Regular expression matching any subset of a given set?

Question

Is it possible to write a regular expression which will match any subset of a given set of characters
a1 ... an ?
I.e. it should match any string whe

Answer 1

Similar to Alan Moore's, using only \1, and doesn't refer to a capturing group before it has been seen:

#!/usr/bin/perl
my $re = qr/^(?:([abc])(?!.*\1))*$/;
foreach (qw(ba pabc abac a cc cba abcd abbbbc), '') {
    print "'$_' ", ($_ =~ $re) ? "matches" : "does not match", " \$re \n";
}

We match any number of blocks (the outer (?:)), where each block must consist of "precisely one character from our preferred set, which is not followed by a string containing that character".

If the string might contain newlines or other funny stuff, it might be necessary to play with some flags to make ^, $ and . behave as intended, but this all depends on the particular RE flavor.

Just for sillyness, one can use a positive look-ahead assertion to effectively AND two regexps, so we can test for any permutation of abc by asserting that the above matches, followed by an ordinary check for 'is N characters long and consists of these characters':

my $re2 = qr/^(?=$re)[abc]{3}$/;
foreach (qw(ba pabc abac a cc abcd abbbbc abc acb bac bca cab cba), '') {
    print "'$_' ", ($_ =~ $re2) ? "matches" : "does not match", " \$re2 \n";
}