NAME
Regexp::Exhaustive - Find all possible matches, including backtracked
and overlapping, of a pattern against a string
SYNOPSIS
use Regexp::Exhaustive qw/ exhaustive /;
print "Subsets:\n";
print "$_\n" for exhaustive('abc' => qr/.+/);
print "\n";
print "Overlapping matching:\n";
print "$_\n" for exhaustive('abc' => qr/(?>.+)/);
print "\n";
print "Heads and tails:\n";
print "@$_\n" for exhaustive('abcde' => qr/^(.+?)(.+)\z/);
print "\n";
print "Triplets:\n";
print "@$_\n" for exhaustive('abcde' => qr/(.)(.)(.)/);
print "\n";
print "Binary count:\n";
print map(length, @$_), "\n"
for exhaustive('111', qr/^(.??)(.??)(.??)/);
__END__
Subsets:
abc
ab
a
bc
b
c
Overlapping matching:
abc
bc
c
Heads and tails:
a bcde
ab cde
abc de
abcd e
Triplets:
a b c
b c d
c d e
Binary count:
000
001
010
011
100
101
110
111
DESCRIPTION
This module does an exhaustive match of a pattern against a string. That
means that it will match all ways possible, including all backtracked
and overlapping matches.
It works a lot like the familiar "m//g" regarding return values.
Beware that exhaustive matching may generate a very large number of
matches. If you only need overlapping matches that's easily achieved.
Overlapping matching has a maximum number of matches being the length of
the string plus one.
This is an initial release, and some things may change for the next
version. If you feel something is missing or poorly designed, now is the
time to voice your opinion.
EXPORTED FUNCTIONS
Nothing is exported by default. The ":ALL" tag exports everything that
can be exported.
"exhaustive(STRING => qr/PATTERN/)"
Exhaustively generates and returns all matches in list context.
Returns the number of matches in scalar context.
If the pattern doesn't contain any capturing subpatterns, the
matched string (equivalent of $&) is returned. If only one capturing
subpattern is seen then $1 is returned. Otherwise $1, $2, etc is
returned grouped using array references.
This is like "m//g" in list context, except "m//g" returns all
capturing subpatterns as a flat list.
This method does not interact with "pos($str)" and can be safely
intermixed with other match operations against the string.
Worthy to note is that it's much more efficient to use "qr/(.)/"
than "qr/./", i.e. making direct use of $1 instead of having
"exhaustive" calculate $&.
"exhaustive(STRING => qr/PATTERN/, qw[ $1 $2 @- $^R ... ])"
Optionally, you can specify which variables to return. Arrays and
hashes will end up as references in the return list.
If two or more variables are specified they will be grouped using
array references.
my @matches = exhaustive('abc' => qr/(a)|(b)/, qw/ $1 $2 $+ /);
for (@matches) {
print join("\t", map { defined() ? $_ : 'undef' } @$_) . "\n";
}
__END__
a undef a
undef b b
Supported variables:
Punctuation: English alias:
$<*digits*>
$` $PREMATCH
$& $MATCH
$' $POSTMATCH
$+ $LAST_PAREN_MATCH
$^N
@- @LAST_MATCH_START
@+ @LAST_MATCH_END
%-
%+
$^R $LAST_REGEXP_CODE_RESULT
Using $`, $&, and $' with "exhaustive" doesn't impose the
performance penalty that those variables otherwise impose. However,
they're usually slower to use than rewriting the pattern to use
capturing subpatterns. In particular, $& is often easy to avoid by
just adding an outer capturing parenthesis and using $1 instead.
"overlapping(STRING => qr/PATTERN/)"
If this function would exist, but it doesn't, it would be like
global match but try to match from every position of the string.
It's like an exhaustive match that doesn't backtrack inside the
pattern.
This function doesn't exist because it isn't needed. Instead use
exhaustive(STRING => qr/(?>PATTERN)/)
i.e. wrap the pattern with the "(?>...)" assertion. This will lock
the match once the pattern has matched, forcing the regex engine to
skip behind the pattern when backtracking, thus moving forward on
the string for the next match.
Using
STRING =~ /(?=PATTERN)/g
will be faster, but has three key differences: (1) assumes "pos()"
is undefined, (2) undefines "pos()", and (3) returns all captured
subpatterns as a flat list. Note that saving away "pos()" and then
restoring it may cause certain global matches to loop infinitely.
$_ = 'foo';
while (/.??/g) {
print pos();
pos() = pos(); # Not really doing anything, or is it?
}
This will loop forever.
DIAGNOSTICS
Use of uninitialized value in &Regexp::Exhaustive::exhaustive
(W|uninitialized) The string given to "exhaustive" was not defined.
The second argument to &Regexp::Exhaustive::exhaustive must be a Regexp
object (qr//)
(F) Self-explanatory.
Uninitialized value passed to &Regexp::Exhaustive::exhaustive as
variable name
(F) An argument for a regex variable name was not defined.
Bad variable name(s) to &Regexp::Exhaustive::exhaustive: %s
(F) You supplied a string that didn't look like a known regex
variable. See "exhaustive" for supported variables.
EXAMPLES
See "SYNOPSIS" for more examples.
Finding all divisors
A commonly known snippet of regex can be used to find out if an integer
is a prime number or not.
sub is_prime {
my ($n) = @_;
my $str = '.' x $n;
return $str =~ /^(?:..+)\1+$/ ? 0 : 1;
}
print '9 is prime: ', is_prime(9), "\n";
print '11 is prime: ', is_prime(11), "\n";
__END__
9 is prime: 0
11 is prime: 1
Equally simple is it, with "Regexp::Exhaustive", to find out not only if
it's a prime number, but which its divisors are.
use Regexp::Exhaustive 'exhaustive';
sub divisors {
my ($i) = @_;
return map length, exhaustive('.' x $i => qr/^(.+?)\1*$/);
}
print "$_\n" for divisors(12);
__END__
1
2
3
4
6
12
Finding the cross product
Set::CrossProduct gives you the cross product of a set, and that's the
good way of doing just that. But as an example, here's how you can find
all possible combinations of two four-sided dice using
"Regexp::Exhaustive". To illustrate the difference between greedy and
non-greedy matches I let the second die be in reversed order.
use Regexp::Exhaustive 'exhaustive';
my $sides = '1234';
my @sets = exhaustive(
"$sides\n$sides"
=>
qr/^.*?(.).*\n.*(.)/
);
print "@$_\n" for @sets;
__END__
1 4
1 3
1 2
1 1
2 4
2 3
2 2
2 1
3 4
3 3
3 2
3 1
4 4
4 3
4 2
4 1
N-ary count
Using "Regexp::Exhaustive" you can generate all values of a certain
digit length using an n-ary count. This is demonstrated for binary
numbers with a length of three. (The length is assumed to be greater
than one.)
use Regexp::Exhaustive 'exhaustive';
sub all_values {
my ($n, @base) = @_;
my $str = (join('', @base) . "\n") x $n;
my $re = qr/.*?(.).*\n/ x $n;
return map { join '', @$_ } exhaustive($str, qr/^$re/);
}
print "$_\n" for all_values(3, qw/ 0 1 /);
__END__
000
001
010
011
100
101
110
111
WARNING
This module uses the experimental "(?{ ... })" assertion. Thus this
module is as experimental as that assertion.
ACKNOWLEDGEMENTS
Thanks to Mike Rosulek for giving useful feedback and suggestions.
AUTHOR
Johan Lodin
COPYRIGHT
Copyright 2005-2007 Johan Lodin. All rights reserved.
This library is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
SEE ALSO
perlre for regular expressions.
perlvar for the special variables.