Extended Backus-Naur Form
Extended BNF

    Any variation on the basic Backus-Naur Form (BNF)
   meta-syntax notation with (some of) the following additional
   constructs: square brackets "[..]" surrounding optional
   items, suffix "*" for Kleene closure (a sequence of zero or
   more of an item), suffix "+" for one or more of an item,
   curly brackets enclosing a list of alternatives, and
   super/subscripts indicating between n and m occurrences.

   All these constructs can be expressed in plain BNF using extra
   productions and have been added for readability and
   succinctness.

   (1995-04-28)