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version 1.1, 2012/02/21 23:05:52	version 1.1.1.5, 2014/06/15 19:46:04
Line 1	Line 1
.TH PCREPARTIAL 3	.TH PCREPARTIAL 3 "02 July 2013" "PCRE 8.34"
.SH NAME	.SH NAME
PCRE - Perl-compatible regular expressions	PCRE - Perl-compatible regular expressions
.SH "PARTIAL MATCHING IN PCRE"	.SH "PARTIAL MATCHING IN PCRE"
.rs	.rs
.sp	.sp
In normal use of PCRE, if the subject string that is passed to	In normal use of PCRE, if the subject string that is passed to a matching
\fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP matches as far as it goes, but is	function matches as far as it goes, but is too short to match the entire
too short to match the entire pattern, PCRE_ERROR_NOMATCH is returned. There	pattern, PCRE_ERROR_NOMATCH is returned. There are circumstances where it might
are circumstances where it might be helpful to distinguish this case from other	be helpful to distinguish this case from other cases in which there is no
cases in which there is no match.	match.
.P	.P
Consider, for example, an application where a human is required to type in data	Consider, for example, an application where a human is required to type in data
for a field with specific formatting requirements. An example might be a date	for a field with specific formatting requirements. An example might be a date
Line 25 entered. Partial matching can also be useful when the	Line 25 entered. Partial matching can also be useful when the
long and is not all available at once.	long and is not all available at once.
.P	.P
PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and	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	PCRE_PARTIAL_HARD options, which can be set when calling any of the matching
\fBpcre_dfa_exec()\fP. For backwards compatibility, PCRE_PARTIAL is a synonym	functions. For backwards compatibility, PCRE_PARTIAL is a synonym for
for PCRE_PARTIAL_SOFT. The essential difference between the two options is	PCRE_PARTIAL_SOFT. The essential difference between the two options is whether
whether or not a partial match is preferred to an alternative complete match,	or not a partial match is preferred to an alternative complete match, though
though the details differ between the two matching functions. If both options	the details differ between the two types of matching function. If both options
are set, PCRE_PARTIAL_HARD takes precedence.	are set, PCRE_PARTIAL_HARD takes precedence.
.P	.P
Setting a partial matching option for \fBpcre_exec()\fP disables the use of any	If you want to use partial matching with just-in-time optimized code, you must
just-in-time code that was set up by calling \fBpcre_study()\fP with the	call \fBpcre_study()\fP, \fBpcre16_study()\fP or \fBpcre32_study()\fP with one
PCRE_STUDY_JIT_COMPILE option. It also disables two of PCRE's standard	or both of these options:
optimizations. PCRE remembers the last literal byte in a pattern, and abandons	.sp
matching immediately if such a byte is not present in the subject string. This	PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE
	PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
	.sp
	PCRE_STUDY_JIT_COMPILE should also be set if you are going to run non-partial
	matches on the same pattern. If the appropriate JIT study mode has not been set
	for a match, the interpretive matching code is used.
	.P
	Setting a partial matching option disables two of PCRE's standard
	optimizations. PCRE remembers the last literal data unit in a pattern, and
	abandons matching immediately if it is not present in the subject string. This
optimization cannot be used for a subject string that might match only	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	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	matching string, and does not bother to run the matching function on shorter
strings. This optimization is also disabled for partial matching.	strings. This optimization is also disabled for partial matching.
.	.
.	.
.SH "PARTIAL MATCHING USING pcre_exec()"	.SH "PARTIAL MATCHING USING pcre_exec() OR pcre[16\|32]_exec()"
.rs	.rs
.sp	.sp
A partial match occurs during a call to \fBpcre_exec()\fP when the end of the	A partial match occurs during a call to \fBpcre_exec()\fP or
subject string is reached successfully, but matching cannot continue because	\fBpcre[16\|32]_exec()\fP when the end of the subject string is reached
more characters are needed. However, at least one character in the subject must	successfully, but matching cannot continue because more characters are needed.
have been inspected. This character need not form part of the final matched	However, at least one character in the subject must have been inspected. This
string; lookbehind assertions and the \eK escape sequence provide ways of	character need not form part of the final matched string; lookbehind assertions
inspecting characters before the start of a matched substring. The requirement	and the \eK escape sequence provide ways of inspecting characters before the
for inspecting at least one character exists because an empty string can always	start of a matched substring. The requirement for inspecting at least one
be matched; without such a restriction there would always be a partial match of	character exists because an empty string can always be matched; without such a
an empty string at the end of the subject.	restriction there would always be a partial match of an empty string at the end
	of the subject.
.P	.P
If there are at least two slots in the offsets vector when \fBpcre_exec()\fP	If there are at least two slots in the offsets vector when a partial match is
returns with a partial match, the first slot is set to the offset of the	returned, the first slot is set to the offset of the earliest character that
earliest character that was inspected when the partial match was found. For	was inspected. For convenience, the second offset points to the end of the
convenience, the second offset points to the end of the subject so that a	subject so that a substring can easily be identified. If there are at least
substring can easily be identified.	three slots in the offsets vector, the third slot is set to the offset of the
	character where matching started.
.P	.P
For the majority of patterns, the first offset identifies the start of the	For the majority of patterns, the contents of the first and third slots will be
partially matched string. However, for patterns that contain lookbehind	the same. However, for patterns that contain lookbehind assertions, or begin
assertions, or \eK, or begin with \eb or \eB, earlier characters have been	with \eb or \eB, characters before the one where matching started may have been
inspected while carrying out the match. For example:	inspected while carrying out the match. For example, consider this pattern:
.sp	.sp
/(?<=abc)123/	/(?<=abc)123/
.sp	.sp
This pattern matches "123", but only if it is preceded by "abc". If the subject	This pattern matches "123", but only if it is preceded by "abc". If the subject
string is "xyzabc12", the offsets after a partial match are for the substring	string is "xyzabc12", the first two offsets after a partial match are for the
"abc12", because all these characters are needed if another match is tried	substring "abc12", because all these characters were inspected. However, the
with extra characters added to the subject.	third offset is set to 6, because that is the offset where matching began.
.P	.P
What happens when a partial match is identified depends on which of the two	What happens when a partial match is identified depends on which of the two
partial matching options are set.	partial matching options are set.
.	.
.	.
.SS "PCRE_PARTIAL_SOFT with pcre_exec()"	.SS "PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre[16\|32]_exec()"
.rs	.rs
.sp	.sp
If PCRE_PARTIAL_SOFT is set when \fBpcre_exec()\fP identifies a partial match,	If PCRE_PARTIAL_SOFT is set when \fBpcre_exec()\fP or \fBpcre[16\|32]_exec()\fP
the partial match is remembered, but matching continues as normal, and other	identifies a partial match, the partial match is remembered, but matching
alternatives in the pattern are tried. If no complete match can be found,	continues as normal, and other alternatives in the pattern are tried. If no
\fBpcre_exec()\fP returns PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH.	complete match can be found, PCRE_ERROR_PARTIAL is returned instead of
	PCRE_ERROR_NOMATCH.
.P	.P
This option is "soft" because it prefers a complete match over a partial match.	This option is "soft" because it prefers a complete match over a partial match.
All the various matching items in a pattern behave as if the subject string is	All the various matching items in a pattern behave as if the subject string is
Line 105 example, there are two partial matches, because "dog"	Line 117 example, there are two partial matches, because "dog"
matches the second alternative.)	matches the second alternative.)
.	.
.	.
.SS "PCRE_PARTIAL_HARD with pcre_exec()"	.SS "PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre[16\|32]_exec()"
.rs	.rs
.sp	.sp
If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP, it returns	If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP or \fBpcre[16\|32]_exec()\fP,
PCRE_ERROR_PARTIAL as soon as a partial match is found, without continuing to	PCRE_ERROR_PARTIAL is returned as soon as a partial match is found, without
search for possible complete matches. This option is "hard" because it prefers	continuing to search for possible complete matches. This option is "hard"
an earlier partial match over a later complete match. For this reason, the	because it prefers an earlier partial match over a later complete match. For
assumption is made that the end of the supplied subject string may not be the	this reason, the assumption is made that the end of the supplied subject string
true end of the available data, and so, if \ez, \eZ, \eb, \eB, or $ are	may not be the true end of the available data, and so, if \ez, \eZ, \eb, \eB,
encountered at the end of the subject, the result is PCRE_ERROR_PARTIAL.	or $ are encountered at the end of the subject, the result is
	PCRE_ERROR_PARTIAL, provided that at least one character in the subject has
	been inspected.
.P	.P
Setting PCRE_PARTIAL_HARD also affects the way \fBpcre_exec()\fP checks UTF-8	Setting PCRE_PARTIAL_HARD also affects the way UTF-8 and UTF-16
subject strings for validity. Normally, an invalid UTF-8 sequence causes the	subject strings are checked for validity. Normally, an invalid sequence
error PCRE_ERROR_BADUTF8. However, in the special case of a truncated UTF-8	causes the error PCRE_ERROR_BADUTF8 or PCRE_ERROR_BADUTF16. However, in the
character at the end of the subject, PCRE_ERROR_SHORTUTF8 is returned when	special case of a truncated character at the end of the subject,
	PCRE_ERROR_SHORTUTF8 or PCRE_ERROR_SHORTUTF16 is returned when
PCRE_PARTIAL_HARD is set.	PCRE_PARTIAL_HARD is set.
.	.
.	.
Line 139 if the pattern is made ungreedy the result is differen	Line 154 if the pattern is made ungreedy the result is differen
.sp	.sp
/dog(sbody)??/	/dog(sbody)??/
.sp	.sp
In this case the result is always a complete match because \fBpcre_exec()\fP	In this case the result is always a complete match because that is found first,
finds that first, and it never continues after finding a match. It might be	and matching never continues after finding a complete match. It might be easier
easier to follow this explanation by thinking of the two patterns like this:	to follow this explanation by thinking of the two patterns like this:
.sp	.sp
/dog(sbody)?/ is the same as /dogsbody\|dog/	/dog(sbody)?/ is the same as /dogsbody\|dog/
/dog(sbody)??/ is the same as /dog\|dogsbody/	/dog(sbody)??/ is the same as /dog\|dogsbody/
.sp	.sp
The second pattern will never match "dogsbody" when \fBpcre_exec()\fP is	The second pattern will never match "dogsbody", because it will always find the
used, because it will always find the shorter match first.	shorter match first.
.	.
.	.
.SH "PARTIAL MATCHING USING pcre_dfa_exec()"	.SH "PARTIAL MATCHING USING pcre_dfa_exec() OR pcre[16\|32]_dfa_exec()"
.rs	.rs
.sp	.sp
The \fBpcre_dfa_exec()\fP function moves along the subject string character by	The DFA functions move along the subject string character by character, without
character, without backtracking, searching for all possible matches	backtracking, searching for all possible matches simultaneously. If the end of
simultaneously. If the end of the subject is reached before the end of the	the subject is reached before the end of the pattern, there is the possibility
pattern, there is the possibility of a partial match, again provided that at	of a partial match, again provided that at least one character has been
least one character has been inspected.	inspected.
.P	.P
When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there	When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there
have been no complete matches. Otherwise, the complete matches are returned.	have been no complete matches. Otherwise, the complete matches are returned.
Line 166 complete matches. The portion of the string that was i	Line 181 complete matches. The portion of the string that was i
partial match was found is set as the first matching string, provided there are	partial match was found is set as the first matching string, provided there are
at least two slots in the offsets vector.	at least two slots in the offsets vector.
.P	.P
Because \fBpcre_dfa_exec()\fP always searches for all possible matches, and	Because the DFA functions always search for all possible matches, and there is
there is no difference between greedy and ungreedy repetition, its behaviour is	no difference between greedy and ungreedy repetition, their behaviour is
different from \fBpcre_exec\fP when PCRE_PARTIAL_HARD is set. Consider the	different from the standard functions when PCRE_PARTIAL_HARD is set. Consider
string "dog" matched against the ungreedy pattern shown above:	the string "dog" matched against the ungreedy pattern shown above:
.sp	.sp
/dog(sbody)??/	/dog(sbody)??/
.sp	.sp
Whereas \fBpcre_exec()\fP stops as soon as it finds the complete match for	Whereas the standard functions stop as soon as they find the complete match for
"dog", \fBpcre_dfa_exec()\fP also finds the partial match for "dogsbody", and	"dog", the DFA functions also find the partial match for "dogsbody", and so
so returns that when PCRE_PARTIAL_HARD is set.	return that when PCRE_PARTIAL_HARD is set.
.	.
.	.
.SH "PARTIAL MATCHING AND WORD BOUNDARIES"	.SH "PARTIAL MATCHING AND WORD BOUNDARIES"
Line 189 results. Consider this pattern:	Line 204 results. Consider this pattern:
.sp	.sp
This matches "cat", provided there is a word boundary at either end. If the	This matches "cat", provided there is a word boundary at either end. If the
subject string is "the cat", the comparison of the final "t" with a following	subject string is "the cat", the comparison of the final "t" with a following
character cannot take place, so a partial match is found. However,	character cannot take place, so a partial match is found. However, normal
\fBpcre_exec()\fP carries on with normal matching, which matches \eb at the end	matching carries on, and \eb matches at the end of the subject when the last
of the subject when the last character is a letter, thus finding a complete	character is a letter, so a complete match is found. The result, therefore, is
match. The result, therefore, is \fInot\fP PCRE_ERROR_PARTIAL. The same thing	\fInot\fP PCRE_ERROR_PARTIAL. Using PCRE_PARTIAL_HARD in this case does yield
happens with \fBpcre_dfa_exec()\fP, because it also finds the complete match.	PCRE_ERROR_PARTIAL, because then the partial match takes precedence.
.P
Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because
then the partial match takes precedence.
.	.
.	.
.SH "FORMERLY RESTRICTED PATTERNS"	.SH "FORMERLY RESTRICTED PATTERNS"
Line 206 For releases of PCRE prior to 8.00, because of the way	Line 218 For releases of PCRE prior to 8.00, because of the way
optimizations were implemented in the \fBpcre_exec()\fP function, the	optimizations were implemented in the \fBpcre_exec()\fP function, the
PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with	PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with
all patterns. From release 8.00 onwards, the restrictions no longer apply, and	all patterns. From release 8.00 onwards, the restrictions no longer apply, and
partial matching with \fBpcre_exec()\fP can be requested for any pattern.	partial matching with can be requested for any pattern.
.P	.P
Items that were formerly restricted were repeated single characters and	Items that were formerly restricted were repeated single characters and
repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not	repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not
Line 239 that uses the date example quoted above:	Line 251 that uses the date example quoted above:
The first data string is matched completely, so \fBpcretest\fP shows the	The first data string is matched completely, so \fBpcretest\fP shows the
matched substrings. The remaining four strings do not match the complete	matched substrings. The remaining four strings do not match the complete
pattern, but the first two are partial matches. Similar output is obtained	pattern, but the first two are partial matches. Similar output is obtained
when \fBpcre_dfa_exec()\fP is used.	if DFA matching is used.
.P	.P
If the escape sequence \eP is present more than once in a \fBpcretest\fP data	If the escape sequence \eP is present more than once in a \fBpcretest\fP data
line, the PCRE_PARTIAL_HARD option is set for the match.	line, the PCRE_PARTIAL_HARD option is set for the match.
.	.
.	.
.SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()"	.SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre[16\|32]_dfa_exec()"
.rs	.rs
.sp	.sp
When a partial match has been found using \fBpcre_dfa_exec()\fP, it is possible	When a partial match has been found using a DFA matching function, it is
to continue the match by providing additional subject data and calling	possible to continue the match by providing additional subject data and calling
\fBpcre_dfa_exec()\fP again with the same compiled regular expression, this	the function again with the same compiled regular expression, this time setting
time setting the PCRE_DFA_RESTART option. You must pass the same working	the PCRE_DFA_RESTART option. You must pass the same working space as before,
space as before, because this is where details of the previous partial match	because this is where details of the previous partial match are stored. Here is
are stored. Here is an example using \fBpcretest\fP, using the \eR escape	an example using \fBpcretest\fP, using the \eR escape sequence to set the
sequence to set the PCRE_DFA_RESTART option (\eD specifies the use of	PCRE_DFA_RESTART option (\eD specifies the use of the DFA matching function):
\fBpcre_dfa_exec()\fP):
.sp	.sp
re> /^\ed?\ed(jan\|feb\|mar\|apr\|may\|jun\|jul\|aug\|sep\|oct\|nov\|dec)\ed\ed$/	re> /^\ed?\ed(jan\|feb\|mar\|apr\|may\|jun\|jul\|aug\|sep\|oct\|nov\|dec)\ed\ed$/
data> 23ja\eP\eD	data> 23ja\eP\eD
Line 269 Notice that when the match is complete, only the last	Line 280 Notice that when the match is complete, only the last
not retain the previously partially-matched string. It is up to the calling	not retain the previously partially-matched string. It is up to the calling
program to do that if it needs to.	program to do that if it needs to.
.P	.P
	That means that, for an unanchored pattern, if a continued match fails, it is
	not possible to try again at a new starting point. All this facility is capable
	of doing is continuing with the previous match attempt. In the previous
	example, if the second set of data is "ug23" the result is no match, even
	though there would be a match for "aug23" if the entire string were given at
	once. Depending on the application, this may or may not be what you want.
	The only way to allow for starting again at the next character is to retain the
	matched part of the subject and try a new complete match.
	.P
You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with	You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
PCRE_DFA_RESTART to continue partial matching over multiple segments. This	PCRE_DFA_RESTART to continue partial matching over multiple segments. This
facility can be used to pass very long subject strings to	facility can be used to pass very long subject strings to the DFA matching
\fBpcre_dfa_exec()\fP.	functions.
.	.
.	.
.SH "MULTI-SEGMENT MATCHING WITH pcre_exec()"	.SH "MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre[16\|32]_exec()"
.rs	.rs
.sp	.sp
From release 8.00, \fBpcre_exec()\fP can also be used to do multi-segment	From release 8.00, the standard matching functions can also be used to do
matching. Unlike \fBpcre_dfa_exec()\fP, it is not possible to restart the	multi-segment matching. Unlike the DFA functions, it is not possible to
previous match with a new segment of data. Instead, new data must be added to	restart the previous match with a new segment of data. Instead, new data must
the previous subject string, and the entire match re-run, starting from the	be added to the previous subject string, and the entire match re-run, starting
point where the partial match occurred. Earlier data can be discarded. It is	from the point where the partial match occurred. Earlier data can be discarded.
best to use PCRE_PARTIAL_HARD in this situation, because it does not treat the	.P
end of a segment as the end of the subject when matching \ez, \eZ, \eb, \eB,	It is best to use PCRE_PARTIAL_HARD in this situation, because it does not
and $. Consider an unanchored pattern that matches dates:	treat the end of a segment as the end of the subject when matching \ez, \eZ,
	\eb, \eB, and $. Consider an unanchored pattern that matches dates:
.sp	.sp
re> /\ed?\ed(jan\|feb\|mar\|apr\|may\|jun\|jul\|aug\|sep\|oct\|nov\|dec)\ed\ed/	re> /\ed?\ed(jan\|feb\|mar\|apr\|may\|jun\|jul\|aug\|sep\|oct\|nov\|dec)\ed\ed/
data> The date is 23ja\eP\eP	data> The date is 23ja\eP\eP
Partial match: 23ja	Partial match: 23ja
.sp	.sp
At this stage, an application could discard the text preceding "23ja", add on	At this stage, an application could discard the text preceding "23ja", add on
text from the next segment, and call \fBpcre_exec()\fP again. Unlike	text from the next segment, and call the matching function again. Unlike the
\fBpcre_dfa_exec()\fP, the entire matching string must always be available, and	DFA matching functions, the entire matching string must always be available,
the complete matching process occurs for each call, so more memory and more	and the complete matching process occurs for each call, so more memory and more
processing time is needed.	processing time is needed.
.P	.P
\fBNote:\fP If the pattern contains lookbehind assertions, or \eK, or starts	\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	with \eb or \eB, the string that is returned for a partial match includes
characters that precede the partially matched string itself, because these must	characters that precede the start of what would be returned for a complete
be retained when adding on more characters for a subsequent matching attempt.	match, because it contains all the characters that were inspected during the
	partial match.
.	.
.	.
.SH "ISSUES WITH MULTI-SEGMENT MATCHING"	.SH "ISSUES WITH MULTI-SEGMENT MATCHING"
Line 315 beginning of a line. There is also a PCRE_NOTEOL optio	Line 337 beginning of a line. There is also a PCRE_NOTEOL optio
doing multi-segment matching you should be using PCRE_PARTIAL_HARD, which	doing multi-segment matching you should be using PCRE_PARTIAL_HARD, which
includes the effect of PCRE_NOTEOL.	includes the effect of PCRE_NOTEOL.
.P	.P
2. Lookbehind assertions at the start of a pattern are catered for in the	2. Lookbehind assertions that have already been obeyed are catered for in the
offsets that are returned for a partial match. However, in theory, a lookbehind	offsets that are returned for a partial match. However a lookbehind assertion
assertion later in the pattern could require even earlier characters to be	later in the pattern could require even earlier characters to be inspected. You
inspected, and it might not have been reached when a partial match occurs. This	can handle this case by using the PCRE_INFO_MAXLOOKBEHIND option of the
is probably an extremely unlikely case; you could guard against it to a certain	\fBpcre_fullinfo()\fP or \fBpcre[16\|32]_fullinfo()\fP functions to obtain the
extent by always including extra characters at the start.	length of the longest lookbehind in the pattern. This length is given in
	characters, not bytes. If you always retain at least that many characters
	before the partially matched string, all should be well. (Of course, near the
	start of the subject, fewer characters may be present; in that case all
	characters should be retained.)
.P	.P
3. Matching a subject string that is split into multiple segments may not	From release 8.33, there is a more accurate way of deciding which characters to
	retain. Instead of subtracting the length of the longest lookbehind from the
	earliest inspected character (\fIoffsets[0]\fP), the match start position
	(\fIoffsets[2]\fP) should be used, and the next match attempt started at the
	\fIoffsets[2]\fP character by setting the \fIstartoffset\fP argument of
	\fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP.
	.P
	For example, if the pattern "(?<=123)abc" is partially
	matched against the string "xx123a", the three offset values returned are 2, 6,
	and 5. This indicates that the matching process that gave a partial match
	started at offset 5, but the characters "123a" were all inspected. The maximum
	lookbehind for that pattern is 3, so taking that away from 5 shows that we need
	only keep "123a", and the next match attempt can be started at offset 3 (that
	is, at "a") when further characters have been added. When the match start is
	not the earliest inspected character, \fBpcretest\fP shows it explicitly:
	.sp
	re> "(?<=123)abc"
	data> xx123a\eP\eP
	Partial match at offset 5: 123a
	.P
	3. Because a partial match must always contain at least one character, what
	might be considered a partial match of an empty string actually gives a "no
	match" result. For example:
	.sp
	re> /c(?<=abc)x/
	data> ab\eP
	No match
	.sp
	If the next segment begins "cx", a match should be found, but this will only
	happen if characters from the previous segment are retained. For this reason, a
	"no match" result should be interpreted as "partial match of an empty string"
	when the pattern contains lookbehinds.
	.P
	4. 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,	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	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	Word Boundaries" above describes an issue that arises if the pattern ends with
Line 343 longer possible. Consider again this \fBpcretest\fP ex	Line 402 longer possible. Consider again this \fBpcretest\fP ex
0: dogsbody	0: dogsbody
1: dog	1: dog
.sp	.sp
The first data line passes the string "dogsb" to \fBpcre_exec()\fP, setting the	The first data line passes the string "dogsb" to a standard matching function,
PCRE_PARTIAL_SOFT option. Although the string is a partial match for	setting the PCRE_PARTIAL_SOFT option. Although the string is a partial match
"dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter string	for "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter
"dog" is a complete match. Similarly, when the subject is presented to	string "dog" is a complete match. Similarly, when the subject is presented to
\fBpcre_dfa_exec()\fP in several parts ("do" and "gsb" being the first two) the	a DFA matching function in several parts ("do" and "gsb" being the first two)
match stops when "dog" has been found, and it is not possible to continue. On	the match stops when "dog" has been found, and it is not possible to continue.
the other hand, if "dogsbody" is presented as a single string,	On the other hand, if "dogsbody" is presented as a single string, a DFA
\fBpcre_dfa_exec()\fP finds both matches.	matching function finds both matches.
.P	.P
Because of these problems, it is best to use PCRE_PARTIAL_HARD when matching	Because of these problems, it is best to use PCRE_PARTIAL_HARD when matching
multi-segment data. The example above then behaves differently:	multi-segment data. The example above then behaves differently:
Line 363 multi-segment data. The example above then behaves dif	Line 422 multi-segment data. The example above then behaves dif
data> gsb\eR\eP\eP\eD	data> gsb\eR\eP\eP\eD
Partial match: gsb	Partial match: gsb
.sp	.sp
4. Patterns that contain alternatives at the top level which do not all	5. Patterns that contain alternatives at the top level which do not all start
start with the same pattern item may not work as expected when	with the same pattern item may not work as expected when PCRE_DFA_RESTART is
PCRE_DFA_RESTART is used with \fBpcre_dfa_exec()\fP. For example, consider this	used. For example, consider this pattern:
pattern:
.sp	.sp
1234\|3789	1234\|3789
.sp	.sp
Line 382 patterns or patterns such as:	Line 440 patterns or patterns such as:
1234\|ABCD	1234\|ABCD
.sp	.sp
where no string can be a partial match for both alternatives. This is not a	where no string can be a partial match for both alternatives. This is not a
problem if \fBpcre_exec()\fP is used, because the entire match has to be rerun	problem if a standard matching function is used, because the entire match has
each time:	to be rerun each time:
.sp	.sp
re> /1234\|3789/	re> /1234\|3789/
data> ABC123\eP\eP	data> ABC123\eP\eP
Line 392 each time:	Line 450 each time:
0: 3789	0: 3789
.sp	.sp
Of course, instead of using PCRE_DFA_RESTART, the same technique of re-running	Of course, instead of using PCRE_DFA_RESTART, the same technique of re-running
the entire match can also be used with \fBpcre_dfa_exec()\fP. Another	the entire match can also be used with the DFA matching functions. Another
possibility is to work with two buffers. If a partial match at offset \fIn\fP	possibility is to work with two buffers. If a partial match at offset \fIn\fP
in the first buffer is followed by "no match" when PCRE_DFA_RESTART is used on	in the first buffer is followed by "no match" when PCRE_DFA_RESTART is used on
the second buffer, you can then try a new match starting at offset \fIn+1\fP in	the second buffer, you can then try a new match starting at offset \fIn+1\fP in
Line 413 Cambridge CB2 3QH, England.	Line 471 Cambridge CB2 3QH, England.
.rs	.rs
.sp	.sp
.nf	.nf
Last updated: 26 August 2011	Last updated: 02 July 2013
Copyright (c) 1997-2011 University of Cambridge.	Copyright (c) 1997-2013 University of Cambridge.
.fi	.fi

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