EBNF (Extended BNF)

• We use (...) for grouping.
• We use $\epsilon$ for the empty word.
• We use [ E ] to stand for ( $\epsilon$ | E )
• We use { E } to stand for ( $\epsilon$ | E | EE | EEE | ...)

We can now write
      Number = Digit { Digit }.
      Digit = "0" | "1" | "2" | "3" | "4" | "5" |  "6" | "7" | "8" | "9".
or
      Sentence = ("Peter" | "Chelsea") ("runs" | "stops").