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revision 111 by ph10, Thu Mar 8 16:53:09 2007 UTC revision 469 by ph10, Mon Oct 19 14:38:48 2009 UTC
# Line 14  man page, in case the conversion went wr Line 14  man page, in case the conversion went wr
14  <br>  <br>
15  <ul>  <ul>
16  <li><a name="TOC1" href="#SEC1">PARTIAL MATCHING IN PCRE</a>  <li><a name="TOC1" href="#SEC1">PARTIAL MATCHING IN PCRE</a>
17  <li><a name="TOC2" href="#SEC2">RESTRICTED PATTERNS FOR PCRE_PARTIAL</a>  <li><a name="TOC2" href="#SEC2">PARTIAL MATCHING USING pcre_exec()</a>
18  <li><a name="TOC3" href="#SEC3">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a>  <li><a name="TOC3" href="#SEC3">PARTIAL MATCHING USING pcre_dfa_exec()</a>
19  <li><a name="TOC4" href="#SEC4">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()</a>  <li><a name="TOC4" href="#SEC4">PARTIAL MATCHING AND WORD BOUNDARIES</a>
20  <li><a name="TOC5" href="#SEC5">AUTHOR</a>  <li><a name="TOC5" href="#SEC5">FORMERLY RESTRICTED PATTERNS</a>
21  <li><a name="TOC6" href="#SEC6">REVISION</a>  <li><a name="TOC6" href="#SEC6">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a>
22    <li><a name="TOC7" href="#SEC7">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()</a>
23    <li><a name="TOC8" href="#SEC8">MULTI-SEGMENT MATCHING WITH pcre_exec()</a>
24    <li><a name="TOC9" href="#SEC9">ISSUES WITH MULTI-SEGMENT MATCHING</a>
25    <li><a name="TOC10" href="#SEC10">AUTHOR</a>
26    <li><a name="TOC11" href="#SEC11">REVISION</a>
27  </ul>  </ul>
28  <br><a name="SEC1" href="#TOC1">PARTIAL MATCHING IN PCRE</a><br>  <br><a name="SEC1" href="#TOC1">PARTIAL MATCHING IN PCRE</a><br>
29  <P>  <P>
# Line 37  in the form <i>ddmmmyy</i>, defined by t Line 42  in the form <i>ddmmmyy</i>, defined by t
42  </pre>  </pre>
43  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
44  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
45  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
46  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
47  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
48  entered.  entered. Partial matching can also sometimes be useful when the subject string
49    is very long and is not all available at once.
50  </P>  </P>
51  <P>  <P>
52  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
53  option, which can be set when calling <b>pcre_exec()</b> or  PCRE_PARTIAL_HARD options, which can be set when calling <b>pcre_exec()</b> or
54  <b>pcre_dfa_exec()</b>. When this flag is set for <b>pcre_exec()</b>, the return  <b>pcre_dfa_exec()</b>. For backwards compatibility, PCRE_PARTIAL is a synonym
55  code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any time  for PCRE_PARTIAL_SOFT. The essential difference between the two options is
56  during the matching process the last part of the subject string matched part of  whether or not a partial match is preferred to an alternative complete match,
57  the pattern. Unfortunately, for non-anchored matching, it is not possible to  though the details differ between the two matching functions. If both options
58  obtain the position of the start of the partial match. No captured data is set  are set, PCRE_PARTIAL_HARD takes precedence.
59  when PCRE_ERROR_PARTIAL is returned.  </P>
60  </P>  <P>
61  <P>  Setting a partial matching option disables two of PCRE's optimizations. PCRE
62  When PCRE_PARTIAL is set for <b>pcre_dfa_exec()</b>, the return code  remembers the last literal byte in a pattern, and abandons matching immediately
63  PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end of the  if such a byte is not present in the subject string. This optimization cannot
64  subject is reached, there have been no complete matches, but there is still at  be used for a subject string that might match only partially. If the pattern
65  least one matching possibility. The portion of the string that provided the  was studied, PCRE knows the minimum length of a matching string, and does not
66  partial match is set as the first matching string.  bother to run the matching function on shorter strings. This optimization is
67    also disabled for partial matching.
68    </P>
69    <br><a name="SEC2" href="#TOC1">PARTIAL MATCHING USING pcre_exec()</a><br>
70    <P>
71    A partial match occurs during a call to <b>pcre_exec()</b> whenever the end of
72    the subject string is reached successfully, but matching cannot continue
73    because more characters are needed. However, at least one character must have
74    been matched. (In other words, a partial match can never be an empty string.)
75    </P>
76    <P>
77    If PCRE_PARTIAL_SOFT is set, the partial match is remembered, but matching
78    continues as normal, and other alternatives in the pattern are tried. If no
79    complete match can be found, <b>pcre_exec()</b> returns PCRE_ERROR_PARTIAL
80    instead of PCRE_ERROR_NOMATCH. If there are at least two slots in the offsets
81    vector, the first of them is set to the offset of the earliest character that
82    was inspected when the partial match was found. For convenience, the second
83    offset points to the end of the string so that a substring can easily be
84    identified.
85    </P>
86    <P>
87    For the majority of patterns, the first offset identifies the start of the
88    partially matched string. However, for patterns that contain lookbehind
89    assertions, or \K, or begin with \b or \B, earlier characters have been
90    inspected while carrying out the match. For example:
91    <pre>
92      /(?&#60;=abc)123/
93    </pre>
94    This pattern matches "123", but only if it is preceded by "abc". If the subject
95    string is "xyzabc12", the offsets after a partial match are for the substring
96    "abc12", because all these characters are needed if another match is tried
97    with extra characters added.
98  </P>  </P>
99  <P>  <P>
100  Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers the  If there is more than one partial match, the first one that was found provides
101  last literal byte in a pattern, and abandons matching immediately if such a  the data that is returned. Consider this pattern:
102  byte is not present in the subject string. This optimization cannot be used  <pre>
103  for a subject string that might match only partially.    /123\w+X|dogY/
104    </pre>
105    If this is matched against the subject string "abc123dog", both
106    alternatives fail to match, but the end of the subject is reached during
107    matching, so PCRE_ERROR_PARTIAL is returned instead of PCRE_ERROR_NOMATCH. The
108    offsets are set to 3 and 9, identifying "123dog" as the first partial match
109    that was found. (In this example, there are two partial matches, because "dog"
110    on its own partially matches the second alternative.)
111  </P>  </P>
 <br><a name="SEC2" href="#TOC1">RESTRICTED PATTERNS FOR PCRE_PARTIAL</a><br>  
112  <P>  <P>
113  Because of the way certain internal optimizations are implemented in the  If PCRE_PARTIAL_HARD is set for <b>pcre_exec()</b>, it returns
114  <b>pcre_exec()</b> function, the PCRE_PARTIAL option cannot be used with all  PCRE_ERROR_PARTIAL as soon as a partial match is found, without continuing to
115  patterns. These restrictions do not apply when <b>pcre_dfa_exec()</b> is used.  search for possible complete matches. The difference between the two options
116  For <b>pcre_exec()</b>, repeated single characters such as  can be illustrated by a pattern such as:
117  <pre>  <pre>
118    a{2,4}    /dog(sbody)?/
119  </pre>  </pre>
120  and repeated single metasequences such as  This matches either "dog" or "dogsbody", greedily (that is, it prefers the
121    longer string if possible). If it is matched against the string "dog" with
122    PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if
123    PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand,
124    if the pattern is made ungreedy the result is different:
125  <pre>  <pre>
126    \d+    /dog(sbody)??/
127  </pre>  </pre>
128  are not permitted if the maximum number of occurrences is greater than one.  In this case the result is always a complete match because <b>pcre_exec()</b>
129  Optional items such as \d? (where the maximum is one) are permitted.  finds that first, and it never continues after finding a match. It might be
130  Quantifiers with any values are permitted after parentheses, so the invalid  easier to follow this explanation by thinking of the two patterns like this:
 examples above can be coded thus:  
131  <pre>  <pre>
132    (a){2,4}    /dog(sbody)?/    is the same as  /dogsbody|dog/
133    (\d)+    /dog(sbody)??/   is the same as  /dog|dogsbody/
134  </pre>  </pre>
135  These constructions run more slowly, but for the kinds of application that are  The second pattern will never match "dogsbody" when <b>pcre_exec()</b> is
136  envisaged for this facility, this is not felt to be a major restriction.  used, because it will always find the shorter match first.
137  </P>  </P>
138    <br><a name="SEC3" href="#TOC1">PARTIAL MATCHING USING pcre_dfa_exec()</a><br>
139  <P>  <P>
140  If PCRE_PARTIAL is set for a pattern that does not conform to the restrictions,  The <b>pcre_dfa_exec()</b> function moves along the subject string character by
141  <b>pcre_exec()</b> returns the error code PCRE_ERROR_BADPARTIAL (-13).  character, without backtracking, searching for all possible matches
142    simultaneously. If the end of the subject is reached before the end of the
143    pattern, there is the possibility of a partial match, again provided that at
144    least one character has matched.
145    </P>
146    <P>
147    When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there
148    have been no complete matches. Otherwise, the complete matches are returned.
149    However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any
150    complete matches. The portion of the string that was inspected when the longest
151    partial match was found is set as the first matching string, provided there are
152    at least two slots in the offsets vector.
153    </P>
154    <P>
155    Because <b>pcre_dfa_exec()</b> always searches for all possible matches, and
156    there is no difference between greedy and ungreedy repetition, its behaviour is
157    different from <b>pcre_exec</b> when PCRE_PARTIAL_HARD is set. Consider the
158    string "dog" matched against the ungreedy pattern shown above:
159    <pre>
160      /dog(sbody)??/
161    </pre>
162    Whereas <b>pcre_exec()</b> stops as soon as it finds the complete match for
163    "dog", <b>pcre_dfa_exec()</b> also finds the partial match for "dogsbody", and
164    so returns that when PCRE_PARTIAL_HARD is set.
165    </P>
166    <br><a name="SEC4" href="#TOC1">PARTIAL MATCHING AND WORD BOUNDARIES</a><br>
167    <P>
168    If a pattern ends with one of sequences \b or \B, which test for word
169    boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
170    results. Consider this pattern:
171    <pre>
172      /\bcat\b/
173    </pre>
174    This matches "cat", provided there is a word boundary at either end. If the
175    subject string is "the cat", the comparison of the final "t" with a following
176    character cannot take place, so a partial match is found. However,
177    <b>pcre_exec()</b> carries on with normal matching, which matches \b at the end
178    of the subject when the last character is a letter, thus finding a complete
179    match. The result, therefore, is <i>not</i> PCRE_ERROR_PARTIAL. The same thing
180    happens with <b>pcre_dfa_exec()</b>, because it also finds the complete match.
181    </P>
182    <P>
183    Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because
184    then the partial match takes precedence.
185    </P>
186    <br><a name="SEC5" href="#TOC1">FORMERLY RESTRICTED PATTERNS</a><br>
187    <P>
188    For releases of PCRE prior to 8.00, because of the way certain internal
189    optimizations were implemented in the <b>pcre_exec()</b> function, the
190    PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with
191    all patterns. From release 8.00 onwards, the restrictions no longer apply, and
192    partial matching with <b>pcre_exec()</b> can be requested for any pattern.
193    </P>
194    <P>
195    Items that were formerly restricted were repeated single characters and
196    repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not
197    conform to the restrictions, <b>pcre_exec()</b> returned the error code
198    PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
199    PCRE_INFO_OKPARTIAL call to <b>pcre_fullinfo()</b> to find out if a compiled
200    pattern can be used for partial matching now always returns 1.
201  </P>  </P>
202  <br><a name="SEC3" href="#TOC1">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a><br>  <br><a name="SEC6" href="#TOC1">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a><br>
203  <P>  <P>
204  If the escape sequence \P is present in a <b>pcretest</b> data line, the  If the escape sequence \P is present in a <b>pcretest</b> data line, the
205  PCRE_PARTIAL flag is used for the match. Here is a run of <b>pcretest</b> that  PCRE_PARTIAL_SOFT option is used for the match. Here is a run of <b>pcretest</b>
206  uses the date example quoted above:  that uses the date example quoted above:
207  <pre>  <pre>
208      re&#62; /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/      re&#62; /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
209    data&#62; 25jun04\P    data&#62; 25jun04\P
210     0: 25jun04     0: 25jun04
211     1: jun     1: jun
212    data&#62; 25dec3\P    data&#62; 25dec3\P
213    Partial match    Partial match: 23dec3
214    data&#62; 3ju\P    data&#62; 3ju\P
215    Partial match    Partial match: 3ju
216    data&#62; 3juj\P    data&#62; 3juj\P
217    No match    No match
218    data&#62; j\P    data&#62; j\P
# Line 114  uses the date example quoted above: Line 220  uses the date example quoted above:
220  </pre>  </pre>
221  The first data string is matched completely, so <b>pcretest</b> shows the  The first data string is matched completely, so <b>pcretest</b> shows the
222  matched substrings. The remaining four strings do not match the complete  matched substrings. The remaining four strings do not match the complete
223  pattern, but the first two are partial matches. The same test, using  pattern, but the first two are partial matches. Similar output is obtained
224  <b>pcre_dfa_exec()</b> matching (by means of the \D escape sequence), produces  when <b>pcre_dfa_exec()</b> is used.
225  the following output:  </P>
226  <pre>  <P>
227      re&#62; /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/  If the escape sequence \P is present more than once in a <b>pcretest</b> data
228    data&#62; 25jun04\P\D  line, the PCRE_PARTIAL_HARD option is set for the match.
    0: 25jun04  
   data&#62; 23dec3\P\D  
   Partial match: 23dec3  
   data&#62; 3ju\P\D  
   Partial match: 3ju  
   data&#62; 3juj\P\D  
   No match  
   data&#62; j\P\D  
   No match  
 </pre>  
 Notice that in this case the portion of the string that was matched is made  
 available.  
229  </P>  </P>
230  <br><a name="SEC4" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()</a><br>  <br><a name="SEC7" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()</a><br>
231  <P>  <P>
232  When a partial match has been found using <b>pcre_dfa_exec()</b>, it is possible  When a partial match has been found using <b>pcre_dfa_exec()</b>, it is possible
233  to continue the match by providing additional subject data and calling  to continue the match by providing additional subject data and calling
234  <b>pcre_dfa_exec()</b> again with the same compiled regular expression, this  <b>pcre_dfa_exec()</b> again with the same compiled regular expression, this
235  time setting the PCRE_DFA_RESTART option. You must also pass the same working  time setting the PCRE_DFA_RESTART option. You must pass the same working
236  space as before, because this is where details of the previous partial match  space as before, because this is where details of the previous partial match
237  are stored. Here is an example using <b>pcretest</b>, using the \R escape  are stored. Here is an example using <b>pcretest</b>, using the \R escape
238  sequence to set the PCRE_DFA_RESTART option (\P and \D are as above):  sequence to set the PCRE_DFA_RESTART option (\D specifies the use of
239    <b>pcre_dfa_exec()</b>):
240  <pre>  <pre>
241      re&#62; /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/      re&#62; /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
242    data&#62; 23ja\P\D    data&#62; 23ja\P\D
# Line 156  not retain the previously partially-matc Line 251  not retain the previously partially-matc
251  program to do that if it needs to.  program to do that if it needs to.
252  </P>  </P>
253  <P>  <P>
254  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
255  over multiple segments. This facility can be used to pass very long subject  PCRE_DFA_RESTART to continue partial matching over multiple segments. This
256  strings to <b>pcre_dfa_exec()</b>. However, some care is needed for certain  facility can be used to pass very long subject strings to
257  types of pattern.  <b>pcre_dfa_exec()</b>.
258    </P>
259    <br><a name="SEC8" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_exec()</a><br>
260    <P>
261    From release 8.00, <b>pcre_exec()</b> can also be used to do multi-segment
262    matching. Unlike <b>pcre_dfa_exec()</b>, it is not possible to restart the
263    previous match with a new segment of data. Instead, new data must be added to
264    the previous subject string, and the entire match re-run, starting from the
265    point where the partial match occurred. Earlier data can be discarded.
266    Consider an unanchored pattern that matches dates:
267    <pre>
268        re&#62; /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
269      data&#62; The date is 23ja\P
270      Partial match: 23ja
271    </pre>
272    At this stage, an application could discard the text preceding "23ja", add on
273    text from the next segment, and call <b>pcre_exec()</b> again. Unlike
274    <b>pcre_dfa_exec()</b>, the entire matching string must always be available, and
275    the complete matching process occurs for each call, so more memory and more
276    processing time is needed.
277    </P>
278    <P>
279    <b>Note:</b> If the pattern contains lookbehind assertions, or \K, or starts
280    with \b or \B, the string that is returned for a partial match will include
281    characters that precede the partially matched string itself, because these must
282    be retained when adding on more characters for a subsequent matching attempt.
283    </P>
284    <br><a name="SEC9" href="#TOC1">ISSUES WITH MULTI-SEGMENT MATCHING</a><br>
285    <P>
286    Certain types of pattern may give problems with multi-segment matching,
287    whichever matching function is used.
288  </P>  </P>
289  <P>  <P>
290  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
# Line 167  to pass the PCRE_NOTBOL or PCRE_NOTEOL o Line 292  to pass the PCRE_NOTBOL or PCRE_NOTEOL o
292  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.
293  </P>  </P>
294  <P>  <P>
295  2. If the pattern contains backward assertions (including \b or \B), you need  2. Lookbehind assertions at the start of a pattern are catered for in the
296  to arrange for some overlap in the subject strings to allow for this. For  offsets that are returned for a partial match. However, in theory, a lookbehind
297  example, you could pass the subject in chunks that are 500 bytes long, but in  assertion later in the pattern could require even earlier characters to be
298  a buffer of 700 bytes, with the starting offset set to 200 and the previous 200  inspected, and it might not have been reached when a partial match occurs. This
299  bytes at the start of the buffer.  is probably an extremely unlikely case; you could guard against it to a certain
300    extent by always including extra characters at the start.
301  </P>  </P>
302  <P>  <P>
303  3. Matching a subject string that is split into multiple segments does not  3. Matching a subject string that is split into multiple segments may not
304  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,
305  The difference arises when there are multiple matching possibilities, because a  especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
306  partial match result is given only when there are no completed matches in a  Word Boundaries" above describes an issue that arises if the pattern ends with
307  call to fBpcre_dfa_exec()\fP. This means that as soon as the shortest match has  \b or \B. Another kind of difference may occur when there are multiple
308    matching possibilities, because a partial match result is given only when there
309    are no completed matches. This means that as soon as the shortest match has
310  been found, continuation to a new subject segment is no longer possible.  been found, continuation to a new subject segment is no longer possible.
311  Consider this <b>pcretest</b> example:  Consider again this <b>pcretest</b> example:
312  <pre>  <pre>
313      re&#62; /dog(sbody)?/      re&#62; /dog(sbody)?/
314      data&#62; dogsb\P
315       0: dog
316    data&#62; do\P\D    data&#62; do\P\D
317    Partial match: do    Partial match: do
318    data&#62; gsb\R\P\D    data&#62; gsb\R\P\D
# Line 191  Consider this <b>pcretest</b> example: Line 321  Consider this <b>pcretest</b> example:
321     0: dogsbody     0: dogsbody
322     1: dog     1: dog
323  </pre>  </pre>
324  The pattern matches the words "dog" or "dogsbody". When the subject is  The first data line passes the string "dogsb" to <b>pcre_exec()</b>, setting the
325  presented in several parts ("do" and "gsb" being the first two) the match stops  PCRE_PARTIAL_SOFT option. Although the string is a partial match for
326  when "dog" has been found, and it is not possible to continue. On the other  "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter string
327  hand, if "dogsbody" is presented as a single string, both matches are found.  "dog" is a complete match. Similarly, when the subject is presented to
328    <b>pcre_dfa_exec()</b> in several parts ("do" and "gsb" being the first two) the
329    match stops when "dog" has been found, and it is not possible to continue. On
330    the other hand, if "dogsbody" is presented as a single string,
331    <b>pcre_dfa_exec()</b> finds both matches.
332  </P>  </P>
333  <P>  <P>
334  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
335  that is going to be matched in this way with a variable repeat.  matching multi-segment data. The example above then behaves differently:
336    <pre>
337        re&#62; /dog(sbody)?/
338      data&#62; dogsb\P\P
339      Partial match: dogsb
340      data&#62; do\P\D
341      Partial match: do
342      data&#62; gsb\R\P\P\D
343      Partial match: gsb
344    
345    </PRE>
346  </P>  </P>
347  <P>  <P>
348  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
349  start with the same pattern item may not work as expected. For example,  start with the same pattern item may not work as expected when
350  consider this pattern:  PCRE_DFA_RESTART is used with <b>pcre_dfa_exec()</b>. For example, consider this
351    pattern:
352  <pre>  <pre>
353    1234|3789    1234|3789
354  </pre>  </pre>
355  If the first part of the subject is "ABC123", a partial match of the first  If the first part of the subject is "ABC123", a partial match of the first
356  alternative is found at offset 3. There is no partial match for the second  alternative is found at offset 3. There is no partial match for the second
357  alternative, because such a match does not start at the same point in the  alternative, because such a match does not start at the same point in the
358  subject string. Attempting to continue with the string "789" does not yield a  subject string. Attempting to continue with the string "7890" does not yield a
359  match because only those alternatives that match at one point in the subject  match because only those alternatives that match at one point in the subject
360  are remembered. The problem arises because the start of the second alternative  are remembered. The problem arises because the start of the second alternative
361  matches within the first alternative. There is no problem with anchored  matches within the first alternative. There is no problem with anchored
# Line 218  patterns or patterns such as: Line 363  patterns or patterns such as:
363  <pre>  <pre>
364    1234|ABCD    1234|ABCD
365  </pre>  </pre>
366  where no string can be a partial match for both alternatives.  where no string can be a partial match for both alternatives. This is not a
367    problem if <b>pcre_exec()</b> is used, because the entire match has to be rerun
368    each time:
369    <pre>
370        re&#62; /1234|3789/
371      data&#62; ABC123\P
372      Partial match: 123
373      data&#62; 1237890
374       0: 3789
375    </pre>
376    Of course, instead of using PCRE_DFA_PARTIAL, the same technique of re-running
377    the entire match can also be used with <b>pcre_dfa_exec()</b>. Another
378    possibility is to work with two buffers. If a partial match at offset <i>n</i>
379    in the first buffer is followed by "no match" when PCRE_DFA_RESTART is used on
380    the second buffer, you can then try a new match starting at offset <i>n+1</i> in
381    the first buffer.
382  </P>  </P>
383  <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>  <br><a name="SEC10" href="#TOC1">AUTHOR</a><br>
384  <P>  <P>
385  Philip Hazel  Philip Hazel
386  <br>  <br>
# Line 229  University Computing Service Line 389  University Computing Service
389  Cambridge CB2 3QH, England.  Cambridge CB2 3QH, England.
390  <br>  <br>
391  </P>  </P>
392  <br><a name="SEC6" href="#TOC1">REVISION</a><br>  <br><a name="SEC11" href="#TOC1">REVISION</a><br>
393  <P>  <P>
394  Last updated: 06 March 2007  Last updated: 19 October 2009
395  <br>  <br>
396  Copyright &copy; 1997-2007 University of Cambridge.  Copyright &copy; 1997-2009 University of Cambridge.
397  <br>  <br>
398  <p>  <p>
399  Return to the <a href="index.html">PCRE index page</a>.  Return to the <a href="index.html">PCRE index page</a>.

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