my $pat = '';
for ($ARGV[0]) {
   /\G ([A-EG-WYZ]+)            /xgc && do { $pat .= $1;       redo };
   /\G X                        /xgc && do { $pat .= '.';      redo };
   /\G F                        /xgc && do { $pat .= '[QRG]';  redo };
   /\G \[ ([A-Z]) \/ ([A-Z]) \] /xgc && do { $pat .= "[$1$2]"; redo };
   /\G \z                       /xgc && last;

   my $pos  = pos();
   my $next = substr($_, $pos, 1);
   die("Unrecognized character \"$next\" at pos $pos\n");
}