.TH PCREMATCHING 3

.SH NAME

PCRE - Perl-compatible regular expressions

.SH "PCRE MATCHING ALGORITHMS"

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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 function.

This works in the same was as Perl's matching function, and provides a

Perl-compatible matching operation.

.P

An alternative algorithm is provided by the \fBpcre_dfa_exec()\fP function;

this operates in a different way, and is not Perl-compatible. It has advantages

and disadvantages compared with the standard algorithm, and these are described

below.

.P

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

.sp

  ^<.*>

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is matched against the string

.sp

  <something> <something else> <something further>

.sp

there are three possible answers. The standard algorithm finds only one of

them, whereas the alternative algorithm finds all three.

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.SH "REGULAR EXPRESSIONS AS TREES"

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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.

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.SH "THE STANDARD MATCHING ALGORITHM"

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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.

.P

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.

.P

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.

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.SH "THE ALTERNATIVE MATCHING ALGORITHM"

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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).

.P

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. In PCRE, there is an option to

stop the algorithm after the first match (which is necessarily the shortest)

has been found.

.P

Note that all the matches that are found start at the same point in the

subject. If the pattern

.sp

  cat(er(pillar)?)

.sp

is matched against the string "the caterpillar catchment", the result will be

the three strings "cat", "cater", and "caterpillar" that start at the fourth

character of the subject. The algorithm does not automatically move on to find

matches that start at later positions.

.P

There are a number of features of PCRE regular expressions that are not

supported by the alternative matching algorithm. They are as follows:

.P

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:

.sp

  ^a++\ew!

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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.

.P

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.

.P

3. Because no substrings are captured, back references within the pattern are

not supported, and cause errors if encountered.

.P

4. For the same reason, conditional expressions that use a backreference as the

condition or test for a specific group recursion are not supported.

.P

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.

.P

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.

.P

7. The \eC escape sequence, which (in the standard algorithm) matches a single

byte, even in UTF-8 mode, is not supported because the alternative algorithm

moves through the subject string one character at a time, for all active paths

through the tree.

.P

8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not

supported. (*FAIL) is supported, and behaves like a failing negative assertion.

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.SH "ADVANTAGES OF THE ALTERNATIVE ALGORITHM"

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Using the alternative matching algorithm provides the following advantages:

.P

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

callouts.

.P

2. There is much better support for partial matching. The restrictions on the

content of the pattern that apply when using the standard algorithm for partial

matching do not apply to the alternative algorithm. For non-anchored patterns,

the starting position of a partial match is available.

.P

3. Because the alternative algorithm scans the subject string just once, and

never needs to backtrack, it is possible to pass very long subject strings to

the matching function in several pieces, checking for partial matching each

time.

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.SH "DISADVANTAGES OF THE ALTERNATIVE ALGORITHM"

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The alternative algorithm suffers from a number of disadvantages:

.P

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.

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2. Capturing parentheses and back references are not supported.

.P

3. Although atomic groups are supported, their use does not provide the

performance advantage that it does for the standard algorithm.

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.SH AUTHOR

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Philip Hazel

University Computing Service

Cambridge CB2 3QH, England.

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.SH REVISION

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Last updated: 19 April 2008

Copyright (c) 1997-2008 University of Cambridge.

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