No subject

  <string element> -> ...
    | \ <intraline whitespace> <line ending> <intraline whitespace>
  <intraline whitespace> -> <character tabulation>
    | <any character whose category is Zs>

Should have a Kleene star representing repetition, thus:

  <string element> -> ...
    | \ <intraline whitespace>* <line ending> <intraline whitespace>*

otherwise you are requiring exactly one space on each side
of a line break within a string.

See my previous message with this title for some
silly questions that I still can't answer.

   -- Keith