while ( my $line = ) { my ($new, $old) = split /\s+/, $line; my $rx = regex($new); print "$new can ", ( $old =~ /$rx/? '': 'NOT ' ), "be made from $old\n"; } sub regex { my %letters; $letters{$_}++ for split //, lc shift; my $regex = join '', map { "(?=(?:.*${_}){$letters{$_}})" } keys %letters; return qr/\A$regex/si; } __DATA__ dog good food fodder lot total fuse useful poor porridge root rotor __END__ dog can be made from good food can NOT be made from fodder lot can be made from total fuse can be made from useful poor can NOT be made from porridge root can be made from rotor #### my $regex = qr{ \A # anchor at start for efficiency (?= # positive lookahead for (?: .* # anything r # with an 'r' after it ){1} # at least once ) (?= # positive lookahead for (?:.*t){1} # a 't' ) (?= # positive lookahead for (?:.*o){2} # for two 'o's ) }six; # ignore case