sub make_pattern {
  my $w = shift;
  my %seen;

  $w =~ s{([A-Z])}{
    if ($seen{$1}) { "\\$seen{$1}" }
    else {
      $seen{$1} = 1 + keys %seen;
      "(" . join("", map "(?!\\$_)", 1 .. ($seen{$1} - 1)) . ".)"
    }
  }ge;

  return qr/^$w$/si;
}