// $X is optional whitespace/comments


    $X = "";


    $X .= "[$space$tab]*";                  // Nab whitespace


    $X .= "(?: $comment [$space$tab]* )*";  // If comment found, allow more spaces








    // Item 10: atom


    $atom_char = "[^($space)<>\@,;:\".$esc$OpenBR$CloseBR$ctrl$NonASCII]";


    $atom = "";


    $atom .= "$atom_char+";    // some number of atom characters ...


    $atom .= "(?!$atom_char)"; // ... not followed by something that


                               //     could be part of an atom





    // Item 11: doublequoted string, unrolled.


    $quoted_str = "";


    $quoted_str .= "\"";                            // "


    $quoted_str .= "$qtext *";                      //   normal


    $quoted_str .= "(?: $quoted_pair $qtext * )*";  //   ( special normal* )*


    $quoted_str .= "\"";                            //        "








    // Item 7: word is an atom or quoted string


    $word = "";


    $word .= "(?:";


    $word .= "$atom";        // Atom


    $word .= "|";            // or


    $word .= "$quoted_str";  // Quoted string


    $word .= ")";





    // Item 12: domain-ref is just an atom


    $domain_ref = $atom;