(* line comment *)

rule        =   [optional] , symbol , { letters } , ( digit | symbol ) ;

optional    =   ? something unnecessary ? ; (* trailing comment *)

symbol      =   '!' | '@' | '#' | '$' | '%' | '&' | '*' ;
digit       =   "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" ;
letter      =   "A" | "B" | "C" | "D" | "E" | "F" | "G"
              | "H" | "I" | "J" | "K" | "L" | "M" | "N"
              | "O" | "P" | "Q" | "R" | "S" | "T" | "U"
              | "V" | "W" | "X" | "Y" | "Z" ;