Excerpt from HOP
sub tokens {
  my $target = shift;
  return sub {
    TOKEN: {
      return ['INTEGER', $1] if $target =˜ /\G (\d+) /gcx;
      return ['PRINT'] if $target =˜ /\G print \b /gcx;
      return ['IDENTIFIER', $1] if $target =˜ /\G ([A-Za-z_]\w*) /gcx;
      return ['OPERATOR', $1] if $target =˜ /\G (\*\*) /gcx;
      return ['OPERATOR', $1] if $target =˜ /\G ([-+*\/=()])/gcx;
      return ['TERMINATOR', $1] if $target =˜ /\G (; \n* | \n+) /gcx;
      redo TOKEN if $target =˜ /\G \s+ /gcx;
      return ['UNKNOWN', $1] if $target =˜ /\G (.) /gcx;
      return;
    }
  };
}
-----------------------------------------------------
cipher text 1        ciphertxt2          ciphertxt3
       |                |                     |
       +-------------+  +----------+          +----
       |             |  |          |          |
key--decryption      | decrypt     |      decrypt
       |             |  |          |          |
-------X             +--X          +----------X
       |                |                     | 
   plaintext 1       plntxt2               plntxt3