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 two 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. If the pattern
was studied, PCRE knows the minimum length of a matching string, and does not
bother to run the matching function on shorter strings. This optimization is
also disabled for partial matching.
.
.
.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. If there are at least two slots in the offsets
vector, the first of them is set to the offset of the earliest character that
was inspected when the partial match was found. For convenience, the second
offset points to the end of the string so that a substring can easily be
identified.
.P
For the majority of patterns, the first offset identifies the start of the
partially matched string. However, for patterns that contain lookbehind
assertions, or \eK, or begin with \eb or \eB, earlier characters have been
inspected while carrying out the match. For example:
.sp
  /(?<=abc)123/
.sp
This pattern matches "123", but only if it is preceded by "abc". If the subject
string is "xyzabc12", the offsets after a partial match are for the substring
"abc12", because all these characters are needed if another match is tried
with extra characters added.
.P
If there is more than one partial match, the first one that was found provides
the data that is returned. 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 first 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 was inspected when the longest
partial match was found 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.
.P
\fBNote:\fP If the pattern contains lookbehind assertions, or \eK, or starts
with \eb or \eB, the string that is returned for a partial match will include
characters that precede the partially matched string itself, because these must
be retained when adding on more characters for a subsequent matching attempt.
.
.
.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. Lookbehind assertions at the start of a pattern are catered for in the
offsets that are returned for a partial match. However, in theory, a lookbehind
assertion later in the pattern could require even earlier characters to be
inspected, and it might not have been reached when a partial match occurs. This
is probably an extremely unlikely case; you could guard against it to a certain
extent by always including extra characters at the start.
.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: 29 September 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	ph10	435	\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	ph10	428	are set, PCRE_PARTIAL_HARD takes precedence.
34	nigel	75	.P
35	ph10	456	Setting a partial matching option disables two of PCRE's optimizations. PCRE
36	ph10	428	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	ph10	461	be used for a subject string that might match only partially. If the pattern
39			was studied, PCRE knows the minimum length of a matching string, and does not
40			bother to run the matching function on shorter strings. This optimization is
41	ph10	456	also disabled for partial matching.
42	ph10	428	.
43			.
44			.SH "PARTIAL MATCHING USING pcre_exec()"
45			.rs
46	ph10	426	.sp
47	ph10	428	A partial match occurs during a call to \fBpcre_exec()\fP whenever the end of
48			the subject string is reached successfully, but matching cannot continue
49			because more characters are needed. However, at least one character must have
50			been matched. (In other words, a partial match can never be an empty string.)
51			.P
52			If PCRE_PARTIAL_SOFT is set, the partial match is remembered, but matching
53			continues as normal, and other alternatives in the pattern are tried. If no
54			complete match can be found, \fBpcre_exec()\fP returns PCRE_ERROR_PARTIAL
55	ph10	435	instead of PCRE_ERROR_NOMATCH. If there are at least two slots in the offsets
56			vector, the first of them is set to the offset of the earliest character that
57			was inspected when the partial match was found. For convenience, the second
58			offset points to the end of the string so that a substring can easily be
59	ph10	456	identified.
60	ph10	435	.P
61			For the majority of patterns, the first offset identifies the start of the
62			partially matched string. However, for patterns that contain lookbehind
63			assertions, or \eK, or begin with \eb or \eB, earlier characters have been
64			inspected while carrying out the match. For example:
65	ph10	428	.sp
66	ph10	435	/(?<=abc)123/
67			.sp
68			This pattern matches "123", but only if it is preceded by "abc". If the subject
69			string is "xyzabc12", the offsets after a partial match are for the substring
70			"abc12", because all these characters are needed if another match is tried
71			with extra characters added.
72			.P
73			If there is more than one partial match, the first one that was found provides
74			the data that is returned. Consider this pattern:
75			.sp
76	ph10	426	/123\ew+X\|dogY/
77			.sp
78			If this is matched against the subject string "abc123dog", both
79	ph10	435	alternatives fail to match, but the end of the subject is reached during
80	ph10	428	matching, so PCRE_ERROR_PARTIAL is returned instead of PCRE_ERROR_NOMATCH. The
81	ph10	435	offsets are set to 3 and 9, identifying "123dog" as the first partial match
82	ph10	428	that was found. (In this example, there are two partial matches, because "dog"
83			on its own partially matches the second alternative.)
84	ph10	426	.P
85	ph10	435	If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP, it returns
86	ph10	428	PCRE_ERROR_PARTIAL as soon as a partial match is found, without continuing to
87			search for possible complete matches. The difference between the two options
88			can be illustrated by a pattern such as:
89			.sp
90			/dog(sbody)?/
91			.sp
92	ph10	435	This matches either "dog" or "dogsbody", greedily (that is, it prefers the
93	ph10	428	longer string if possible). If it is matched against the string "dog" with
94	ph10	435	PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if
95			PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand,
96	ph10	428	if the pattern is made ungreedy the result is different:
97			.sp
98			/dog(sbody)??/
99			.sp
100	ph10	435	In this case the result is always a complete match because \fBpcre_exec()\fP
101			finds that first, and it never continues after finding a match. It might be
102	ph10	428	easier to follow this explanation by thinking of the two patterns like this:
103			.sp
104			/dog(sbody)?/ is the same as /dogsbody\|dog/
105			/dog(sbody)??/ is the same as /dog\|dogsbody/
106			.sp
107	ph10	435	The second pattern will never match "dogsbody" when \fBpcre_exec()\fP is
108	ph10	428	used, because it will always find the shorter match first.
109			.
110			.
111			.SH "PARTIAL MATCHING USING pcre_dfa_exec()"
112			.rs
113			.sp
114	ph10	435	The \fBpcre_dfa_exec()\fP function moves along the subject string character by
115			character, without backtracking, searching for all possible matches
116			simultaneously. If the end of the subject is reached before the end of the
117	ph10	428	pattern, there is the possibility of a partial match, again provided that at
118			least one character has matched.
119	nigel	77	.P
120	ph10	428	When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there
121			have been no complete matches. Otherwise, the complete matches are returned.
122			However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any
123	ph10	435	complete matches. The portion of the string that was inspected when the longest
124			partial match was found is set as the first matching string, provided there are
125			at least two slots in the offsets vector.
126	ph10	428	.P
127	ph10	435	Because \fBpcre_dfa_exec()\fP always searches for all possible matches, and
128	ph10	428	there is no difference between greedy and ungreedy repetition, its behaviour is
129	ph10	435	different from \fBpcre_exec\fP when PCRE_PARTIAL_HARD is set. Consider the
130	ph10	428	string "dog" matched against the ungreedy pattern shown above:
131			.sp
132			/dog(sbody)??/
133			.sp
134	ph10	435	Whereas \fBpcre_exec()\fP stops as soon as it finds the complete match for
135	ph10	428	"dog", \fBpcre_dfa_exec()\fP also finds the partial match for "dogsbody", and
136			so returns that when PCRE_PARTIAL_HARD is set.
137	nigel	75	.
138			.
139	ph10	428	.SH "PARTIAL MATCHING AND WORD BOUNDARIES"
140	nigel	75	.rs
141			.sp
142	ph10	435	If a pattern ends with one of sequences \ew or \eW, which test for word
143			boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
144	ph10	428	results. Consider this pattern:
145			.sp
146			/\ebcat\eb/
147			.sp
148			This matches "cat", provided there is a word boundary at either end. If the
149			subject string is "the cat", the comparison of the final "t" with a following
150	ph10	435	character cannot take place, so a partial match is found. However,
151			\fBpcre_exec()\fP carries on with normal matching, which matches \eb at the end
152			of the subject when the last character is a letter, thus finding a complete
153			match. The result, therefore, is \fInot\fP PCRE_ERROR_PARTIAL. The same thing
154	ph10	428	happens with \fBpcre_dfa_exec()\fP, because it also finds the complete match.
155			.P
156	ph10	435	Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because
157	ph10	428	then the partial match takes precedence.
158			.
159			.
160			.SH "FORMERLY RESTRICTED PATTERNS"
161			.rs
162			.sp
163	ph10	426	For releases of PCRE prior to 8.00, because of the way certain internal
164			optimizations were implemented in the \fBpcre_exec()\fP function, the
165	ph10	428	PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with
166			all patterns. From release 8.00 onwards, the restrictions no longer apply, and
167			partial matching with \fBpcre_exec()\fP can be requested for any pattern.
168	nigel	75	.P
169	ph10	426	Items that were formerly restricted were repeated single characters and
170			repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not
171			conform to the restrictions, \fBpcre_exec()\fP returned the error code
172			PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
173			PCRE_INFO_OKPARTIAL call to \fBpcre_fullinfo()\fP to find out if a compiled
174			pattern can be used for partial matching now always returns 1.
175	nigel	75	.
176			.
177			.SH "EXAMPLE OF PARTIAL MATCHING USING PCRETEST"
178			.rs
179			.sp
180			If the escape sequence \eP is present in a \fBpcretest\fP data line, the
181	ph10	428	PCRE_PARTIAL_SOFT option is used for the match. Here is a run of \fBpcretest\fP
182			that uses the date example quoted above:
183	nigel	75	.sp
184			re> /^\ed?\ed(jan\|feb\|mar\|apr\|may\|jun\|jul\|aug\|sep\|oct\|nov\|dec)\ed\ed$/
185	nigel	87	data> 25jun04\eP
186	nigel	75	0: 25jun04
187			1: jun
188	nigel	87	data> 25dec3\eP
189	ph10	426	Partial match: 23dec3
190	nigel	87	data> 3ju\eP
191	ph10	426	Partial match: 3ju
192	nigel	87	data> 3juj\eP
193	nigel	75	No match
194	nigel	87	data> j\eP
195	nigel	75	No match
196			.sp
197			The first data string is matched completely, so \fBpcretest\fP shows the
198			matched substrings. The remaining four strings do not match the complete
199	ph10	426	pattern, but the first two are partial matches. Similar output is obtained
200			when \fBpcre_dfa_exec()\fP is used.
201	ph10	428	.P
202			If the escape sequence \eP is present more than once in a \fBpcretest\fP data
203			line, the PCRE_PARTIAL_HARD option is set for the match.
204	ph10	426	.
205	ph10	435	.
206	nigel	77	.SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()"
207			.rs
208			.sp
209			When a partial match has been found using \fBpcre_dfa_exec()\fP, it is possible
210			to continue the match by providing additional subject data and calling
211	nigel	93	\fBpcre_dfa_exec()\fP again with the same compiled regular expression, this
212	ph10	428	time setting the PCRE_DFA_RESTART option. You must pass the same working
213	nigel	93	space as before, because this is where details of the previous partial match
214			are stored. Here is an example using \fBpcretest\fP, using the \eR escape
215	ph10	428	sequence to set the PCRE_DFA_RESTART option (\eD specifies the use of
216			\fBpcre_dfa_exec()\fP):
217	nigel	77	.sp
218	ph10	155	re> /^\ed?\ed(jan\|feb\|mar\|apr\|may\|jun\|jul\|aug\|sep\|oct\|nov\|dec)\ed\ed$/
219	nigel	77	data> 23ja\eP\eD
220			Partial match: 23ja
221			data> n05\eR\eD
222			0: n05
223			.sp
224			The first call has "23ja" as the subject, and requests partial matching; the
225			second call has "n05" as the subject for the continued (restarted) match.
226			Notice that when the match is complete, only the last part is shown; PCRE does
227			not retain the previously partially-matched string. It is up to the calling
228			program to do that if it needs to.
229	nigel	75	.P
230	ph10	428	You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
231			PCRE_DFA_RESTART to continue partial matching over multiple segments. This
232			facility can be used to pass very long subject strings to
233			\fBpcre_dfa_exec()\fP.
234	ph10	426	.
235			.
236			.SH "MULTI-SEGMENT MATCHING WITH pcre_exec()"
237			.rs
238			.sp
239	ph10	435	From release 8.00, \fBpcre_exec()\fP can also be used to do multi-segment
240			matching. Unlike \fBpcre_dfa_exec()\fP, it is not possible to restart the
241			previous match with a new segment of data. Instead, new data must be added to
242			the previous subject string, and the entire match re-run, starting from the
243	ph10	426	point where the partial match occurred. Earlier data can be discarded.
244			Consider an unanchored pattern that matches dates:
245			.sp
246			re> /\ed?\ed(jan\|feb\|mar\|apr\|may\|jun\|jul\|aug\|sep\|oct\|nov\|dec)\ed\ed/
247			data> The date is 23ja\eP
248			Partial match: 23ja
249			.sp
250	ph10	435	The this stage, an application could discard the text preceding "23ja", add on
251			text from the next segment, and call \fBpcre_exec()\fP again. Unlike
252			\fBpcre_dfa_exec()\fP, the entire matching string must always be available, and
253			the complete matching process occurs for each call, so more memory and more
254	ph10	426	processing time is needed.
255	ph10	435	.P
256			\fBNote:\fP If the pattern contains lookbehind assertions, or \eK, or starts
257			with \eb or \eB, the string that is returned for a partial match will include
258			characters that precede the partially matched string itself, because these must
259			be retained when adding on more characters for a subsequent matching attempt.
260	ph10	426	.
261	ph10	435	.
262	ph10	426	.SH "ISSUES WITH MULTI-SEGMENT MATCHING"
263			.rs
264			.sp
265	ph10	435	Certain types of pattern may give problems with multi-segment matching,
266	ph10	426	whichever matching function is used.
267	nigel	77	.P
268			1. If the pattern contains tests for the beginning or end of a line, you need
269			to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropriate, when the
270			subject string for any call does not contain the beginning or end of a line.
271			.P
272	ph10	435	2. Lookbehind assertions at the start of a pattern are catered for in the
273			offsets that are returned for a partial match. However, in theory, a lookbehind
274			assertion later in the pattern could require even earlier characters to be
275			inspected, and it might not have been reached when a partial match occurs. This
276			is probably an extremely unlikely case; you could guard against it to a certain
277			extent by always including extra characters at the start.
278	nigel	77	.P
279	ph10	428	3. Matching a subject string that is split into multiple segments may not
280			always produce exactly the same result as matching over one single long string,
281	ph10	435	especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and
282			Word Boundaries" above describes an issue that arises if the pattern ends with
283	ph10	428	\eb or \eB. Another kind of difference may occur when there are multiple
284			matching possibilities, because a partial match result is given only when there
285			are no completed matches. This means that as soon as the shortest match has
286			been found, continuation to a new subject segment is no longer possible.
287			Consider again this \fBpcretest\fP example:
288	nigel	77	.sp
289			re> /dog(sbody)?/
290	ph10	426	data> dogsb\eP
291	ph10	435	0: dog
292	nigel	77	data> do\eP\eD
293			Partial match: do
294			data> gsb\eR\eP\eD
295			0: g
296			data> dogsbody\eD
297			0: dogsbody
298			1: dog
299			.sp
300	ph10	428	The first data line passes the string "dogsb" to \fBpcre_exec()\fP, setting the
301			PCRE_PARTIAL_SOFT option. Although the string is a partial match for
302			"dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter string
303			"dog" is a complete match. Similarly, when the subject is presented to
304			\fBpcre_dfa_exec()\fP in several parts ("do" and "gsb" being the first two) the
305			match stops when "dog" has been found, and it is not possible to continue. On
306			the other hand, if "dogsbody" is presented as a single string,
307			\fBpcre_dfa_exec()\fP finds both matches.
308	nigel	77	.P
309	ph10	428	Because of these problems, it is probably best to use PCRE_PARTIAL_HARD when
310			matching multi-segment data. The example above then behaves differently:
311			.sp
312			re> /dog(sbody)?/
313			data> dogsb\eP\eP
314	ph10	435	Partial match: dogsb
315	ph10	428	data> do\eP\eD
316			Partial match: do
317			data> gsb\eR\eP\eP\eD
318	ph10	435	Partial match: gsb
319	ph10	428	.sp
320	nigel	87	.P
321			4. Patterns that contain alternatives at the top level which do not all
322	ph10	435	start with the same pattern item may not work as expected when
323	ph10	426	\fBpcre_dfa_exec()\fP is used. For example, consider this pattern:
324	nigel	87	.sp
325			1234\|3789
326			.sp
327			If the first part of the subject is "ABC123", a partial match of the first
328			alternative is found at offset 3. There is no partial match for the second
329			alternative, because such a match does not start at the same point in the
330	ph10	426	subject string. Attempting to continue with the string "7890" does not yield a
331	nigel	87	match because only those alternatives that match at one point in the subject
332			are remembered. The problem arises because the start of the second alternative
333			matches within the first alternative. There is no problem with anchored
334			patterns or patterns such as:
335			.sp
336			1234\|ABCD
337			.sp
338	ph10	426	where no string can be a partial match for both alternatives. This is not a
339	ph10	435	problem if \fPpcre_exec()\fP is used, because the entire match has to be rerun
340	ph10	426	each time:
341			.sp
342			re> /1234\|3789/
343			data> ABC123\eP
344			Partial match: 123
345			data> 1237890
346			0: 3789
347	ph10	435	.sp
348	nigel	77	.
349			.
350	ph10	99	.SH AUTHOR
351			.rs
352			.sp
353			.nf
354			Philip Hazel
355			University Computing Service
356			Cambridge CB2 3QH, England.
357			.fi
358			.
359			.
360			.SH REVISION
361			.rs
362			.sp
363			.nf
364	ph10	456	Last updated: 29 September 2009
365	ph10	426	Copyright (c) 1997-2009 University of Cambridge.
366	ph10	99	.fi
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