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PCRE2MATCHING(3) Library Functions Manual PCRE2MATCHING(3)
PCRE2 - Perl-compatible regular expressions (revised API)
This document describes the two different algorithms that are
available in PCRE2 for matching a compiled regular expression
against a given subject string. The "standard" algorithm is the
one provided by the pcre2_match() function. This works in the
same as Perl's matching function, and provide a Perl-compatible
matching operation. The just-in-time (JIT) optimization that is
described in the pcre2jit documentation is compatible with this
function.
An alternative algorithm is provided by the pcre2_dfa_match()
function; it operates in a different way, and is not Perl-
compatible. This alternative has advantages and disadvantages
compared with the standard algorithm, and these are described
below.
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 PCRE2.
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 alternations and the
greedy or 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
backreferences.
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 simultaneously).
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 the output vector 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 the size of vector needed to contain all the results
depends on the number of simultaneous matches, not on the number
of parentheses in the pattern. Using
pcre2_match_data_create_from_pattern() to create the match data
block is therefore not advisable when doing DFA matching.
Note also 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 is 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.
PCRE2's "auto-possessification" optimization usually applies to
character repeats at the end of a pattern (as well as
internally). For example, the pattern "a\d+" is compiled as if it
were "a\d++" 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\d+?") or set the PCRE2_NO_AUTO_POSSESS option
when compiling.
There are a number of features of PCRE2 regular expressions that
are not supported or behave differently in the alternative
matching function. Those that are not supported cause an error if
encountered.
1. Because the algorithm finds all possible matches, the greedy
or ungreedy nature of repetition quantifiers is not relevant
(though it may affect auto-possessification, as just described).
During matching, 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++\w!
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
PCRE2'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, backreferences within the
pattern are not supported.
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. Again for the same reason, script runs are not supported.
6. Because many paths through the tree may be active, the \K
escape sequence, which resets the start of the match when
encountered (but may be on some paths and not on others), is not
supported.
7. Callouts are supported, but the value of the capture_top field
is always 1, and the value of the capture_last field is always 0.
8. The \C escape sequence, which (in the standard algorithm)
always matches a single code unit, even in a UTF mode, is not
supported in these modes, because the alternative algorithm moves
through the subject string one character (not code unit) at a
time, for all active paths through the tree.
9. Except for (*FAIL), the backtracking control verbs such as
(*PRUNE) are not supported. (*FAIL) is supported, and behaves
like a failing negative assertion.
10. The PCRE2_MATCH_INVALID_UTF option for pcre2_compile() is not
supported by pcre2_dfa_match().
The main advantage of the alternative algorithm is that 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 at the same point using the
standard algorithm, you have to do kludgy things with callouts.
Partial matching is possible with this algorithm, though it has
some limitations. The pcre2partial documentation gives details of
partial matching and discusses multi-segment matching.
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, backreferences, script runs, and
matching within invalid UTF string are not supported.
3. Although atomic groups are supported, their use does not
provide the performance advantage that it does for the standard
algorithm.
4. JIT optimization is not supported.
Philip Hazel
Retired from University Computing Service
Cambridge, England.
Last updated: 19 January 2024
Copyright (c) 1997-2024 University of Cambridge.
This page is part of the PCRE (Perl Compatible Regular
Expressions) project. Information about the project can be found
at ⟨http://www.pcre.org/⟩. If you have a bug report for this
manual page, see
⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩. This page was
obtained from the tarball fetched from
⟨https://github.com/PhilipHazel/pcre2.git⟩ on 2024-06-14. If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original
manual page), send a mail to [email protected]
PCRE2 10.43 19 January 2024 PCRE2MATCHING(3)
Pages that refer to this page: pcre2test(1), pcre2api(3), pcre2pattern(3), pcre2syntax(3)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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