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Linux 2.6 - man page for pcrepartial (linux section 3)


       PCRE - Perl-compatible regular expressions


       In  normal  use	of  PCRE,  if  the  subject  string  that  is  passed  to  pcre_exec() or
       pcre_dfa_exec() 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 dis-
       tinguish this case from other cases in which there is no match.

       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 ddmm-
       myy, defined by this pattern:


       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 pcre_exec() or pcre_dfa_exec(). For backwards  com-
       patibility,  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.

       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.


       A  partial match occurs during a call to pcre_exec() 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  \K  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 pcre_exec() returns with a par-
       tial  match,  the  first  slot  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 subject so that a substring can easily be identified.

       For  the  majority  of  patterns,  the  first offset identifies the start of the partially
       matched string. However, for patterns that contain lookbehind assertions, or \K, or  begin
       with  \b  or  \B, earlier characters have been inspected while carrying out the match. For


       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  to  the

       What happens when a partial match is identified depends on which of the two partial match-
       ing options are set.

   PCRE_PARTIAL_SOFT with pcre_exec()

       If PCRE_PARTIAL_SOFT is set when pcre_exec() 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_exec() returns  PCRE_ERROR_PARTIAL  instead

       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  com-
       plete.  For  example, \z, \Z, and $ match at the end of the subject, as normal, and for \b
       and \B the end of the subject is treated as a non-alphanumeric.

       If there is more than one partial match, the first one that was found  provides	the  data
       that is returned. Consider this pattern:


       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.)

   PCRE_PARTIAL_HARD with pcre_exec()

       If  PCRE_PARTIAL_HARD  is  set for pcre_exec(), it returns PCRE_ERROR_PARTIAL 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 \z, \Z, \b, \B, or $ are encountered
       at the end of the subject, the result is PCRE_ERROR_PARTIAL.

       Setting PCRE_PARTIAL_HARD also affects the way pcre_exec() checks  UTF-8  subject  strings
       for  validity.  Normally,  an  invalid UTF-8 sequence causes the error PCRE_ERROR_BADUTF8.
       However, in the special case of a truncated UTF-8 character at the  end	of  the  subject,
       PCRE_ERROR_SHORTUTF8 is returned when PCRE_PARTIAL_HARD is set.

   Comparing hard and soft partial matching

       The  difference	between  the two partial matching options can be illustrated by a pattern
       such as:


       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 dif-


       In this case the result is always a complete match because pcre_exec() finds  that  first,
       and  it	never continues after finding a match. It might be easier to follow this explana-
       tion 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" when pcre_exec() is used, because  it  will
       always find the shorter match first.


       The  pcre_dfa_exec() function moves along the subject string character by character, with-
       out 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 inspected.

       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_PAR-
       TIAL_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 pcre_dfa_exec() always searches for all possible matches, and there is no  differ-
       ence  between  greedy  and  ungreedy repetition, its behaviour is different from pcre_exec
       when PCRE_PARTIAL_HARD is set. Consider the string "dog" matched against the ungreedy pat-
       tern shown above:


       Whereas	 pcre_exec()   stops   as  soon  as  it  finds	the  complete  match  for  "dog",
       pcre_dfa_exec() also finds the partial match for "dogsbody",  and  so  returns  that  when
       PCRE_PARTIAL_HARD is set.


       If  a pattern ends with one of sequences \b or \B, which test for word boundaries, partial
       matching with PCRE_PARTIAL_SOFT can give counter-intuitive results. Consider this pattern:


       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, pcre_exec() carries on with normal	matching,
       which matches \b at the end of the subject when the last character is a letter, thus find-
       ing a complete match. The result, therefore, is not  PCRE_ERROR_PARTIAL.  The  same  thing
       happens with pcre_dfa_exec(), because it also finds the complete match.

       Using  PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because then the par-
       tial match takes precedence.


       For releases of PCRE prior to 8.00, because of the way certain internal optimizations were
       implemented in the pcre_exec() function, the PCRE_PARTIAL option (predecessor of PCRE_PAR-
       TIAL_SOFT) could not be used with all patterns. From release 8.00  onwards,  the  restric-
       tions no longer apply, and partial matching with pcre_exec() can be requested for any pat-

       Items that were formerly restricted were repeated single characters and	repeated  metase-
       quences.  If  PCRE_PARTIAL was set for a pattern that did not conform to the restrictions,
       pcre_exec() returned the error code PCRE_ERROR_BADPARTIAL (-13). This  error  code  is  no
       longer  in  use. The PCRE_INFO_OKPARTIAL call to pcre_fullinfo() to find out if a compiled
       pattern can be used for partial matching now always returns 1.


       If the escape sequence \P is present in a pcretest data line, the PCRE_PARTIAL_SOFT option
       is used for the match. Here is a run of pcretest that uses the date example quoted above:

	   re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
	 data> 25jun04\P
	  0: 25jun04
	  1: jun
	 data> 25dec3\P
	 Partial match: 23dec3
	 data> 3ju\P
	 Partial match: 3ju
	 data> 3juj\P
	 No match
	 data> j\P
	 No match

       The first data string is matched completely, so pcretest 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 pcre_dfa_exec() is used.

       If the escape sequence \P is present more than once in a pcretest data line, the PCRE_PAR-
       TIAL_HARD option is set for the match.


       When a partial match has been found using pcre_dfa_exec(), it is possible to continue  the
       match by providing additional subject data and calling pcre_dfa_exec() 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 pcretest, using the \R escape sequence  to  set
       the PCRE_DFA_RESTART option (\D specifies the use of pcre_dfa_exec()):

	   re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
	 data> 23ja\P\D
	 Partial match: 23ja
	 data> n05\R\D
	  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.

       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 pcre_dfa_exec().


       From release 8.00, pcre_exec() can also be  used  to  do  multi-segment	matching.  Unlike
       pcre_dfa_exec(),  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 dis-
       carded. 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 \z, \Z, \b, \B, and $. Con-
       sider an unanchored pattern that matches dates:

	   re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
	 data> The date is 23ja\P\P
	 Partial match: 23ja

       At this stage, an application could discard the text preceding "23ja", add  on  text  from
       the  next segment, and call pcre_exec() again. Unlike pcre_dfa_exec(), 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.

       Note:  If  the pattern contains lookbehind assertions, or \K, or starts with \b or \B, the
       string that is returned for a partial match will include characters that precede the  par-
       tially  matched	string itself, because these must be retained when adding on more charac-
       ters for a subsequent matching attempt.


       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 match-
       ing you should be using PCRE_PARTIAL_HARD, which includes the effect of PCRE_NOTEOL.

       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 pat-
       tern 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

       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_PAR-
       TIAL_SOFT  is  used. The section "Partial Matching and Word Boundaries" above describes an
       issue that arises if the pattern ends with \b or \B. 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 pos-
       sible. Consider again this pcretest example:

	   re> /dog(sbody)?/
	 data> dogsb\P
	  0: dog
	 data> do\P\D
	 Partial match: do
	 data> gsb\R\P\D
	  0: g
	 data> dogsbody\D
	  0: dogsbody
	  1: dog

       The first data line passes the  string  "dogsb"	to  pcre_exec(),  setting  the	PCRE_PAR-
       TIAL_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 pcre_dfa_exec() 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, pcre_dfa_exec() 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\P\P
	 Partial match: dogsb
	 data> do\P\D
	 Partial match: do
	 data> gsb\R\P\P\D
	 Partial match: gsb

       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	PCRE_DFA_RESTART  is  used   with
       pcre_dfa_exec(). For example, consider this pattern:


       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  pat-
       terns or patterns such as:


       where  no  string  can  be a partial match for both alternatives. This is not a problem if
       pcre_exec() is used, because the entire match has to be rerun each time:

	   re> /1234|3789/
	 data> ABC123\P\P
	 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 pcre_dfa_exec(). Another possibility is to work with two buf-
       fers. If a partial match at offset n 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 n+1 in the first buffer.


       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


       Last updated: 07 November 2010
       Copyright (c) 1997-2010 University of Cambridge.


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