资料来源 : Free On-Line Dictionary of Computing

Extended Backus-Naur Form
     
         Any variation on the basic {Backus-Naur Form} (BNF)
        {meta-syntax} notation with (some of) the following additional
        constructs: {square bracket}s "[..]" 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 bracket}s 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
        {production}s and have been added for readability and
        succinctness.
     
        (1995-04-28)