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revision 427 by ph10, Wed Aug 26 15:38:32 2009 UTC revision 428 by ph10, Mon Aug 31 17:10:26 2009 UTC
# Line 18  in the form \fIddmmmyy\fP, defined by th Line 18  in the form \fIddmmmyy\fP, defined by th
18  .sp  .sp
19  If the application sees the user's keystrokes one by one, and can check that  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  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  as soon as a mistake is made, by beeping and not reflecting the character that
22  character that has been typed. This immediate feedback is likely to be a better  has been typed, for example. This immediate feedback is likely to be a better
23  user interface than a check that is delayed until the entire string has been  user interface than a check that is delayed until the entire string has been
24  entered.  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  .P
27  PCRE supports the concept of partial matching by means of the PCRE_PARTIAL  PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and
28  option, 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.  \fBpcre_dfa_exec()\fP. For backwards compatibility, PCRE_PARTIAL is a synonym
30  .P  for PCRE_PARTIAL_SOFT. The essential difference between the two options is
31  When PCRE_PARTIAL is set for \fBpcre_exec()\fP, the return code  whether or not a partial match is preferred to an alternative complete match,
32  PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any time during  though the details differ between the two matching functions. If both options
33  the matching process the last part of the subject string matched part of the  are set, PCRE_PARTIAL_HARD takes precedence.
34  pattern. If there are at least two slots in the offsets vector, they are filled  .P
35  in with the offsets of the longest found string that partially matched. No  Setting a partial matching option disables one of PCRE's optimizations. PCRE
36  other captured data is set when PCRE_ERROR_PARTIAL is returned. The second  remembers the last literal byte in a pattern, and abandons matching immediately
37  offset is always that for the end of the subject. Consider this pattern:  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.
39    .
40    .
41    .SH "PARTIAL MATCHING USING pcre_exec()"
42    .rs
43    .sp
44    A partial match occurs during a call to \fBpcre_exec()\fP whenever the end of
45    the subject string is reached successfully, but matching cannot continue
46    because more characters are needed. However, at least one character must have
47    been matched. (In other words, a partial match can never be an empty string.)
48    .P
49    If PCRE_PARTIAL_SOFT is set, the partial match is remembered, but matching
50    continues as normal, and other alternatives in the pattern are tried. If no
51    complete match can be found, \fBpcre_exec()\fP returns PCRE_ERROR_PARTIAL
52    instead of PCRE_ERROR_NOMATCH, and if there are at least two slots in the
53    offsets vector, they are filled in with the offsets of the longest string that
54    partially matched. Consider this pattern:
55  .sp  .sp
56    /123\ew+X|dogY/    /123\ew+X|dogY/
57  .sp  .sp
58  If this is matched against the subject string "abc123dog", both  If this is matched against the subject string "abc123dog", both
59  alternatives fail to match, but the end of the subject is reached, so  alternatives fail to match, but the end of the subject is reached during
60  PCRE_ERROR_PARTIAL is returned instead of PCRE_ERROR_NOMATCH if the  matching, so PCRE_ERROR_PARTIAL is returned instead of PCRE_ERROR_NOMATCH. The
61  PCRE_PARTIAL option is set. The offsets are set to 3 and 9, identifying  offsets are set to 3 and 9, identifying "123dog" as the longest partial match
62  "123dog" as the longest partial match that was found. (In this example, there  that was found. (In this example, there are two partial matches, because "dog"
63  are two partial matches, because "dog" on its own partially matches the second  on its own partially matches the second alternative.)
64  alternative.)  .P
65  .P  If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP, it returns
66  When PCRE_PARTIAL is set for \fBpcre_dfa_exec()\fP, the return code  PCRE_ERROR_PARTIAL as soon as a partial match is found, without continuing to
67  PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end of the  search for possible complete matches. The difference between the two options
68  subject is reached, there have been no complete matches, but there is still at  can be illustrated by a pattern such as:
69  least one matching possibility. The portion of the string that provided the  .sp
70  longest partial match is set as the first matching string, provided there are    /dog(sbody)?/
71  at least two slots in the offsets vector.  .sp
72  .P  This matches either "dog" or "dogsbody", greedily (that is, it prefers the
73  Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers the  longer string if possible). If it is matched against the string "dog" with
74  last literal byte in a pattern, and abandons matching immediately if such a  PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if
75  byte is not present in the subject string. This optimization cannot be used  PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand,
76  for a subject string that might match only partially.  if the pattern is made ungreedy the result is different:
77    .sp
78      /dog(sbody)??/
79    .sp
80    In this case the result is always a complete match because \fBpcre_exec()\fP
81    finds that first, and it never continues after finding a match. It might be
82    easier to follow this explanation by thinking of the two patterns like this:
83    .sp
84      /dog(sbody)?/    is the same as  /dogsbody|dog/
85      /dog(sbody)??/   is the same as  /dog|dogsbody/
86    .sp
87    The second pattern will never match "dogsbody" when \fBpcre_exec()\fP is
88    used, because it will always find the shorter match first.
89    .
90    .
91    .SH "PARTIAL MATCHING USING pcre_dfa_exec()"
92    .rs
93    .sp
94    The \fBpcre_dfa_exec()\fP function moves along the subject string character by
95    character, without backtracking, searching for all possible matches
96    simultaneously. If the end of the subject is reached before the end of the
97    pattern, there is the possibility of a partial match, again provided that at
98    least one character has matched.
99    .P
100    When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there
101    have been no complete matches. Otherwise, the complete matches are returned.
102    However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any
103    complete matches. The portion of the string that provided the longest partial
104    match is set as the first matching string, provided there are at least two
105    slots in the offsets vector.
106    .P
107    Because \fBpcre_dfa_exec()\fP always searches for all possible matches, and
108    there is no difference between greedy and ungreedy repetition, its behaviour is
109    different from \fBpcre_exec\fP when PCRE_PARTIAL_HARD is set. Consider the
110    string "dog" matched against the ungreedy pattern shown above:
111    .sp
112      /dog(sbody)??/
113    .sp
114    Whereas \fBpcre_exec()\fP stops as soon as it finds the complete match for
115    "dog", \fBpcre_dfa_exec()\fP also finds the partial match for "dogsbody", and
116    so returns that when PCRE_PARTIAL_HARD is set.
117    .
118    .
119    .SH "PARTIAL MATCHING AND WORD BOUNDARIES"
120    .rs
121    .sp
122    If a pattern ends with one of sequences \ew or \eW, which test for word
123    boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
124    results. Consider this pattern:
125    .sp
126      /\ebcat\eb/
127    .sp
128    This matches "cat", provided there is a word boundary at either end. If the
129    subject string is "the cat", the comparison of the final "t" with a following
130    character cannot take place, so a partial match is found. However,
131    \fBpcre_exec()\fP carries on with normal matching, which matches \eb at the end
132    of the subject when the last character is a letter, thus finding a complete
133    match. The result, therefore, is \fInot\fP PCRE_ERROR_PARTIAL. The same thing
134    happens with \fBpcre_dfa_exec()\fP, because it also finds the complete match.
135    .P
136    Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because
137    then the partial match takes precedence.
138  .  .
139  .  .
140  .SH "FORMERLY RESTRICTED PATTERNS FOR PCRE_PARTIAL"  .SH "FORMERLY RESTRICTED PATTERNS"
141  .rs  .rs
142  .sp  .sp
143  For releases of PCRE prior to 8.00, because of the way certain internal  For releases of PCRE prior to 8.00, because of the way certain internal
144  optimizations were implemented in the \fBpcre_exec()\fP function, the  optimizations were implemented in the \fBpcre_exec()\fP function, the
145  PCRE_PARTIAL option could not be used with all patterns. From release 8.00  PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with
146  onwards, the restrictions no longer apply, and partial matching can be  all patterns. From release 8.00 onwards, the restrictions no longer apply, and
147  requested for any pattern.  partial matching with \fBpcre_exec()\fP can be requested for any pattern.
148  .P  .P
149  Items that were formerly restricted were repeated single characters and  Items that were formerly restricted were repeated single characters and
150  repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not  repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not
# Line 79  pattern can be used for partial matching Line 158  pattern can be used for partial matching
158  .rs  .rs
159  .sp  .sp
160  If the escape sequence \eP is present in a \fBpcretest\fP data line, the  If the escape sequence \eP is present in a \fBpcretest\fP data line, the
161  PCRE_PARTIAL flag is used for the match. Here is a run of \fBpcretest\fP that  PCRE_PARTIAL_SOFT option is used for the match. Here is a run of \fBpcretest\fP
162  uses the date example quoted above:  that uses the date example quoted above:
163  .sp  .sp
164      re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/      re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
165    data> 25jun04\eP    data> 25jun04\eP
# Line 99  The first data string is matched complet Line 178  The first data string is matched complet
178  matched substrings. The remaining four strings do not match the complete  matched substrings. The remaining four strings do not match the complete
179  pattern, but the first two are partial matches. Similar output is obtained  pattern, but the first two are partial matches. Similar output is obtained
180  when \fBpcre_dfa_exec()\fP is used.  when \fBpcre_dfa_exec()\fP is used.
181    .P
182    If the escape sequence \eP is present more than once in a \fBpcretest\fP data
183    line, the PCRE_PARTIAL_HARD option is set for the match.
184  .  .
185  .  .
 .SH "ISSUES WITH PARTIAL MATCHING"  
 .rs  
 .sp  
 Certain types of pattern may behave in unintuitive ways when partial matching  
 is requested, whichever matching function is used. For example, matching a  
 pattern that ends with (*FAIL), or any other assertion that causes a match to  
 fail without inspecting any data, yields PCRE_ERROR_PARTIAL rather than  
 PCRE_ERROR_NOMATCH:  
 .sp  
     re> /a+(*FAIL)/  
   data> aaa\eP  
   Partial match: aaa  
 .sp  
 Although (*FAIL) itself could possibly be made a special case, there are other  
 assertions, for example (?!), which behave in the same way, and it is not  
 possible to catch all cases. For consistency, therefore, there are no  
 exceptions to the rule that PCRE_ERROR_PARTIAL is returned instead of  
 PCRE_ERROR_NOMATCH if at any time during the match the end of the subject  
 string was reached.  
 .  
 .  
186  .SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()"  .SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()"
187  .rs  .rs
188  .sp  .sp
189  When a partial match has been found using \fBpcre_dfa_exec()\fP, it is possible  When a partial match has been found using \fBpcre_dfa_exec()\fP, it is possible
190  to continue the match by providing additional subject data and calling  to continue the match by providing additional subject data and calling
191  \fBpcre_dfa_exec()\fP again with the same compiled regular expression, this  \fBpcre_dfa_exec()\fP again with the same compiled regular expression, this
192  time setting the PCRE_DFA_RESTART option. You must also pass the same working  time setting the PCRE_DFA_RESTART option. You must pass the same working
193  space as before, because this is where details of the previous partial match  space as before, because this is where details of the previous partial match
194  are stored. Here is an example using \fBpcretest\fP, using the \eR escape  are stored. Here is an example using \fBpcretest\fP, using the \eR escape
195  sequence to set the PCRE_DFA_RESTART option (\eP sets the PCRE_PARTIAL option,  sequence to set the PCRE_DFA_RESTART option (\eD specifies the use of
196  and \eD specifies the use of \fBpcre_dfa_exec()\fP):  \fBpcre_dfa_exec()\fP):
197  .sp  .sp
198      re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/      re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
199    data> 23ja\eP\eD    data> 23ja\eP\eD
# Line 146  Notice that when the match is complete, Line 207  Notice that when the match is complete,
207  not retain the previously partially-matched string. It is up to the calling  not retain the previously partially-matched string. It is up to the calling
208  program to do that if it needs to.  program to do that if it needs to.
209  .P  .P
210  You can set PCRE_PARTIAL with PCRE_DFA_RESTART to continue partial matching  You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
211  over multiple segments. This facility can be used to pass very long subject  PCRE_DFA_RESTART to continue partial matching over multiple segments. This
212  strings to \fBpcre_dfa_exec()\fP.  facility can be used to pass very long subject strings to
213    \fBpcre_dfa_exec()\fP.
214  .  .
215  .  .
216  .SH "MULTI-SEGMENT MATCHING WITH pcre_exec()"  .SH "MULTI-SEGMENT MATCHING WITH pcre_exec()"
# Line 183  to pass the PCRE_NOTBOL or PCRE_NOTEOL o Line 245  to pass the PCRE_NOTBOL or PCRE_NOTEOL o
245  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.
246  .P  .P
247  2. If the pattern contains backward assertions (including \eb or \eB), you need  2. If the pattern contains backward assertions (including \eb or \eB), you need
248  to arrange for some overlap in the subject strings to allow for this. For  to arrange for some overlap in the subject strings to allow for them to be
249  example, using \fBpcre_dfa_exec()\fP, you could pass the subject in chunks that  correctly tested at the start of each substring. For example, using
250  are 500 bytes long, but in a buffer of 700 bytes, with the starting offset set  \fBpcre_dfa_exec()\fP, you could pass the subject in chunks that are 500 bytes
251  to 200 and the previous 200 bytes at the start of the buffer.  long, but in a buffer of 700 bytes, with the starting offset set to 200 and the
252  .P  previous 200 bytes at the start of the buffer.
253  3. Matching a subject string that is split into multiple segments does not  .P
254  always produce exactly the same result as matching over one single long string.  3. Matching a subject string that is split into multiple segments may not
255  The difference arises when there are multiple matching possibilities, because a  always produce exactly the same result as matching over one single long string,
256  partial match result is given only when there are no completed matches. This  especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
257  means that as soon as the shortest match has been found, continuation to a new  Word Boundaries" above describes an issue that arises if the pattern ends with
258  subject segment is no longer possible. Consider this \fBpcretest\fP example:  \eb or \eB. Another kind of difference may occur when there are multiple
259    matching possibilities, because a partial match result is given only when there
260    are no completed matches. This means that as soon as the shortest match has
261    been found, continuation to a new subject segment is no longer possible.
262    Consider again this \fBpcretest\fP example:
263  .sp  .sp
264      re> /dog(sbody)?/      re> /dog(sbody)?/
265    data> dogsb\eP    data> dogsb\eP
# Line 206  subject segment is no longer possible. C Line 272  subject segment is no longer possible. C
272     0: dogsbody     0: dogsbody
273     1: dog     1: dog
274  .sp  .sp
275  The pattern matches "dog" or "dogsbody". The first data line passes the string  The first data line passes the string "dogsb" to \fBpcre_exec()\fP, setting the
276  "dogsb" to \fBpcre_exec()\fP, setting the PCRE_PARTIAL option. Although the  PCRE_PARTIAL_SOFT option. Although the string is a partial match for
277  string is a partial match for "dogsbody", the result is not PCRE_ERROR_PARTIAL,  "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter string
278  because the shorter string "dog" is a complete match. Similarly, when the  "dog" is a complete match. Similarly, when the subject is presented to
279  subject is presented to \fBpcre_dfa_exec()\fP in several parts ("do" and "gsb"  \fBpcre_dfa_exec()\fP in several parts ("do" and "gsb" being the first two) the
280  being the first two) the match stops when "dog" has been found, and it is not  match stops when "dog" has been found, and it is not possible to continue. On
281  possible to continue. On the other hand, if "dogsbody" is presented as a single  the other hand, if "dogsbody" is presented as a single string,
282  string, \fBpcre_dfa_exec()\fP finds both matches.  \fBpcre_dfa_exec()\fP finds both matches.
283  .P  .P
284  Because of this phenomenon, it does not usually make sense to end a pattern  Because of these problems, it is probably best to use PCRE_PARTIAL_HARD when
285  that is going to be matched in this way with a variable repeat.  matching multi-segment data. The example above then behaves differently:
286    .sp
287        re> /dog(sbody)?/
288      data> dogsb\eP\eP
289      Partial match: dogsb
290      data> do\eP\eD
291      Partial match: do
292      data> gsb\eR\eP\eP\eD
293      Partial match: gsb
294    .sp
295  .P  .P
296  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
297  start with the same pattern item may not work as expected when  start with the same pattern item may not work as expected when
# Line 261  Cambridge CB2 3QH, England. Line 336  Cambridge CB2 3QH, England.
336  .rs  .rs
337  .sp  .sp
338  .nf  .nf
339  Last updated: 26 August 2009  Last updated: 31 August 2009
340  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2009 University of Cambridge.
341  .fi  .fi

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