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revision 428 by ph10, Mon Aug 31 17:10:26 2009 UTC revision 461 by ph10, Mon Oct 5 10:59:35 2009 UTC
# Line 26  is very long and is not all available at Line 26  is very long and is not all available at
26  .P  .P
27  PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and  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  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  \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  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,  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  though the details differ between the two matching functions. If both options
33  are set, PCRE_PARTIAL_HARD takes precedence.  are set, PCRE_PARTIAL_HARD takes precedence.
34  .P  .P
35  Setting a partial matching option disables one of PCRE's optimizations. PCRE  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  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  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.  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()"  .SH "PARTIAL MATCHING USING pcre_exec()"
# Line 49  been matched. (In other words, a partial Line 52  been matched. (In other words, a partial
52  If PCRE_PARTIAL_SOFT is set, the partial match is remembered, but matching  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  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  complete match can be found, \fBpcre_exec()\fP returns PCRE_ERROR_PARTIAL
55  instead of PCRE_ERROR_NOMATCH, and if there are at least two slots in the  instead of PCRE_ERROR_NOMATCH. If there are at least two slots in the offsets
56  offsets vector, they are filled in with the offsets of the longest string that  vector, the first of them is set to the offset of the earliest character that
57  partially matched. Consider this pattern:  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  .sp
76    /123\ew+X|dogY/    /123\ew+X|dogY/
77  .sp  .sp
78  If this is matched against the subject string "abc123dog", both  If this is matched against the subject string "abc123dog", both
79  alternatives fail to match, but the end of the subject is reached during  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  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 longest partial match  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"  that was found. (In this example, there are two partial matches, because "dog"
83  on its own partially matches the second alternative.)  on its own partially matches the second alternative.)
84  .P  .P
85  If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP, it returns  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  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  search for possible complete matches. The difference between the two options
88  can be illustrated by a pattern such as:  can be illustrated by a pattern such as:
89  .sp  .sp
90    /dog(sbody)?/    /dog(sbody)?/
91  .sp  .sp
92  This matches either "dog" or "dogsbody", greedily (that is, it prefers the  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  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  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,  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:  if the pattern is made ungreedy the result is different:
97  .sp  .sp
98    /dog(sbody)??/    /dog(sbody)??/
99  .sp  .sp
100  In this case the result is always a complete match because \fBpcre_exec()\fP  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  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:  easier to follow this explanation by thinking of the two patterns like this:
103  .sp  .sp
104    /dog(sbody)?/    is the same as  /dogsbody|dog/    /dog(sbody)?/    is the same as  /dogsbody|dog/
105    /dog(sbody)??/   is the same as  /dog|dogsbody/    /dog(sbody)??/   is the same as  /dog|dogsbody/
106  .sp  .sp
107  The second pattern will never match "dogsbody" when \fBpcre_exec()\fP is  The second pattern will never match "dogsbody" when \fBpcre_exec()\fP is
108  used, because it will always find the shorter match first.  used, because it will always find the shorter match first.
109  .  .
110  .  .
111  .SH "PARTIAL MATCHING USING pcre_dfa_exec()"  .SH "PARTIAL MATCHING USING pcre_dfa_exec()"
112  .rs  .rs
113  .sp  .sp
114  The \fBpcre_dfa_exec()\fP function moves along the subject string character by  The \fBpcre_dfa_exec()\fP function moves along the subject string character by
115  character, without backtracking, searching for all possible matches  character, without backtracking, searching for all possible matches
116  simultaneously. If the end of the subject is reached before the end of the  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  pattern, there is the possibility of a partial match, again provided that at
118  least one character has matched.  least one character has matched.
119  .P  .P
120  When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there  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.  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  However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any
123  complete matches. The portion of the string that provided the longest partial  complete matches. The portion of the string that was inspected when the longest
124  match is set as the first matching string, provided there are at least two  partial match was found is set as the first matching string, provided there are
125  slots in the offsets vector.  at least two slots in the offsets vector.
126  .P  .P
127  Because \fBpcre_dfa_exec()\fP always searches for all possible matches, and  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  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  different from \fBpcre_exec\fP when PCRE_PARTIAL_HARD is set. Consider the
130  string "dog" matched against the ungreedy pattern shown above:  string "dog" matched against the ungreedy pattern shown above:
131  .sp  .sp
132    /dog(sbody)??/    /dog(sbody)??/
133  .sp  .sp
134  Whereas \fBpcre_exec()\fP stops as soon as it finds the complete match for  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  "dog", \fBpcre_dfa_exec()\fP also finds the partial match for "dogsbody", and
136  so returns that when PCRE_PARTIAL_HARD is set.  so returns that when PCRE_PARTIAL_HARD is set.
137  .  .
# Line 119  so returns that when PCRE_PARTIAL_HARD i Line 139  so returns that when PCRE_PARTIAL_HARD i
139  .SH "PARTIAL MATCHING AND WORD BOUNDARIES"  .SH "PARTIAL MATCHING AND WORD BOUNDARIES"
140  .rs  .rs
141  .sp  .sp
142  If a pattern ends with one of sequences \ew or \eW, which test for word  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  boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
144  results. Consider this pattern:  results. Consider this pattern:
145  .sp  .sp
146    /\ebcat\eb/    /\ebcat\eb/
147  .sp  .sp
148  This matches "cat", provided there is a word boundary at either end. If the  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  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,  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  \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  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  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.  happens with \fBpcre_dfa_exec()\fP, because it also finds the complete match.
155  .P  .P
156  Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because  Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because
157  then the partial match takes precedence.  then the partial match takes precedence.
158  .  .
159  .  .
# Line 182  when \fBpcre_dfa_exec()\fP is used. Line 202  when \fBpcre_dfa_exec()\fP is used.
202  If the escape sequence \eP is present more than once in a \fBpcretest\fP data  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.  line, the PCRE_PARTIAL_HARD option is set for the match.
204  .  .
205  .  .
206  .SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()"  .SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()"
207  .rs  .rs
208  .sp  .sp
# Line 216  facility can be used to pass very long s Line 236  facility can be used to pass very long s
236  .SH "MULTI-SEGMENT MATCHING WITH pcre_exec()"  .SH "MULTI-SEGMENT MATCHING WITH pcre_exec()"
237  .rs  .rs
238  .sp  .sp
239  From release 8.00, \fBpcre_exec()\fP can also be used to do multi-segment  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  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  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  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.  point where the partial match occurred. Earlier data can be discarded.
244  Consider an unanchored pattern that matches dates:  Consider an unanchored pattern that matches dates:
245  .sp  .sp
# Line 227  Consider an unanchored pattern that matc Line 247  Consider an unanchored pattern that matc
247    data> The date is 23ja\eP    data> The date is 23ja\eP
248    Partial match: 23ja    Partial match: 23ja
249  .sp  .sp
250  The this stage, an application could discard the text preceding "23ja", add on  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  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  \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  the complete matching process occurs for each call, so more memory and more
254  processing time is needed.  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"  .SH "ISSUES WITH MULTI-SEGMENT MATCHING"
263  .rs  .rs
264  .sp  .sp
265  Certain types of pattern may give problems with multi-segment matching,  Certain types of pattern may give problems with multi-segment matching,
266  whichever matching function is used.  whichever matching function is used.
267  .P  .P
268  1. If the pattern contains tests for the beginning or end of a line, you need  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  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.  subject string for any call does not contain the beginning or end of a line.
271  .P  .P
272  2. If the pattern contains backward assertions (including \eb or \eB), you need  2. Lookbehind assertions at the start of a pattern are catered for in the
273  to arrange for some overlap in the subject strings to allow for them to be  offsets that are returned for a partial match. However, in theory, a lookbehind
274  correctly tested at the start of each substring. For example, using  assertion later in the pattern could require even earlier characters to be
275  \fBpcre_dfa_exec()\fP, you could pass the subject in chunks that are 500 bytes  inspected, and it might not have been reached when a partial match occurs. This
276  long, but in a buffer of 700 bytes, with the starting offset set to 200 and the  is probably an extremely unlikely case; you could guard against it to a certain
277  previous 200 bytes at the start of the buffer.  extent by always including extra characters at the start.
278  .P  .P
279  3. Matching a subject string that is split into multiple segments may not  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,  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  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  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  \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  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  are no completed matches. This means that as soon as the shortest match has
# Line 263  Consider again this \fBpcretest\fP examp Line 288  Consider again this \fBpcretest\fP examp
288  .sp  .sp
289      re> /dog(sbody)?/      re> /dog(sbody)?/
290    data> dogsb\eP    data> dogsb\eP
291     0: dog     0: dog
292    data> do\eP\eD    data> do\eP\eD
293    Partial match: do    Partial match: do
294    data> gsb\eR\eP\eD    data> gsb\eR\eP\eD
# Line 286  matching multi-segment data. The example Line 311  matching multi-segment data. The example
311  .sp  .sp
312      re> /dog(sbody)?/      re> /dog(sbody)?/
313    data> dogsb\eP\eP    data> dogsb\eP\eP
314    Partial match: dogsb    Partial match: dogsb
315    data> do\eP\eD    data> do\eP\eD
316    Partial match: do    Partial match: do
317    data> gsb\eR\eP\eP\eD    data> gsb\eR\eP\eP\eD
318    Partial match: gsb    Partial match: gsb
319  .sp  .sp
320  .P  .P
321  4. Patterns that contain alternatives at the top level which do not all  4. Patterns that contain alternatives at the top level which do not all
322  start with the same pattern item may not work as expected when  start with the same pattern item may not work as expected when
323  \fBpcre_dfa_exec()\fP is used. For example, consider this pattern:  \fBpcre_dfa_exec()\fP is used. For example, consider this pattern:
324  .sp  .sp
325    1234|3789    1234|3789
# Line 311  patterns or patterns such as: Line 336  patterns or patterns such as:
336    1234|ABCD    1234|ABCD
337  .sp  .sp
338  where no string can be a partial match for both alternatives. This is not a  where no string can be a partial match for both alternatives. This is not a
339  problem if \fPpcre_exec()\fP is used, because the entire match has to be rerun  problem if \fPpcre_exec()\fP is used, because the entire match has to be rerun
340  each time:  each time:
341  .sp  .sp
342      re> /1234|3789/      re> /1234|3789/
# Line 319  each time: Line 344  each time:
344    Partial match: 123    Partial match: 123
345    data> 1237890    data> 1237890
346     0: 3789     0: 3789
347  .sp  .sp
348  .  .
349  .  .
350  .SH AUTHOR  .SH AUTHOR
# Line 336  Cambridge CB2 3QH, England. Line 361  Cambridge CB2 3QH, England.
361  .rs  .rs
362  .sp  .sp
363  .nf  .nf
364  Last updated: 31 August 2009  Last updated: 29 September 2009
365  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2009 University of Cambridge.
366  .fi  .fi
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