sub template_to_regexp {
  my($template) = @_;
  my $seen_count;
  my %seen_at;
  my $regexp = '';
  for my $i (0 .. length($template) - 1) {
    my $chr = substr($template, $i, 1);
    my $seen = $seen_at{$chr};

    if (defined $seen) {
      $regexp .= "\\$seen";
      next;
    }

    # else it's a new template character
    $regexp .= sprintf '(?!%s)', join '|', map "\\$_", 1 .. $seen_count
        if $seen_count;
    $seen_at{$chr} = ++$seen_count;
    $regexp .= '(.)';
  }
  return qr{$regexp};
}