trunk/doc/pcrepartial.3

.TH PCREPARTIAL 3
.SH NAME
PCRE - Perl-compatible regular expressions
.SH "PARTIAL MATCHING IN PCRE"
.rs
.sp
In normal use of PCRE, if the subject string that is passed to
\fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP matches as far as it goes, but is
too short to match the entire pattern, PCRE_ERROR_NOMATCH is returned. There
are circumstances where it might be helpful to distinguish this case from other
cases in which there is no match.
.P
Consider, for example, an application where a human is required to type in data
for a field with specific formatting requirements. An example might be a date
in the form \fIddmmmyy\fP, defined by this pattern:
.sp
  ^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$
.sp
If the application sees the user's keystrokes one by one, and can check that
what has been typed so far is potentially valid, it is able to raise an error
as soon as a mistake is made, by beeping and not reflecting the character that
has been typed, for example. This immediate feedback is likely to be a better
user interface than a check that is delayed until the entire string has been
entered. Partial matching can also sometimes be useful when the subject string
is very long and is not all available at once.
.P
PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and
PCRE_PARTIAL_HARD options, which can be set when calling \fBpcre_exec()\fP or
\fBpcre_dfa_exec()\fP. For backwards compatibility, PCRE_PARTIAL is a synonym 
for PCRE_PARTIAL_SOFT. The essential difference between the two options is 
whether or not a partial match is preferred to an alternative complete match, 
though the details differ between the two matching functions. If both options 
are set, PCRE_PARTIAL_HARD takes precedence.
.P
Setting a partial matching option disables one of PCRE's optimizations. PCRE
remembers the last literal byte in a pattern, and abandons matching immediately
if such a byte is not present in the subject string. This optimization cannot
be used for a subject string that might match only partially.
.
.
.SH "PARTIAL MATCHING USING pcre_exec()"
.rs
.sp
A partial match occurs during a call to \fBpcre_exec()\fP whenever the end of
the subject string is reached successfully, but matching cannot continue
because more characters are needed. However, at least one character must have
been matched. (In other words, a partial match can never be an empty string.)
.P
If PCRE_PARTIAL_SOFT is set, the partial match is remembered, but matching
continues as normal, and other alternatives in the pattern are tried. If no
complete match can be found, \fBpcre_exec()\fP returns PCRE_ERROR_PARTIAL
instead of PCRE_ERROR_NOMATCH, and if there are at least two slots in the
offsets vector, they are filled in with the offsets of the longest string that
partially matched. Consider this pattern:
.sp
  /123\ew+X|dogY/
.sp
If this is matched against the subject string "abc123dog", both
alternatives fail to match, but the end of the subject is reached during 
matching, so PCRE_ERROR_PARTIAL is returned instead of PCRE_ERROR_NOMATCH. The
offsets are set to 3 and 9, identifying "123dog" as the longest partial match
that was found. (In this example, there are two partial matches, because "dog"
on its own partially matches the second alternative.)
.P
If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP, it returns 
PCRE_ERROR_PARTIAL as soon as a partial match is found, without continuing to
search for possible complete matches. The difference between the two options
can be illustrated by a pattern such as:
.sp
  /dog(sbody)?/
.sp
This matches either "dog" or "dogsbody", greedily (that is, it prefers the 
longer string if possible). If it is matched against the string "dog" with
PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if 
PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand, 
if the pattern is made ungreedy the result is different:
.sp
  /dog(sbody)??/
.sp
In this case the result is always a complete match because \fBpcre_exec()\fP 
finds that first, and it never continues after finding a match. It might be 
easier to follow this explanation by thinking of the two patterns like this:
.sp
  /dog(sbody)?/    is the same as  /dogsbody|dog/
  /dog(sbody)??/   is the same as  /dog|dogsbody/
.sp
The second pattern will never match "dogsbody" when \fBpcre_exec()\fP is 
used, because it will always find the shorter match first.
.
.
.SH "PARTIAL MATCHING USING pcre_dfa_exec()"
.rs
.sp
The \fBpcre_dfa_exec()\fP function moves along the subject string character by 
character, without backtracking, searching for all possible matches 
simultaneously. If the end of the subject is reached before the end of the 
pattern, there is the possibility of a partial match, again provided that at
least one character has matched.
.P
When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there
have been no complete matches. Otherwise, the complete matches are returned.
However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any
complete matches. The portion of the string that provided the longest partial
match is set as the first matching string, provided there are at least two
slots in the offsets vector.
.P
Because \fBpcre_dfa_exec()\fP always searches for all possible matches, and 
there is no difference between greedy and ungreedy repetition, its behaviour is
different from \fBpcre_exec\fP when PCRE_PARTIAL_HARD is set. Consider the 
string "dog" matched against the ungreedy pattern shown above:
.sp
  /dog(sbody)??/
.sp
Whereas \fBpcre_exec()\fP stops as soon as it finds the complete match for 
"dog", \fBpcre_dfa_exec()\fP also finds the partial match for "dogsbody", and
so returns that when PCRE_PARTIAL_HARD is set.
.
.
.SH "PARTIAL MATCHING AND WORD BOUNDARIES"
.rs
.sp
If a pattern ends with one of sequences \ew or \eW, which test for word 
boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive 
results. Consider this pattern:
.sp
  /\ebcat\eb/
.sp
This matches "cat", provided there is a word boundary at either end. If the
subject string is "the cat", the comparison of the final "t" with a following
character cannot take place, so a partial match is found. However, 
\fBpcre_exec()\fP carries on with normal matching, which matches \eb at the end 
of the subject when the last character is a letter, thus finding a complete 
match. The result, therefore, is \fInot\fP PCRE_ERROR_PARTIAL. The same thing 
happens with \fBpcre_dfa_exec()\fP, because it also finds the complete match.
.P
Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because 
then the partial match takes precedence.
.
.
.SH "FORMERLY RESTRICTED PATTERNS"
.rs
.sp
For releases of PCRE prior to 8.00, because of the way certain internal
optimizations were implemented in the \fBpcre_exec()\fP function, the
PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with
all patterns. From release 8.00 onwards, the restrictions no longer apply, and
partial matching with \fBpcre_exec()\fP can be requested for any pattern.
.P
Items that were formerly restricted were repeated single characters and
repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not
conform to the restrictions, \fBpcre_exec()\fP returned the error code
PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
PCRE_INFO_OKPARTIAL call to \fBpcre_fullinfo()\fP to find out if a compiled
pattern can be used for partial matching now always returns 1.
.
.
.SH "EXAMPLE OF PARTIAL MATCHING USING PCRETEST"
.rs
.sp
If the escape sequence \eP is present in a \fBpcretest\fP data line, the
PCRE_PARTIAL_SOFT option is used for the match. Here is a run of \fBpcretest\fP
that uses the date example quoted above:
.sp
    re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
  data> 25jun04\eP
   0: 25jun04
   1: jun
  data> 25dec3\eP
  Partial match: 23dec3
  data> 3ju\eP
  Partial match: 3ju
  data> 3juj\eP
  No match
  data> j\eP
  No match
.sp
The first data string is matched completely, so \fBpcretest\fP shows the
matched substrings. The remaining four strings do not match the complete
pattern, but the first two are partial matches. Similar output is obtained
when \fBpcre_dfa_exec()\fP is used.
.P
If the escape sequence \eP is present more than once in a \fBpcretest\fP data
line, the PCRE_PARTIAL_HARD option is set for the match.
.
.                                                          
.SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()"
.rs
.sp
When a partial match has been found using \fBpcre_dfa_exec()\fP, it is possible
to continue the match by providing additional subject data and calling
\fBpcre_dfa_exec()\fP again with the same compiled regular expression, this
time setting the PCRE_DFA_RESTART option. You must pass the same working
space as before, because this is where details of the previous partial match
are stored. Here is an example using \fBpcretest\fP, using the \eR escape
sequence to set the PCRE_DFA_RESTART option (\eD specifies the use of
\fBpcre_dfa_exec()\fP):
.sp
    re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
  data> 23ja\eP\eD
  Partial match: 23ja
  data> n05\eR\eD
   0: n05
.sp
The first call has "23ja" as the subject, and requests partial matching; the
second call has "n05" as the subject for the continued (restarted) match.
Notice that when the match is complete, only the last part is shown; PCRE does
not retain the previously partially-matched string. It is up to the calling
program to do that if it needs to.
.P
You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
PCRE_DFA_RESTART to continue partial matching over multiple segments. This
facility can be used to pass very long subject strings to
\fBpcre_dfa_exec()\fP.
.
.
.SH "MULTI-SEGMENT MATCHING WITH pcre_exec()"
.rs
.sp
From release 8.00, \fBpcre_exec()\fP can also be used to do multi-segment 
matching. Unlike \fBpcre_dfa_exec()\fP, it is not possible to restart the 
previous match with a new segment of data. Instead, new data must be added to 
the previous subject string, and the entire match re-run, starting from the 
point where the partial match occurred. Earlier data can be discarded.
Consider an unanchored pattern that matches dates:
.sp
    re> /\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed/
  data> The date is 23ja\eP
  Partial match: 23ja
.sp
The this stage, an application could discard the text preceding "23ja", add on 
text from the next segment, and call \fBpcre_exec()\fP again. Unlike 
\fBpcre_dfa_exec()\fP, the entire matching string must always be available, and 
the complete matching process occurs for each call, so more memory and more 
processing time is needed.
.
.                                                          
.SH "ISSUES WITH MULTI-SEGMENT MATCHING"
.rs
.sp
Certain types of pattern may give problems with multi-segment matching, 
whichever matching function is used.
.P
1. If the pattern contains tests for the beginning or end of a line, you need
to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropriate, when the
subject string for any call does not contain the beginning or end of a line.
.P
2. If the pattern contains backward assertions (including \eb or \eB), you need
to arrange for some overlap in the subject strings to allow for them to be
correctly tested at the start of each substring. For example, using
\fBpcre_dfa_exec()\fP, you could pass the subject in chunks that are 500 bytes
long, but in a buffer of 700 bytes, with the starting offset set to 200 and the
previous 200 bytes at the start of the buffer.
.P
3. Matching a subject string that is split into multiple segments may not
always produce exactly the same result as matching over one single long string,
especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and 
Word Boundaries" above describes an issue that arises if the pattern ends with 
\eb or \eB. Another kind of difference may occur when there are multiple
matching possibilities, because a partial match result is given only when there
are no completed matches. This means that as soon as the shortest match has
been found, continuation to a new subject segment is no longer possible.
Consider again this \fBpcretest\fP example:
.sp
    re> /dog(sbody)?/
  data> dogsb\eP
   0: dog    
  data> do\eP\eD
  Partial match: do
  data> gsb\eR\eP\eD
   0: g
  data> dogsbody\eD
   0: dogsbody
   1: dog
.sp
The first data line passes the string "dogsb" to \fBpcre_exec()\fP, setting the
PCRE_PARTIAL_SOFT option. Although the string is a partial match for
"dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter string
"dog" is a complete match. Similarly, when the subject is presented to
\fBpcre_dfa_exec()\fP in several parts ("do" and "gsb" being the first two) the
match stops when "dog" has been found, and it is not possible to continue. On
the other hand, if "dogsbody" is presented as a single string,
\fBpcre_dfa_exec()\fP finds both matches.
.P
Because of these problems, it is probably best to use PCRE_PARTIAL_HARD when
matching multi-segment data. The example above then behaves differently:
.sp
    re> /dog(sbody)?/
  data> dogsb\eP\eP
  Partial match: dogsb 
  data> do\eP\eD
  Partial match: do
  data> gsb\eR\eP\eP\eD
  Partial match: gsb    
.sp
.P
4. Patterns that contain alternatives at the top level which do not all
start with the same pattern item may not work as expected when 
\fBpcre_dfa_exec()\fP is used. For example, consider this pattern:
.sp
  1234|3789
.sp
If the first part of the subject is "ABC123", a partial match of the first
alternative is found at offset 3. There is no partial match for the second
alternative, because such a match does not start at the same point in the
subject string. Attempting to continue with the string "7890" does not yield a
match because only those alternatives that match at one point in the subject
are remembered. The problem arises because the start of the second alternative
matches within the first alternative. There is no problem with anchored
patterns or patterns such as:
.sp
  1234|ABCD
.sp
where no string can be a partial match for both alternatives. This is not a
problem if \fPpcre_exec()\fP is used, because the entire match has to be rerun 
each time:
.sp
    re> /1234|3789/
  data> ABC123\eP
  Partial match: 123
  data> 1237890
   0: 3789
.sp        
.
.
.SH AUTHOR
.rs
.sp
.nf
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
.fi
.
.
.SH REVISION
.rs
.sp
.nf
Last updated: 31 August 2009
Copyright (c) 1997-2009 University of Cambridge.
.fi
1	nigel	79	.TH PCREPARTIAL 3
2	nigel	75	.SH NAME
3			PCRE - Perl-compatible regular expressions
4			.SH "PARTIAL MATCHING IN PCRE"
5			.rs
6			.sp
7			In normal use of PCRE, if the subject string that is passed to
8	nigel	77	\fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP matches as far as it goes, but is
9			too short to match the entire pattern, PCRE_ERROR_NOMATCH is returned. There
10			are circumstances where it might be helpful to distinguish this case from other
11			cases in which there is no match.
12	nigel	75	.P
13			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
15			in the form \fIddmmmyy\fP, defined by this pattern:
16			.sp
17			^\ed?\ed(jan\|feb\|mar\|apr\|may\|jun\|jul\|aug\|sep\|oct\|nov\|dec)\ed\ed$
18			.sp
19			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
21	ph10	428	as soon as a mistake is made, by beeping and not reflecting the character that
22			has been typed, for example. This immediate feedback is likely to be a better
23	nigel	75	user interface than a check that is delayed until the entire string has been
24	ph10	428	entered. Partial matching can also sometimes be useful when the subject string
25			is very long and is not all available at once.
26	nigel	75	.P
27	ph10	428	PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and
28			PCRE_PARTIAL_HARD options, which can be set when calling \fBpcre_exec()\fP or
29			\fBpcre_dfa_exec()\fP. For backwards compatibility, PCRE_PARTIAL is a synonym
30			for PCRE_PARTIAL_SOFT. The essential difference between the two options is
31			whether or not a partial match is preferred to an alternative complete match,
32			though the details differ between the two matching functions. If both options
33			are set, PCRE_PARTIAL_HARD takes precedence.
34	nigel	75	.P
35	ph10	428	Setting a partial matching option disables one of PCRE's optimizations. PCRE
36			remembers the last literal byte in a pattern, and abandons matching immediately
37			if such a byte is not present in the subject string. This optimization cannot
38			be used for a subject string that might match only partially.
39			.
40			.
41			.SH "PARTIAL MATCHING USING pcre_exec()"
42			.rs
43	ph10	426	.sp
44	ph10	428	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
56	ph10	426	/123\ew+X\|dogY/
57			.sp
58			If this is matched against the subject string "abc123dog", both
59	ph10	428	alternatives fail to match, but the end of the subject is reached during
60			matching, so PCRE_ERROR_PARTIAL is returned instead of PCRE_ERROR_NOMATCH. The
61			offsets are set to 3 and 9, identifying "123dog" as the longest partial match
62			that was found. (In this example, there are two partial matches, because "dog"
63			on its own partially matches the second alternative.)
64	ph10	426	.P
65	ph10	428	If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP, it returns
66			PCRE_ERROR_PARTIAL as soon as a partial match is found, without continuing to
67			search for possible complete matches. The difference between the two options
68			can be illustrated by a pattern such as:
69			.sp
70			/dog(sbody)?/
71			.sp
72			This matches either "dog" or "dogsbody", greedily (that is, it prefers the
73			longer string if possible). If it is matched against the string "dog" with
74			PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if
75			PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand,
76			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	nigel	77	.P
100	ph10	428	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	nigel	75	.
118			.
119	ph10	428	.SH "PARTIAL MATCHING AND WORD BOUNDARIES"
120	nigel	75	.rs
121			.sp
122	ph10	428	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"
141			.rs
142			.sp
143	ph10	426	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
145	ph10	428	PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with
146			all patterns. From release 8.00 onwards, the restrictions no longer apply, and
147			partial matching with \fBpcre_exec()\fP can be requested for any pattern.
148	nigel	75	.P
149	ph10	426	Items that were formerly restricted were repeated single characters and
150			repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not
151			conform to the restrictions, \fBpcre_exec()\fP returned the error code
152			PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
153			PCRE_INFO_OKPARTIAL call to \fBpcre_fullinfo()\fP to find out if a compiled
154			pattern can be used for partial matching now always returns 1.
155	nigel	75	.
156			.
157			.SH "EXAMPLE OF PARTIAL MATCHING USING PCRETEST"
158			.rs
159			.sp
160			If the escape sequence \eP is present in a \fBpcretest\fP data line, the
161	ph10	428	PCRE_PARTIAL_SOFT option is used for the match. Here is a run of \fBpcretest\fP
162			that uses the date example quoted above:
163	nigel	75	.sp
164			re> /^\ed?\ed(jan\|feb\|mar\|apr\|may\|jun\|jul\|aug\|sep\|oct\|nov\|dec)\ed\ed$/
165	nigel	87	data> 25jun04\eP
166	nigel	75	0: 25jun04
167			1: jun
168	nigel	87	data> 25dec3\eP
169	ph10	426	Partial match: 23dec3
170	nigel	87	data> 3ju\eP
171	ph10	426	Partial match: 3ju
172	nigel	87	data> 3juj\eP
173	nigel	75	No match
174	nigel	87	data> j\eP
175	nigel	75	No match
176			.sp
177			The first data string is matched completely, so \fBpcretest\fP shows the
178			matched substrings. The remaining four strings do not match the complete
179	ph10	426	pattern, but the first two are partial matches. Similar output is obtained
180			when \fBpcre_dfa_exec()\fP is used.
181	ph10	428	.P
182			If the escape sequence \eP is present more than once in a \fBpcretest\fP data
183			line, the PCRE_PARTIAL_HARD option is set for the match.
184	ph10	426	.
185			.
186	nigel	77	.SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()"
187			.rs
188			.sp
189			When a partial match has been found using \fBpcre_dfa_exec()\fP, it is possible
190			to continue the match by providing additional subject data and calling
191	nigel	93	\fBpcre_dfa_exec()\fP again with the same compiled regular expression, this
192	ph10	428	time setting the PCRE_DFA_RESTART option. You must pass the same working
193	nigel	93	space as before, because this is where details of the previous partial match
194			are stored. Here is an example using \fBpcretest\fP, using the \eR escape
195	ph10	428	sequence to set the PCRE_DFA_RESTART option (\eD specifies the use of
196			\fBpcre_dfa_exec()\fP):
197	nigel	77	.sp
198	ph10	155	re> /^\ed?\ed(jan\|feb\|mar\|apr\|may\|jun\|jul\|aug\|sep\|oct\|nov\|dec)\ed\ed$/
199	nigel	77	data> 23ja\eP\eD
200			Partial match: 23ja
201			data> n05\eR\eD
202			0: n05
203			.sp
204			The first call has "23ja" as the subject, and requests partial matching; the
205			second call has "n05" as the subject for the continued (restarted) match.
206			Notice that when the match is complete, only the last part is shown; PCRE does
207			not retain the previously partially-matched string. It is up to the calling
208			program to do that if it needs to.
209	nigel	75	.P
210	ph10	428	You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
211			PCRE_DFA_RESTART to continue partial matching over multiple segments. This
212			facility can be used to pass very long subject strings to
213			\fBpcre_dfa_exec()\fP.
214	ph10	426	.
215			.
216			.SH "MULTI-SEGMENT MATCHING WITH pcre_exec()"
217			.rs
218			.sp
219			From release 8.00, \fBpcre_exec()\fP can also be used to do multi-segment
220			matching. Unlike \fBpcre_dfa_exec()\fP, it is not possible to restart the
221			previous match with a new segment of data. Instead, new data must be added to
222			the previous subject string, and the entire match re-run, starting from the
223			point where the partial match occurred. Earlier data can be discarded.
224			Consider an unanchored pattern that matches dates:
225			.sp
226			re> /\ed?\ed(jan\|feb\|mar\|apr\|may\|jun\|jul\|aug\|sep\|oct\|nov\|dec)\ed\ed/
227			data> The date is 23ja\eP
228			Partial match: 23ja
229			.sp
230			The this stage, an application could discard the text preceding "23ja", add on
231			text from the next segment, and call \fBpcre_exec()\fP again. Unlike
232			\fBpcre_dfa_exec()\fP, the entire matching string must always be available, and
233			the complete matching process occurs for each call, so more memory and more
234			processing time is needed.
235			.
236			.
237			.SH "ISSUES WITH MULTI-SEGMENT MATCHING"
238			.rs
239			.sp
240			Certain types of pattern may give problems with multi-segment matching,
241			whichever matching function is used.
242	nigel	77	.P
243			1. If the pattern contains tests for the beginning or end of a line, you need
244			to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropriate, when the
245			subject string for any call does not contain the beginning or end of a line.
246			.P
247			2. If the pattern contains backward assertions (including \eb or \eB), you need
248	ph10	428	to arrange for some overlap in the subject strings to allow for them to be
249			correctly tested at the start of each substring. For example, using
250			\fBpcre_dfa_exec()\fP, you could pass the subject in chunks that are 500 bytes
251			long, but in a buffer of 700 bytes, with the starting offset set to 200 and the
252			previous 200 bytes at the start of the buffer.
253	nigel	77	.P
254	ph10	428	3. Matching a subject string that is split into multiple segments may not
255			always produce exactly the same result as matching over one single long string,
256			especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
257			Word Boundaries" above describes an issue that arises if the pattern ends with
258			\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	nigel	77	.sp
264			re> /dog(sbody)?/
265	ph10	426	data> dogsb\eP
266			0: dog
267	nigel	77	data> do\eP\eD
268			Partial match: do
269			data> gsb\eR\eP\eD
270			0: g
271			data> dogsbody\eD
272			0: dogsbody
273			1: dog
274			.sp
275	ph10	428	The first data line passes the string "dogsb" to \fBpcre_exec()\fP, setting the
276			PCRE_PARTIAL_SOFT option. Although the string is a partial match for
277			"dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter string
278			"dog" is a complete match. Similarly, when the subject is presented to
279			\fBpcre_dfa_exec()\fP in several parts ("do" and "gsb" being the first two) the
280			match stops when "dog" has been found, and it is not possible to continue. On
281			the other hand, if "dogsbody" is presented as a single string,
282			\fBpcre_dfa_exec()\fP finds both matches.
283	nigel	77	.P
284	ph10	428	Because of these problems, it is probably best to use PCRE_PARTIAL_HARD when
285			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	nigel	87	.P
296			4. Patterns that contain alternatives at the top level which do not all
297	ph10	426	start with the same pattern item may not work as expected when
298			\fBpcre_dfa_exec()\fP is used. For example, consider this pattern:
299	nigel	87	.sp
300			1234\|3789
301			.sp
302			If the first part of the subject is "ABC123", a partial match of the first
303			alternative is found at offset 3. There is no partial match for the second
304			alternative, because such a match does not start at the same point in the
305	ph10	426	subject string. Attempting to continue with the string "7890" does not yield a
306	nigel	87	match because only those alternatives that match at one point in the subject
307			are remembered. The problem arises because the start of the second alternative
308			matches within the first alternative. There is no problem with anchored
309			patterns or patterns such as:
310			.sp
311			1234\|ABCD
312			.sp
313	ph10	426	where no string can be a partial match for both alternatives. This is not a
314			problem if \fPpcre_exec()\fP is used, because the entire match has to be rerun
315			each time:
316			.sp
317			re> /1234\|3789/
318			data> ABC123\eP
319			Partial match: 123
320			data> 1237890
321			0: 3789
322			.sp
323	nigel	77	.
324			.
325	ph10	99	.SH AUTHOR
326			.rs
327			.sp
328			.nf
329			Philip Hazel
330			University Computing Service
331			Cambridge CB2 3QH, England.
332			.fi
333			.
334			.
335			.SH REVISION
336			.rs
337			.sp
338			.nf
339	ph10	428	Last updated: 31 August 2009
340	ph10	426	Copyright (c) 1997-2009 University of Cambridge.
341	ph10	99	.fi
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