Dictionary.net

Any variation on the basic Backus-Naur Form (BNF)meta-syntax notation with (some of) the following additionalconstructs: square brackets "[..]" surrounding optionalitems, suffix "*" for Kleene closure (a sequence of zero ormore of an item), suffix "+" for one or more of an item,curly brackets enclosing a list of alternatives, andsuper/subscripts indicating between n and m occurrences.All these constructs can be expressed in plain BNF using extraproductions and have been added for readability andsuccinctness.