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+.TH PCREPARTIAL 3 "02 July 2013" "PCRE 8.34"
+PCRE - Perl-compatible regular expressions
+In normal use of PCRE, if the subject string that is passed to a matching
+function 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
+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:
+ ^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$
+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 be useful when the subject string is very
+long and is not all available at once.
+PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and
+PCRE_PARTIAL_HARD options, which can be set when calling any of the matching
+functions. 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 types of matching function. If both options
+are set, PCRE_PARTIAL_HARD takes precedence.
+If you want to use partial matching with just-in-time optimized code, you must
+call \fBpcre_study()\fP, \fBpcre16_study()\fP or \fBpcre32_study()\fP with one
+or both of these options:
+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.
+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
+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() OR pcre[16|32]_exec()"
+A partial match occurs during a call to \fBpcre_exec()\fP or
+\fBpcre[16|32]_exec()\fP when the end of the subject string is reached
+successfully, but matching cannot continue because more characters are needed.
+However, at least one character in the subject must have been inspected. This
+character need not form part of the final matched string; lookbehind assertions
+and the \eK escape sequence provide ways of inspecting characters before the
+start of a matched substring. The requirement for inspecting at least one
+character exists because an empty string can always be matched; without such a
+restriction there would always be a partial match of an empty string at the end
+of the subject.
+If there are at least two slots in the offsets vector when a partial match is
+returned, the first slot is set to the offset of the earliest character that
+was inspected. For convenience, the second offset points to the end of the
+subject so that a substring can easily be identified. If there are at least
+three slots in the offsets vector, the third slot is set to the offset of the
+character where matching started.
+For the majority of patterns, the contents of the first and third slots will be
+the same. However, for patterns that contain lookbehind assertions, or begin
+with \eb or \eB, characters before the one where matching started may have been
+inspected while carrying out the match. For example, consider this pattern:
+ /(?<=abc)123/
+This pattern matches "123", but only if it is preceded by "abc". If the subject
+string is "xyzabc12", the first two offsets after a partial match are for the
+substring "abc12", because all these characters were inspected. However, the
+third offset is set to 6, because that is the offset where matching began.
+What happens when a partial match is identified depends on which of the two
+partial matching options are set.
+.SS "PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre[16|32]_exec()"
+If PCRE_PARTIAL_SOFT is set when \fBpcre_exec()\fP or \fBpcre[16|32]_exec()\fP
+identifies a partial match, 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, PCRE_ERROR_PARTIAL is returned instead of
+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
+potentially complete. For example, \ez, \eZ, and $ match at the end of the
+subject, as normal, and for \eb and \eB the end of the subject is treated as a
+If there is more than one partial match, the first one that was found provides
+the data that is returned. Consider this pattern:
+ /123\ew+X|dogY/
+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. 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.)
+.SS "PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre[16|32]_exec()"
+If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP or \fBpcre[16|32]_exec()\fP,
+PCRE_ERROR_PARTIAL is returned as soon as a partial match is found, without
+continuing to search for possible complete matches. This option is "hard"
+because it prefers an earlier partial match over a later complete match. For
+this reason, the assumption is made that the end of the supplied subject string
+may not be the true end of the available data, and so, if \ez, \eZ, \eb, \eB,
+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.
+Setting PCRE_PARTIAL_HARD also affects the way UTF-8 and UTF-16
+subject strings are checked for validity. Normally, an invalid sequence
+causes the error PCRE_ERROR_BADUTF8 or PCRE_ERROR_BADUTF16. However, in the
+special case of a truncated character at the end of the subject,
+.SS "Comparing hard and soft partial matching"
+The difference between the two partial matching options can be illustrated by a
+pattern such as:
+ /dog(sbody)?/
+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:
+ /dog(sbody)??/
+In this case the result is always a complete match because that is found first,
+and matching never continues after finding a complete match. It might be easier
+to follow this explanation by thinking of the two patterns like this:
+ /dog(sbody)?/ is the same as /dogsbody|dog/
+ /dog(sbody)??/ is the same as /dog|dogsbody/
+The second pattern will never match "dogsbody", because it will always find the
+shorter match first.
+.SH "PARTIAL MATCHING USING pcre_dfa_exec() OR pcre[16|32]_dfa_exec()"
+The DFA functions move 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 been
+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.
+Because the DFA functions always search for all possible matches, and there is
+no difference between greedy and ungreedy repetition, their behaviour is
+different from the standard functions when PCRE_PARTIAL_HARD is set. Consider
+the string "dog" matched against the ungreedy pattern shown above:
+ /dog(sbody)??/
+Whereas the standard functions stop as soon as they find the complete match for
+"dog", the DFA functions also find the partial match for "dogsbody", and so
+return that when PCRE_PARTIAL_HARD is set.
+If a pattern ends with one of sequences \eb or \eB, which test for word
+boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive
+results. Consider this pattern:
+ /\ebcat\eb/
+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, normal
+matching carries on, and \eb matches at the end of the subject when the last
+character is a letter, so a complete match is found. The result, therefore, is
+\fInot\fP PCRE_ERROR_PARTIAL. Using PCRE_PARTIAL_HARD in this case does yield
+PCRE_ERROR_PARTIAL, because then the partial match takes precedence.
+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 can be requested for any pattern.
+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.
+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:
+ re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
+ data> 25jun04\eP
+ 0: 25jun04
+ 1: jun
+ data> 25dec3\eP
+ Partial match: 23dec3
+ data> 3ju\eP
+ Partial match: 3ju
+ data> 3juj\eP
+ No match
+ data> j\eP
+ No match
+The first data string is matched completely, so \fBpcretest\fP shows the
+matched substrings. The remaining four strings do not match the complete
+pattern, but the first two are partial matches. Similar output is obtained
+if DFA matching is used.
+If the escape sequence \eP is present more than once in a \fBpcretest\fP data
+line, the PCRE_PARTIAL_HARD option is set for the match.
+.SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre[16|32]_dfa_exec()"
+When a partial match has been found using a DFA matching function, it is
+possible to continue the match by providing additional subject data and calling
+the function again with the same compiled regular expression, this time setting
+the PCRE_DFA_RESTART option. You must pass the same working space as before,
+because this is where details of the previous partial match are stored. Here is
+an example using \fBpcretest\fP, using the \eR escape sequence to set the
+PCRE_DFA_RESTART option (\eD specifies the use of the DFA matching function):
+ re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/
+ data> 23ja\eP\eD
+ Partial match: 23ja
+ data> n05\eR\eD
+ 0: n05
+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.
+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.
+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 the DFA matching
+.SH "MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre[16|32]_exec()"
+From release 8.00, the standard matching functions can also be used to do
+multi-segment matching. Unlike the DFA functions, 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.
+It is best to use PCRE_PARTIAL_HARD in this situation, because it does not
+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:
+ re> /\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed/
+ data> The date is 23ja\eP\eP
+ Partial match: 23ja
+At this stage, an application could discard the text preceding "23ja", add on
+text from the next segment, and call the matching function again. Unlike the
+DFA matching functions, 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.
+\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 includes
+characters that precede the start of what would be returned for a complete
+match, because it contains all the characters that were inspected during the
+partial match.
+Certain types of pattern may give problems with multi-segment matching,
+whichever matching function is used.
+1. If the pattern contains a test for the beginning of a line, you need to pass
+the PCRE_NOTBOL option when the subject string for any call does start at the
+beginning of a line. There is also a PCRE_NOTEOL option, but in practice when
+doing multi-segment matching you should be using PCRE_PARTIAL_HARD, which
+includes the effect of PCRE_NOTEOL.
+2. Lookbehind assertions that have already been obeyed are catered for in the
+offsets that are returned for a partial match. However a lookbehind assertion
+later in the pattern could require even earlier characters to be inspected. You
+can handle this case by using the PCRE_INFO_MAXLOOKBEHIND option of the
+\fBpcre_fullinfo()\fP or \fBpcre[16|32]_fullinfo()\fP functions to obtain the
+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.)
+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.
+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:
+ re> "(?<=123)abc"
+ data> xx123a\eP\eP
+ Partial match at offset 5: 123a
+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:
+ re> /c(?<=abc)x/
+ data> ab\eP
+ No match
+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.
+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,
+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 (for PCRE_PARTIAL_SOFT) 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:
+ 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
+The first data line passes the string "dogsb" to a standard matching function,
+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
+a DFA matching function 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, a DFA
+matching function finds both matches.
+Because of these problems, it is best to use PCRE_PARTIAL_HARD when matching
+multi-segment data. The example above then behaves differently:
+ 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
+5. Patterns that contain alternatives at the top level which do not all start
+with the same pattern item may not work as expected when PCRE_DFA_RESTART is
+used. For example, consider this pattern:
+ 1234|3789
+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:
+ 1234|ABCD
+where no string can be a partial match for both alternatives. This is not a
+problem if a standard matching function is used, because the entire match has
+to be rerun each time:
+ re> /1234|3789/
+ data> ABC123\eP\eP
+ Partial match: 123
+ data> 1237890
+ 0: 3789
+Of course, instead of using PCRE_DFA_RESTART, the same technique of re-running
+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
+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 first buffer.
+Philip Hazel
+University Computing Service
+Cambridge CB2 3QH, England.
+Last updated: 02 July 2013
+Copyright (c) 1997-2013 University of Cambridge.