not work as expected when
323 ph10 426 \fBpcre_dfa_exec()\fP is used. For example, consider this pattern:
324 nigel 87 .sp
325     1234|3789
326     .sp
327     If the first part of the subject is "ABC123", a partial match of the first
328     alternative is found at offset 3. There is no partial match for the second
329     alternative, because such a match does not start at the same point in the
330 ph10 426 subject string. Attempting to continue with the string "7890" does not yield a
331 nigel 87 match because only those alternatives that match at one point in the subject
332     are remembered. The problem arises because the start of the second alternative
333     matches within the first alternative. There is no problem with anchored
334     patterns or patterns such as:
335     .sp
336     1234|ABCD
337     .sp
338 ph10 426 where no string can be a partial match for both alternatives. This is not a
339 ph10 435 problem if \fPpcre_exec()\fP is used, because the entire match has to be rerun
340 ph10 426 each time:
341     .sp
342     re> /1234|3789/
343     data> ABC123\eP
344     Partial match: 123
345     data> 1237890
346     0: 3789
347 ph10 435 .sp
348 nigel 77 .
349     .
350 ph10 99 .SH AUTHOR
351     .rs
352     .sp
353     .nf
354     Philip Hazel
355     University Computing Service
356     Cambridge CB2 3QH, England.
357     .fi
358     .
359     .
360     .SH REVISION
361     .rs
362     .sp
363     .nf
364 ph10 456 Last updated: 29 September 2009
365 ph10 426 Copyright (c) 1997-2009 University of Cambridge.
366 ph10 99 .fi

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