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- Committer: Bazaar Package Importer
- Author(s): Mark Baker
- Date: 2010-05-07 21:18:05 UTC
- mfrom: (1.1.9 upstream)
- Revision ID: james.westby@ubuntu.com-20100507211805-hgyyr2003bmbu54m
Tags: 8.02-1
New upstream release
- files added:
- doc/html/pcredemo.html
- m4
- files modified:
- 132html
added
removed
doc/pcrepartial.3
18
18
.sp
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If the application sees the user's keystrokes one by one, and can check that
20
20
what has been typed so far is potentially valid, it is able to raise an error
21
as soon as a mistake is made, possibly beeping and not reflecting the
22
character that has been typed. This immediate feedback is likely to be a better
21
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
23
user interface than a check that is delayed until the entire string has been
24
entered.
25
.P
26
PCRE supports the concept of partial matching by means of the PCRE_PARTIAL
27
option, which can be set when calling \fBpcre_exec()\fP or
28
\fBpcre_dfa_exec()\fP. When this flag is set for \fBpcre_exec()\fP, the return
29
code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any time
30
during the matching process the last part of the subject string matched part of
31
the pattern. Unfortunately, for non-anchored matching, it is not possible to
32
obtain the position of the start of the partial match. No captured data is set
33
when PCRE_ERROR_PARTIAL is returned.
34
.P
35
When PCRE_PARTIAL is set for \fBpcre_dfa_exec()\fP, the return code
36
PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end of the
37
subject is reached, there have been no complete matches, but there is still at
38
least one matching possibility. The portion of the string that provided the
39
partial match is set as the first matching string.
40
.P
41
Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers the
42
last literal byte in a pattern, and abandons matching immediately if such a
43
byte is not present in the subject string. This optimization cannot be used
44
for a subject string that might match only partially.
45
.
46
.
47
.SH "RESTRICTED PATTERNS FOR PCRE_PARTIAL"
48
.rs
49
.sp
50
Because of the way certain internal optimizations are implemented in the
51
\fBpcre_exec()\fP function, the PCRE_PARTIAL option cannot be used with all
52
patterns. These restrictions do not apply when \fBpcre_dfa_exec()\fP is used.
53
For \fBpcre_exec()\fP, repeated single characters such as
54
.sp
55
a{2,4}
56
.sp
57
and repeated single metasequences such as
58
.sp
59
\ed+
60
.sp
61
are not permitted if the maximum number of occurrences is greater than one.
62
Optional items such as \ed? (where the maximum is one) are permitted.
63
Quantifiers with any values are permitted after parentheses, so the invalid
64
examples above can be coded thus:
65
.sp
66
(a){2,4}
67
(\ed)+
68
.sp
69
These constructions run more slowly, but for the kinds of application that are
70
envisaged for this facility, this is not felt to be a major restriction.
71
.P
72
If PCRE_PARTIAL is set for a pattern that does not conform to the restrictions,
73
\fBpcre_exec()\fP returns the error code PCRE_ERROR_BADPARTIAL (-13).
74
You can use the PCRE_INFO_OKPARTIAL call to \fBpcre_fullinfo()\fP to find out
75
if a compiled pattern can be used for partial matching.
24
entered. Partial matching can also sometimes be useful when the subject string
25
is very long and is not all available at once.
26
.P
27
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
\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
are set, PCRE_PARTIAL_HARD takes precedence.
34
.P
35
Setting a partial matching option disables two of PCRE's optimizations. PCRE
36
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
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
.
43
.
44
.SH "PARTIAL MATCHING USING pcre_exec()"
45
.rs
46
.sp
47
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
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
identified.
60
.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
.sp
66
/(?<=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
/123\ew+X|dogY/
77
.sp
78
If this is matched against the subject string "abc123dog", both
79
alternatives fail to match, but the end of the subject is reached during
80
matching, so PCRE_ERROR_PARTIAL is returned instead of PCRE_ERROR_NOMATCH. The
81
offsets are set to 3 and 9, identifying "123dog" as the first partial match
82
that was found. (In this example, there are two partial matches, because "dog"
83
on its own partially matches the second alternative.)
84
.P
85
If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP, it returns
86
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
This matches either "dog" or "dogsbody", greedily (that is, it prefers the
93
longer string if possible). If it is matched against the string "dog" with
94
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
if the pattern is made ungreedy the result is different:
97
.sp
98
/dog(sbody)??/
99
.sp
100
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
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
The second pattern will never match "dogsbody" when \fBpcre_exec()\fP is
108
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
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
pattern, there is the possibility of a partial match, again provided that at
118
least one character has matched.
119
.P
120
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
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
.P
127
Because \fBpcre_dfa_exec()\fP always searches for all possible matches, and
128
there is no difference between greedy and ungreedy repetition, its behaviour is
129
different from \fBpcre_exec\fP when PCRE_PARTIAL_HARD is set. Consider the
130
string "dog" matched against the ungreedy pattern shown above:
131
.sp
132
/dog(sbody)??/
133
.sp
134
Whereas \fBpcre_exec()\fP stops as soon as it finds the complete match for
135
"dog", \fBpcre_dfa_exec()\fP also finds the partial match for "dogsbody", and
136
so returns that when PCRE_PARTIAL_HARD is set.
137
.
138
.
139
.SH "PARTIAL MATCHING AND WORD BOUNDARIES"
140
.rs
141
.sp
142
If a pattern ends with one of sequences \eb or \eB, which test for word
143
boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
144
results. Consider this pattern:
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.sp
146
/\ebcat\eb/
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.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
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
happens with \fBpcre_dfa_exec()\fP, because it also finds the complete match.
155
.P
156
Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because
157
then the partial match takes precedence.
158
.
159
.
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.SH "FORMERLY RESTRICTED PATTERNS"
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.rs
162
.sp
163
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
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
.P
169
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.
76
175
.
77
176
.
78
177
.SH "EXAMPLE OF PARTIAL MATCHING USING PCRETEST"
79
178
.rs
80
179
.sp
81
180
If the escape sequence \eP is present in a \fBpcretest\fP data line, the
82
PCRE_PARTIAL flag is used for the match. Here is a run of \fBpcretest\fP that
83
uses the date example quoted above:
181
PCRE_PARTIAL_SOFT option is used for the match. Here is a run of \fBpcretest\fP
182
that uses the date example quoted above:
84
183
.sp
85
184
re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
86
185
data> 25jun04\eP
87
186
0: 25jun04
88
187
1: jun
89
188
data> 25dec3\eP
90
Partial match
189
Partial match: 23dec3
91
190
data> 3ju\eP
92
Partial match
191
Partial match: 3ju
93
192
data> 3juj\eP
94
193
No match
95
194
data> j\eP
97
196
.sp
98
197
The first data string is matched completely, so \fBpcretest\fP shows the
99
198
matched substrings. The remaining four strings do not match the complete
100
pattern, but the first two are partial matches. The same test, using
101
\fBpcre_dfa_exec()\fP matching (by means of the \eD escape sequence), produces
102
the following output:
103
.sp
104
re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
105
data> 25jun04\eP\eD
106
0: 25jun04
107
data> 23dec3\eP\eD
108
Partial match: 23dec3
109
data> 3ju\eP\eD
110
Partial match: 3ju
111
data> 3juj\eP\eD
112
No match
113
data> j\eP\eD
114
No match
115
.sp
116
Notice that in this case the portion of the string that was matched is made
117
available.
199
pattern, but the first two are partial matches. Similar output is obtained
200
when \fBpcre_dfa_exec()\fP is used.
201
.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.
118
204
.
119
205
.
120
206
.SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()"
123
209
When a partial match has been found using \fBpcre_dfa_exec()\fP, it is possible
124
210
to continue the match by providing additional subject data and calling
125
211
\fBpcre_dfa_exec()\fP again with the same compiled regular expression, this
126
time setting the PCRE_DFA_RESTART option. You must also pass the same working
212
time setting the PCRE_DFA_RESTART option. You must pass the same working
127
213
space as before, because this is where details of the previous partial match
128
214
are stored. Here is an example using \fBpcretest\fP, using the \eR escape
129
sequence to set the PCRE_DFA_RESTART option (\eP and \eD are as above):
215
sequence to set the PCRE_DFA_RESTART option (\eD specifies the use of
216
\fBpcre_dfa_exec()\fP):
130
217
.sp
131
218
re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
132
219
data> 23ja\eP\eD
140
227
not retain the previously partially-matched string. It is up to the calling
141
228
program to do that if it needs to.
142
229
.P
143
You can set PCRE_PARTIAL with PCRE_DFA_RESTART to continue partial matching
144
over multiple segments. This facility can be used to pass very long subject
145
strings to \fBpcre_dfa_exec()\fP. However, some care is needed for certain
146
types of pattern.
230
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
.
235
.
236
.SH "MULTI-SEGMENT MATCHING WITH pcre_exec()"
237
.rs
238
.sp
239
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
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
At 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
processing time is needed.
255
.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
.
261
.
262
.SH "ISSUES WITH MULTI-SEGMENT MATCHING"
263
.rs
264
.sp
265
Certain types of pattern may give problems with multi-segment matching,
266
whichever matching function is used.
147
267
.P
148
268
1. If the pattern contains tests for the beginning or end of a line, you need
149
269
to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropriate, when the
150
270
subject string for any call does not contain the beginning or end of a line.
151
271
.P
152
2. If the pattern contains backward assertions (including \eb or \eB), you need
153
to arrange for some overlap in the subject strings to allow for this. For
154
example, you could pass the subject in chunks that are 500 bytes long, but in
155
a buffer of 700 bytes, with the starting offset set to 200 and the previous 200
156
bytes at the start of the buffer.
272
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.
157
278
.P
158
3. Matching a subject string that is split into multiple segments does not
159
always produce exactly the same result as matching over one single long string.
160
The difference arises when there are multiple matching possibilities, because a
161
partial match result is given only when there are no completed matches in a
162
call to \fBpcre_dfa_exec()\fP. This means that as soon as the shortest match has
279
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
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
\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
163
286
been found, continuation to a new subject segment is no longer possible.
164
Consider this \fBpcretest\fP example:
287
Consider again this \fBpcretest\fP example:
165
288
.sp
166
289
re> /dog(sbody)?/
290
data> dogsb\eP
291
0: dog
167
292
data> do\eP\eD
168
293
Partial match: do
169
294
data> gsb\eR\eP\eD
172
297
0: dogsbody
173
298
1: dog
174
299
.sp
175
The pattern matches the words "dog" or "dogsbody". When the subject is
176
presented in several parts ("do" and "gsb" being the first two) the match stops
177
when "dog" has been found, and it is not possible to continue. On the other
178
hand, if "dogsbody" is presented as a single string, both matches are found.
300
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.
179
308
.P
180
Because of this phenomenon, it does not usually make sense to end a pattern
181
that is going to be matched in this way with a variable repeat.
309
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
Partial match: dogsb
315
data> do\eP\eD
316
Partial match: do
317
data> gsb\eR\eP\eP\eD
318
Partial match: gsb
319
.sp
182
320
.P
183
321
4. Patterns that contain alternatives at the top level which do not all
184
start with the same pattern item may not work as expected. For example,
185
consider this pattern:
322
start with the same pattern item may not work as expected when
323
PCRE_DFA_RESTART is used with \fBpcre_dfa_exec()\fP. For example, consider this
324
pattern:
186
325
.sp
187
326
1234|3789
188
327
.sp
189
328
If the first part of the subject is "ABC123", a partial match of the first
190
329
alternative is found at offset 3. There is no partial match for the second
191
330
alternative, because such a match does not start at the same point in the
192
subject string. Attempting to continue with the string "789" does not yield a
331
subject string. Attempting to continue with the string "7890" does not yield a
193
332
match because only those alternatives that match at one point in the subject
194
333
are remembered. The problem arises because the start of the second alternative
195
334
matches within the first alternative. There is no problem with anchored
197
336
.sp
198
337
1234|ABCD
199
338
.sp
200
where no string can be a partial match for both alternatives.
339
where no string can be a partial match for both alternatives. This is not a
340
problem if \fBpcre_exec()\fP is used, because the entire match has to be rerun
341
each time:
342
.sp
343
re> /1234|3789/
344
data> ABC123\eP
345
Partial match: 123
346
data> 1237890
347
0: 3789
348
.sp
349
Of course, instead of using PCRE_DFA_PARTIAL, the same technique of re-running
350
the entire match can also be used with \fBpcre_dfa_exec()\fP. Another
351
possibility is to work with two buffers. If a partial match at offset \fIn\fP
352
in the first buffer is followed by "no match" when PCRE_DFA_RESTART is used on
353
the second buffer, you can then try a new match starting at offset \fIn+1\fP in
354
the first buffer.
201
355
.
202
356
.
203
357
.SH AUTHOR
214
368
.rs
215
369
.sp
216
370
.nf
217
Last updated: 04 June 2007
218
Copyright (c) 1997-2007 University of Cambridge.
371
Last updated: 19 October 2009
372
Copyright (c) 1997-2009 University of Cambridge.
219
373
.fi
Loggerhead is a web-based interface for Breezy
Version: 2.0.1