/[pcre]/code/trunk/doc/pcrepartial.3
ViewVC logotype

Diff of /code/trunk/doc/pcrepartial.3

Parent Directory Parent Directory | Revision Log Revision Log | View Patch Patch

revision 469 by ph10, Mon Oct 19 14:38:48 2009 UTC revision 858 by ph10, Sun Jan 8 17:55:38 2012 UTC
# Line 4  PCRE - Perl-compatible regular expressio Line 4  PCRE - Perl-compatible regular expressio
4  .SH "PARTIAL MATCHING IN PCRE"  .SH "PARTIAL MATCHING IN PCRE"
5  .rs  .rs
6  .sp  .sp
7  In normal use of PCRE, if the subject string that is passed to  In normal use of PCRE, if the subject string that is passed to a matching
8  \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP matches as far as it goes, but is  function matches as far as it goes, but is too short to match the entire
9  too short to match the entire pattern, PCRE_ERROR_NOMATCH is returned. There  pattern, PCRE_ERROR_NOMATCH is returned. There are circumstances where it might
10  are circumstances where it might be helpful to distinguish this case from other  be helpful to distinguish this case from other cases in which there is no
11  cases in which there is no match.  match.
12  .P  .P
13  Consider, for example, an application where a human is required to type in data  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  for a field with specific formatting requirements. An example might be a date
# Line 21  what has been typed so far is potentiall Line 21  what has been typed so far is potentiall
21  as soon as a mistake is made, by beeping and not reflecting the character that  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  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. Partial matching can also sometimes be useful when the subject string  entered. Partial matching can also be useful when the subject string is very
25  is very long and is not all available at once.  long and is not all available at once.
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 any of the matching
29  \fBpcre_dfa_exec()\fP. For backwards compatibility, PCRE_PARTIAL is a synonym  functions. For backwards compatibility, PCRE_PARTIAL is a synonym for
30  for PCRE_PARTIAL_SOFT. The essential difference between the two options is  PCRE_PARTIAL_SOFT. The essential difference between the two options is whether
31  whether or not a partial match is preferred to an alternative complete match,  or not a partial match is preferred to an alternative complete match, though
32  though the details differ between the two matching functions. If both options  the details differ between the two types of matching function. 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 two of PCRE's optimizations. PCRE  Setting a partial matching option disables the use of any just-in-time code
36  remembers the last literal byte in a pattern, and abandons matching immediately  that was set up by studying the compiled pattern with the
37  if such a byte is not present in the subject string. This optimization cannot  PCRE_STUDY_JIT_COMPILE option. It also disables two of PCRE's standard
38  be used for a subject string that might match only partially. If the pattern  optimizations. PCRE remembers the last literal data unit in a pattern, and
39  was studied, PCRE knows the minimum length of a matching string, and does not  abandons matching immediately if it is not present in the subject string. This
40  bother to run the matching function on shorter strings. This optimization is  optimization cannot be used for a subject string that might match only
41  also disabled for partial matching.  partially. If the pattern was studied, PCRE knows the minimum length of a
42  .  matching string, and does not bother to run the matching function on shorter
43  .  strings. This optimization is also disabled for partial matching.
44  .SH "PARTIAL MATCHING USING pcre_exec()"  .
45  .rs  .
46  .sp  .SH "PARTIAL MATCHING USING pcre_exec() OR pcre16_exec()"
47  A partial match occurs during a call to \fBpcre_exec()\fP whenever the end of  .rs
48  the subject string is reached successfully, but matching cannot continue  .sp
49  because more characters are needed. However, at least one character must have  A partial match occurs during a call to \fBpcre_exec()\fP or
50  been matched. (In other words, a partial match can never be an empty string.)  \fBpcre16_exec()\fP when the end of the subject string is reached successfully,
51  .P  but matching cannot continue because more characters are needed. However, at
52  If PCRE_PARTIAL_SOFT is set, the partial match is remembered, but matching  least one character in the subject must have been inspected. This character
53  continues as normal, and other alternatives in the pattern are tried. If no  need not form part of the final matched string; lookbehind assertions and the
54  complete match can be found, \fBpcre_exec()\fP returns PCRE_ERROR_PARTIAL  \eK escape sequence provide ways of inspecting characters before the start of a
55  instead of PCRE_ERROR_NOMATCH. If there are at least two slots in the offsets  matched substring. The requirement for inspecting at least one character exists
56  vector, the first of them is set to the offset of the earliest character that  because an empty string can always be matched; without such a restriction there
57  was inspected when the partial match was found. For convenience, the second  would always be a partial match of an empty string at the end of the subject.
58  offset points to the end of the string so that a substring can easily be  .P
59  identified.  If there are at least two slots in the offsets vector when a partial match is
60    returned, the first slot is set to the offset of the earliest character that
61    was inspected. For convenience, the second offset points to the end of the
62    subject so that a substring can easily be identified.
63  .P  .P
64  For the majority of patterns, the first offset identifies the start of the  For the majority of patterns, the first offset identifies the start of the
65  partially matched string. However, for patterns that contain lookbehind  partially matched string. However, for patterns that contain lookbehind
# Line 68  inspected while carrying out the match. Line 71  inspected while carrying out the match.
71  This pattern matches "123", but only if it is preceded by "abc". If the subject  This pattern matches "123", but only if it is preceded by "abc". If the subject
72  string is "xyzabc12", the offsets after a partial match are for the substring  string is "xyzabc12", the offsets after a partial match are for the substring
73  "abc12", because all these characters are needed if another match is tried  "abc12", because all these characters are needed if another match is tried
74  with extra characters added.  with extra characters added to the subject.
75    .P
76    What happens when a partial match is identified depends on which of the two
77    partial matching options are set.
78    .
79    .
80    .SS "PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre16_exec()"
81    .rs
82    .sp
83    If PCRE_PARTIAL_SOFT is set when \fBpcre_exec()\fP or \fBpcre16_exec()\fP
84    identifies a partial match, the partial match is remembered, but matching
85    continues as normal, and other alternatives in the pattern are tried. If no
86    complete match can be found, PCRE_ERROR_PARTIAL is returned instead of
87    PCRE_ERROR_NOMATCH.
88    .P
89    This option is "soft" because it prefers a complete match over a partial match.
90    All the various matching items in a pattern behave as if the subject string is
91    potentially complete. For example, \ez, \eZ, and $ match at the end of the
92    subject, as normal, and for \eb and \eB the end of the subject is treated as a
93    non-alphanumeric.
94  .P  .P
95  If there is more than one partial match, the first one that was found provides  If there is more than one partial match, the first one that was found provides
96  the data that is returned. Consider this pattern:  the data that is returned. Consider this pattern:
# Line 77  the data that is returned. Consider this Line 99  the data that is returned. Consider this
99  .sp  .sp
100  If this is matched against the subject string "abc123dog", both  If this is matched against the subject string "abc123dog", both
101  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
102  matching, so PCRE_ERROR_PARTIAL is returned instead of PCRE_ERROR_NOMATCH. The  matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set to 3 and 9,
103  offsets are set to 3 and 9, identifying "123dog" as the first partial match  identifying "123dog" as the first partial match that was found. (In this
104  that was found. (In this example, there are two partial matches, because "dog"  example, there are two partial matches, because "dog" on its own partially
105  on its own partially matches the second alternative.)  matches the second alternative.)
106  .P  .
107  If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP, it returns  .
108  PCRE_ERROR_PARTIAL as soon as a partial match is found, without continuing to  .SS "PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre16_exec()"
109  search for possible complete matches. The difference between the two options  .rs
110  can be illustrated by a pattern such as:  .sp
111    If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP or \fBpcre16_exec()\fP,
112    PCRE_ERROR_PARTIAL is returned as soon as a partial match is found, without
113    continuing to search for possible complete matches. This option is "hard"
114    because it prefers an earlier partial match over a later complete match. For
115    this reason, the assumption is made that the end of the supplied subject string
116    may not be the true end of the available data, and so, if \ez, \eZ, \eb, \eB,
117    or $ are encountered at the end of the subject, the result is
118    PCRE_ERROR_PARTIAL.
119    .P
120    Setting PCRE_PARTIAL_HARD also affects the way UTF-8 and UTF-16
121    subject strings are checked for validity. Normally, an invalid sequence
122    causes the error PCRE_ERROR_BADUTF8 or PCRE_ERROR_BADUTF16. However, in the
123    special case of a truncated character at the end of the subject,
124    PCRE_ERROR_SHORTUTF8 or PCRE_ERROR_SHORTUTF16 is returned when
125    PCRE_PARTIAL_HARD is set.
126    .
127    .
128    .SS "Comparing hard and soft partial matching"
129    .rs
130    .sp
131    The difference between the two partial matching options can be illustrated by a
132    pattern such as:
133  .sp  .sp
134    /dog(sbody)?/    /dog(sbody)?/
135  .sp  .sp
# Line 97  if the pattern is made ungreedy the resu Line 141  if the pattern is made ungreedy the resu
141  .sp  .sp
142    /dog(sbody)??/    /dog(sbody)??/
143  .sp  .sp
144  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 that is found first,
145  finds that first, and it never continues after finding a match. It might be  and matching never continues after finding a complete match. It might be easier
146  easier to follow this explanation by thinking of the two patterns like this:  to follow this explanation by thinking of the two patterns like this:
147  .sp  .sp
148    /dog(sbody)?/    is the same as  /dogsbody|dog/    /dog(sbody)?/    is the same as  /dogsbody|dog/
149    /dog(sbody)??/   is the same as  /dog|dogsbody/    /dog(sbody)??/   is the same as  /dog|dogsbody/
150  .sp  .sp
151  The second pattern will never match "dogsbody" when \fBpcre_exec()\fP is  The second pattern will never match "dogsbody", because it will always find the
152  used, because it will always find the shorter match first.  shorter match first.
153  .  .
154  .  .
155  .SH "PARTIAL MATCHING USING pcre_dfa_exec()"  .SH "PARTIAL MATCHING USING pcre_dfa_exec() OR pcre16_dfa_exec()"
156  .rs  .rs
157  .sp  .sp
158  The \fBpcre_dfa_exec()\fP function moves along the subject string character by  The DFA functions move along the subject string character by character, without
159  character, without backtracking, searching for all possible matches  backtracking, searching for all possible matches simultaneously. If the end of
160  simultaneously. If the end of the subject is reached before the end of the  the subject is reached before the end of the pattern, there is the possibility
161  pattern, there is the possibility of a partial match, again provided that at  of a partial match, again provided that at least one character has been
162  least one character has matched.  inspected.
163  .P  .P
164  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
165  have been no complete matches. Otherwise, the complete matches are returned.  have been no complete matches. Otherwise, the complete matches are returned.
# Line 124  complete matches. The portion of the str Line 168  complete matches. The portion of the str
168  partial match was found is set as the first matching string, provided there are  partial match was found is set as the first matching string, provided there are
169  at least two slots in the offsets vector.  at least two slots in the offsets vector.
170  .P  .P
171  Because \fBpcre_dfa_exec()\fP always searches for all possible matches, and  Because the DFA functions always search for all possible matches, and there is
172  there is no difference between greedy and ungreedy repetition, its behaviour is  no difference between greedy and ungreedy repetition, their behaviour is
173  different from \fBpcre_exec\fP when PCRE_PARTIAL_HARD is set. Consider the  different from the standard functions when PCRE_PARTIAL_HARD is set. Consider
174  string "dog" matched against the ungreedy pattern shown above:  the string "dog" matched against the ungreedy pattern shown above:
175  .sp  .sp
176    /dog(sbody)??/    /dog(sbody)??/
177  .sp  .sp
178  Whereas \fBpcre_exec()\fP stops as soon as it finds the complete match for  Whereas the standard functions stop as soon as they find the complete match for
179  "dog", \fBpcre_dfa_exec()\fP also finds the partial match for "dogsbody", and  "dog", the DFA functions also find the partial match for "dogsbody", and so
180  so returns that when PCRE_PARTIAL_HARD is set.  return that when PCRE_PARTIAL_HARD is set.
181  .  .
182  .  .
183  .SH "PARTIAL MATCHING AND WORD BOUNDARIES"  .SH "PARTIAL MATCHING AND WORD BOUNDARIES"
# Line 147  results. Consider this pattern: Line 191  results. Consider this pattern:
191  .sp  .sp
192  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
193  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
194  character cannot take place, so a partial match is found. However,  character cannot take place, so a partial match is found. However, normal
195  \fBpcre_exec()\fP carries on with normal matching, which matches \eb at the end  matching carries on, and \eb matches at the end of the subject when the last
196  of the subject when the last character is a letter, thus finding a complete  character is a letter, so a complete match is found. The result, therefore, is
197  match. The result, therefore, is \fInot\fP PCRE_ERROR_PARTIAL. The same thing  \fInot\fP PCRE_ERROR_PARTIAL. Using PCRE_PARTIAL_HARD in this case does yield
198  happens with \fBpcre_dfa_exec()\fP, because it also finds the complete match.  PCRE_ERROR_PARTIAL, because then the partial match takes precedence.
 .P  
 Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because  
 then the partial match takes precedence.  
199  .  .
200  .  .
201  .SH "FORMERLY RESTRICTED PATTERNS"  .SH "FORMERLY RESTRICTED PATTERNS"
# Line 164  For releases of PCRE prior to 8.00, beca Line 205  For releases of PCRE prior to 8.00, beca
205  optimizations were implemented in the \fBpcre_exec()\fP function, the  optimizations were implemented in the \fBpcre_exec()\fP function, the
206  PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with  PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with
207  all patterns. From release 8.00 onwards, the restrictions no longer apply, and  all patterns. From release 8.00 onwards, the restrictions no longer apply, and
208  partial matching with \fBpcre_exec()\fP can be requested for any pattern.  partial matching with can be requested for any pattern.
209  .P  .P
210  Items that were formerly restricted were repeated single characters and  Items that were formerly restricted were repeated single characters and
211  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 197  that uses the date example quoted above: Line 238  that uses the date example quoted above:
238  The first data string is matched completely, so \fBpcretest\fP shows the  The first data string is matched completely, so \fBpcretest\fP shows the
239  matched substrings. The remaining four strings do not match the complete  matched substrings. The remaining four strings do not match the complete
240  pattern, but the first two are partial matches. Similar output is obtained  pattern, but the first two are partial matches. Similar output is obtained
241  when \fBpcre_dfa_exec()\fP is used.  if DFA matching is used.
242  .P  .P
243  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
244  line, the PCRE_PARTIAL_HARD option is set for the match.  line, the PCRE_PARTIAL_HARD option is set for the match.
245  .  .
246  .  .
247  .SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()"  .SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre16_dfa_exec()"
248  .rs  .rs
249  .sp  .sp
250  When a partial match has been found using \fBpcre_dfa_exec()\fP, it is possible  When a partial match has been found using a DFA matching function, it is
251  to continue the match by providing additional subject data and calling  possible to continue the match by providing additional subject data and calling
252  \fBpcre_dfa_exec()\fP again with the same compiled regular expression, this  the function again with the same compiled regular expression, this time setting
253  time setting the PCRE_DFA_RESTART option. You must pass the same working  the PCRE_DFA_RESTART option. You must pass the same working space as before,
254  space as before, because this is where details of the previous partial match  because this is where details of the previous partial match are stored. Here is
255  are stored. Here is an example using \fBpcretest\fP, using the \eR escape  an example using \fBpcretest\fP, using the \eR escape sequence to set the
256  sequence to set the PCRE_DFA_RESTART option (\eD specifies the use of  PCRE_DFA_RESTART option (\eD specifies the use of the DFA matching function):
 \fBpcre_dfa_exec()\fP):  
257  .sp  .sp
258      re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/      re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
259    data> 23ja\eP\eD    data> 23ja\eP\eD
# Line 229  program to do that if it needs to. Line 269  program to do that if it needs to.
269  .P  .P
270  You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with  You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
271  PCRE_DFA_RESTART to continue partial matching over multiple segments. This  PCRE_DFA_RESTART to continue partial matching over multiple segments. This
272  facility can be used to pass very long subject strings to  facility can be used to pass very long subject strings to the DFA matching
273  \fBpcre_dfa_exec()\fP.  functions.
274  .  .
275  .  .
276  .SH "MULTI-SEGMENT MATCHING WITH pcre_exec()"  .SH "MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre16_exec()"
277  .rs  .rs
278  .sp  .sp
279  From release 8.00, \fBpcre_exec()\fP can also be used to do multi-segment  From release 8.00, the standard matching functions can also be used to do
280  matching. Unlike \fBpcre_dfa_exec()\fP, it is not possible to restart the  multi-segment matching. Unlike the DFA functions, it is not possible to
281  previous match with a new segment of data. Instead, new data must be added to  restart the previous match with a new segment of data. Instead, new data must
282  the previous subject string, and the entire match re-run, starting from the  be added to the previous subject string, and the entire match re-run, starting
283  point where the partial match occurred. Earlier data can be discarded.  from the point where the partial match occurred. Earlier data can be discarded.
284  Consider an unanchored pattern that matches dates:  .P
285    It is best to use PCRE_PARTIAL_HARD in this situation, because it does not
286    treat the end of a segment as the end of the subject when matching \ez, \eZ,
287    \eb, \eB, and $. Consider an unanchored pattern that matches dates:
288  .sp  .sp
289      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/
290    data> The date is 23ja\eP    data> The date is 23ja\eP\eP
291    Partial match: 23ja    Partial match: 23ja
292  .sp  .sp
293  At this stage, an application could discard the text preceding "23ja", add on  At this stage, an application could discard the text preceding "23ja", add on
294  text from the next segment, and call \fBpcre_exec()\fP again. Unlike  text from the next segment, and call the matching function again. Unlike the
295  \fBpcre_dfa_exec()\fP, the entire matching string must always be available, and  DFA matching functions the entire matching string must always be available, and
296  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
297  processing time is needed.  processing time is needed.
298  .P  .P
299  \fBNote:\fP If the pattern contains lookbehind assertions, or \eK, or starts  \fBNote:\fP If the pattern contains lookbehind assertions, or \eK, or starts
300  with \eb or \eB, the string that is returned for a partial match will include  with \eb or \eB, the string that is returned for a partial match includes
301  characters that precede the partially matched string itself, because these must  characters that precede the partially matched string itself, because these must
302  be retained when adding on more characters for a subsequent matching attempt.  be retained when adding on more characters for a subsequent matching attempt.
303  .  .
# Line 265  be retained when adding on more characte Line 308  be retained when adding on more characte
308  Certain types of pattern may give problems with multi-segment matching,  Certain types of pattern may give problems with multi-segment matching,
309  whichever matching function is used.  whichever matching function is used.
310  .P  .P
311  1. If the pattern contains tests for the beginning or end of a line, you need  1. If the pattern contains a test for the beginning of a line, you need to pass
312  to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropriate, when the  the PCRE_NOTBOL option when the subject string for any call does start at the
313  subject string for any call does not contain the beginning or end of a line.  beginning of a line. There is also a PCRE_NOTEOL option, but in practice when
314    doing multi-segment matching you should be using PCRE_PARTIAL_HARD, which
315    includes the effect of PCRE_NOTEOL.
316  .P  .P
317  2. Lookbehind assertions at the start of a pattern are catered for in the  2. Lookbehind assertions at the start of a pattern are catered for in the
318  offsets that are returned for a partial match. However, in theory, a lookbehind  offsets that are returned for a partial match. However, in theory, a lookbehind
# Line 281  always produce exactly the same result a Line 326  always produce exactly the same result a
326  especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and  especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
327  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
328  \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
329  matching possibilities, because a partial match result is given only when there  matching possibilities, because (for PCRE_PARTIAL_SOFT) a partial match result
330  are no completed matches. This means that as soon as the shortest match has  is given only when there are no completed matches. This means that as soon as
331  been found, continuation to a new subject segment is no longer possible.  the shortest match has been found, continuation to a new subject segment is no
332  Consider again this \fBpcretest\fP example:  longer possible. Consider again this \fBpcretest\fP example:
333  .sp  .sp
334      re> /dog(sbody)?/      re> /dog(sbody)?/
335    data> dogsb\eP    data> dogsb\eP
# Line 297  Consider again this \fBpcretest\fP examp Line 342  Consider again this \fBpcretest\fP examp
342     0: dogsbody     0: dogsbody
343     1: dog     1: dog
344  .sp  .sp
345  The first data line passes the string "dogsb" to \fBpcre_exec()\fP, setting the  The first data line passes the string "dogsb" to a standard matching function,
346  PCRE_PARTIAL_SOFT option. Although the string is a partial match for  setting the PCRE_PARTIAL_SOFT option. Although the string is a partial match
347  "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter string  for "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter
348  "dog" is a complete match. Similarly, when the subject is presented to  string "dog" is a complete match. Similarly, when the subject is presented to
349  \fBpcre_dfa_exec()\fP in several parts ("do" and "gsb" being the first two) the  a DFA matching function in several parts ("do" and "gsb" being the first two)
350  match stops when "dog" has been found, and it is not possible to continue. On  the match stops when "dog" has been found, and it is not possible to continue.
351  the other hand, if "dogsbody" is presented as a single string,  On the other hand, if "dogsbody" is presented as a single string, a DFA
352  \fBpcre_dfa_exec()\fP finds both matches.  matching function finds both matches.
353  .P  .P
354  Because of these problems, it is probably best to use PCRE_PARTIAL_HARD when  Because of these problems, it is best to use PCRE_PARTIAL_HARD when matching
355  matching multi-segment data. The example above then behaves differently:  multi-segment data. The example above then behaves differently:
356  .sp  .sp
357      re> /dog(sbody)?/      re> /dog(sbody)?/
358    data> dogsb\eP\eP    data> dogsb\eP\eP
# Line 317  matching multi-segment data. The example Line 362  matching multi-segment data. The example
362    data> gsb\eR\eP\eP\eD    data> gsb\eR\eP\eP\eD
363    Partial match: gsb    Partial match: gsb
364  .sp  .sp
365  .P  4. Patterns that contain alternatives at the top level which do not all start
366  4. Patterns that contain alternatives at the top level which do not all  with the same pattern item may not work as expected when PCRE_DFA_RESTART is
367  start with the same pattern item may not work as expected when  used. For example, consider this pattern:
 PCRE_DFA_RESTART is used with \fBpcre_dfa_exec()\fP. For example, consider this  
 pattern:  
368  .sp  .sp
369    1234|3789    1234|3789
370  .sp  .sp
# Line 337  patterns or patterns such as: Line 380  patterns or patterns such as:
380    1234|ABCD    1234|ABCD
381  .sp  .sp
382  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
383  problem if \fBpcre_exec()\fP is used, because the entire match has to be rerun  problem if a standard matching function is used, because the entire match has
384  each time:  to be rerun each time:
385  .sp  .sp
386      re> /1234|3789/      re> /1234|3789/
387    data> ABC123\eP    data> ABC123\eP\eP
388    Partial match: 123    Partial match: 123
389    data> 1237890    data> 1237890
390     0: 3789     0: 3789
391  .sp  .sp
392  Of course, instead of using PCRE_DFA_PARTIAL, the same technique of re-running  Of course, instead of using PCRE_DFA_RESTART, the same technique of re-running
393  the entire match can also be used with \fBpcre_dfa_exec()\fP. Another  the entire match can also be used with the DFA matching functions. Another
394  possibility is to work with two buffers. If a partial match at offset \fIn\fP  possibility is to work with two buffers. If a partial match at offset \fIn\fP
395  in the first buffer is followed by "no match" when PCRE_DFA_RESTART is used on  in the first buffer is followed by "no match" when PCRE_DFA_RESTART is used on
396  the second buffer, you can then try a new match starting at offset \fIn+1\fP in  the second buffer, you can then try a new match starting at offset \fIn+1\fP in
# Line 368  Cambridge CB2 3QH, England. Line 411  Cambridge CB2 3QH, England.
411  .rs  .rs
412  .sp  .sp
413  .nf  .nf
414  Last updated: 19 October 2009  Last updated: 08 January 2012
415  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2012 University of Cambridge.
416  .fi  .fi
Legend:
Removed from v.469  
changed lines
  Added in v.858
webmaster@exim.org
 ViewVC Help
Powered by ViewVC 1.1.12