I was going to create a nasty little test using length, tr///s, m//, and a couple of evals, but I just couldn't bring myself to complete it. Instead, I reached for CPAN and Math::Combinatorics. 

use strict;
use warnings;

use Math::Combinatorics qw( permute );
use Test::More;

my @match_wanted = qw( abc bca cab cba bac acb );
my @match_unwanted = qw( aab abbc acc );
my @charset = qw( a b c );

my %valid = map { join( '', @$_ ) => 1 } permute( @charset );

sub match
{
    my $s = shift;
    return( exists $valid{$s} ? 1 : 0 );
}

plan 'tests' => ( scalar @match_wanted + scalar @match_unwanted );

ok(   match( $_ ), "matches '$_'" )        for @match_wanted;
ok( ! match( $_ ), "does not match '$_'" ) for @match_unwanted;

[download]
Output: 
1..9
ok 1 - matches 'abc'
ok 2 - matches 'bca'
ok 3 - matches 'cab'
ok 4 - matches 'cba'
ok 5 - matches 'bac'
ok 6 - matches 'acb'
ok 7 - does not match 'aab'
ok 8 - does not match 'abbc'
ok 9 - does not match 'acc'

[download]

Thanks and ++ to kyle for the framework.

In reply to Re: REgular expression to check the string that allows "a","b" and "c" to occur only once in any order. (permutations) by bobf
in thread REgular expression to check the string that allows "a","b" and "c" to occur only once in any order. by isha

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