use warnings;
use strict;
use Algorithm::Combinatorics 'variations_with_repetition';
use Test::More;

my $re = qr{ \A
		( .* )
		( (??{ length $1 ? "[^".quotemeta($1)."]" : "." }) )
		( (?: (?! \2 ) . )* )
	\z }msx;

for my $len (1..7) {
	my $iter = variations_with_repetition([qw/a b c/], $len);
	while ( my $c = $iter->next ) {
		my $str = join '', @$c;
		my $exp = do {
			my %h; $h{$_}++ for @$c;
			my %r = reverse %h;
			exists $r{1} };
		is $str=~$re, $exp, $str.($exp?' =~':' !~').' re'
			or diag explain
				[ \%-, map [ $-[$_], eval "\$$_", $+[$_] ], 1..@+ ];
	}
}

done_testing;