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+.TH PCREMATCHING 3 "12 November 2013" "PCRE 8.34"
+PCRE - Perl-compatible regular expressions
+This document describes the two different algorithms that are available in PCRE
+for matching a compiled regular expression against a given subject string. The
+"standard" algorithm is the one provided by the \fBpcre_exec()\fP,
+\fBpcre16_exec()\fP and \fBpcre32_exec()\fP functions. These work in the same
+as as Perl's matching function, and provide a Perl-compatible matching operation.
+The just-in-time (JIT) optimization that is described in the
+.\" HREF
+documentation is compatible with these functions.
+An alternative algorithm is provided by the \fBpcre_dfa_exec()\fP,
+\fBpcre16_dfa_exec()\fP and \fBpcre32_dfa_exec()\fP functions; they operate in
+a different way, and are not Perl-compatible. This alternative has advantages
+and disadvantages compared with the standard algorithm, and these are described
+When there is only one possible way in which a given subject string can match a
+pattern, the two algorithms give the same answer. A difference arises, however,
+when there are multiple possibilities. For example, if the pattern
+ ^<.*>
+is matched against the string
+ <something> <something else> <something further>
+there are three possible answers. The standard algorithm finds only one of
+them, whereas the alternative algorithm finds all three.
+The set of strings that are matched by a regular expression can be represented
+as a tree structure. An unlimited repetition in the pattern makes the tree of
+infinite size, but it is still a tree. Matching the pattern to a given subject
+string (from a given starting point) can be thought of as a search of the tree.
+There are two ways to search a tree: depth-first and breadth-first, and these
+correspond to the two matching algorithms provided by PCRE.
+In the terminology of Jeffrey Friedl's book "Mastering Regular
+Expressions", the standard algorithm is an "NFA algorithm". It conducts a
+depth-first search of the pattern tree. That is, it proceeds along a single
+path through the tree, checking that the subject matches what is required. When
+there is a mismatch, the algorithm tries any alternatives at the current point,
+and if they all fail, it backs up to the previous branch point in the tree, and
+tries the next alternative branch at that level. This often involves backing up
+(moving to the left) in the subject string as well. The order in which
+repetition branches are tried is controlled by the greedy or ungreedy nature of
+the quantifier.
+If a leaf node is reached, a matching string has been found, and at that point
+the algorithm stops. Thus, if there is more than one possible match, this
+algorithm returns the first one that it finds. Whether this is the shortest,
+the longest, or some intermediate length depends on the way the greedy and
+ungreedy repetition quantifiers are specified in the pattern.
+Because it ends up with a single path through the tree, it is relatively
+straightforward for this algorithm to keep track of the substrings that are
+matched by portions of the pattern in parentheses. This provides support for
+capturing parentheses and back references.
+This algorithm conducts a breadth-first search of the tree. Starting from the
+first matching point in the subject, it scans the subject string from left to
+right, once, character by character, and as it does this, it remembers all the
+paths through the tree that represent valid matches. In Friedl's terminology,
+this is a kind of "DFA algorithm", though it is not implemented as a
+traditional finite state machine (it keeps multiple states active
+Although the general principle of this matching algorithm is that it scans the
+subject string only once, without backtracking, there is one exception: when a
+lookaround assertion is encountered, the characters following or preceding the
+current point have to be independently inspected.
+The scan continues until either the end of the subject is reached, or there are
+no more unterminated paths. At this point, terminated paths represent the
+different matching possibilities (if there are none, the match has failed).
+Thus, if there is more than one possible match, this algorithm finds all of
+them, and in particular, it finds the longest. The matches are returned in
+decreasing order of length. There is an option to stop the algorithm after the
+first match (which is necessarily the shortest) is found.
+Note that all the matches that are found start at the same point in the
+subject. If the pattern
+ cat(er(pillar)?)?
+is matched against the string "the caterpillar catchment", the result will be
+the three strings "caterpillar", "cater", and "cat" that start at the fifth
+character of the subject. The algorithm does not automatically move on to find
+matches that start at later positions.
+PCRE's "auto-possessification" optimization usually applies to character
+repeats at the end of a pattern (as well as internally). For example, the
+pattern "a\ed+" is compiled as if it were "a\ed++" because there is no point
+even considering the possibility of backtracking into the repeated digits. For
+DFA matching, this means that only one possible match is found. If you really
+do want multiple matches in such cases, either use an ungreedy repeat
+("a\ed+?") or set the PCRE_NO_AUTO_POSSESS option when compiling.
+There are a number of features of PCRE regular expressions that are not
+supported by the alternative matching algorithm. They are as follows:
+1. Because the algorithm finds all possible matches, the greedy or ungreedy
+nature of repetition quantifiers is not relevant. Greedy and ungreedy
+quantifiers are treated in exactly the same way. However, possessive
+quantifiers can make a difference when what follows could also match what is
+quantified, for example in a pattern like this:
+ ^a++\ew!
+This pattern matches "aaab!" but not "aaa!", which would be matched by a
+non-possessive quantifier. Similarly, if an atomic group is present, it is
+matched as if it were a standalone pattern at the current point, and the
+longest match is then "locked in" for the rest of the overall pattern.
+2. When dealing with multiple paths through the tree simultaneously, it is not
+straightforward to keep track of captured substrings for the different matching
+possibilities, and PCRE's implementation of this algorithm does not attempt to
+do this. This means that no captured substrings are available.
+3. Because no substrings are captured, back references within the pattern are
+not supported, and cause errors if encountered.
+4. For the same reason, conditional expressions that use a backreference as the
+condition or test for a specific group recursion are not supported.
+5. Because many paths through the tree may be active, the \eK escape sequence,
+which resets the start of the match when encountered (but may be on some paths
+and not on others), is not supported. It causes an error if encountered.
+6. Callouts are supported, but the value of the \fIcapture_top\fP field is
+always 1, and the value of the \fIcapture_last\fP field is always -1.
+7. The \eC escape sequence, which (in the standard algorithm) always matches a
+single data unit, even in UTF-8, UTF-16 or UTF-32 modes, is not supported in
+these modes, because the alternative algorithm moves through the subject string
+one character (not data unit) at a time, for all active paths through the tree.
+8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not
+supported. (*FAIL) is supported, and behaves like a failing negative assertion.
+Using the alternative matching algorithm provides the following advantages:
+1. All possible matches (at a single point in the subject) are automatically
+found, and in particular, the longest match is found. To find more than one
+match using the standard algorithm, you have to do kludgy things with
+2. Because the alternative algorithm scans the subject string just once, and
+never needs to backtrack (except for lookbehinds), it is possible to pass very
+long subject strings to the matching function in several pieces, checking for
+partial matching each time. Although it is possible to do multi-segment
+matching using the standard algorithm by retaining partially matched
+substrings, it is more complicated. The
+.\" HREF
+documentation gives details of partial matching and discusses multi-segment
+The alternative algorithm suffers from a number of disadvantages:
+1. It is substantially slower than the standard algorithm. This is partly
+because it has to search for all possible matches, but is also because it is
+less susceptible to optimization.
+2. Capturing parentheses and back references are not supported.
+3. Although atomic groups are supported, their use does not provide the
+performance advantage that it does for the standard algorithm.
+Philip Hazel
+University Computing Service
+Cambridge CB2 3QH, England.
+Last updated: 12 November 2013
+Copyright (c) 1997-2012 University of Cambridge.