sub find_substring
{
	my $input = shift;
	my $length = length $input;
	my $i = 0;
	my $possible;
	while( 1 ) { 
		$possible = substr $input, 0, $i+1; # increase length by 1
		$i = index $input, $possible, $i+1; # find next occurence of candidate
		return $input if $i < 0;            # if not found return full string => no repetition
		$possible = substr $input, 0, $i;   # this is the minimum length candidate
		return $possible if $input eq substr($possible x (1 + int($length / $i)), 0, $length); # success
    }
}