[Haskell-cafe] Building ? using kleene closure {not haskell specific}

Hello gentle Haskell folks, I happened to read "Beautiful code"'s chapter 1 today and found Brian Kerninghan's regex implementation. In it he only shows the * meta character. I can easily understand how + can be built but am having trouble with building ? (zero or one). I'd really appreciate it if some one could help me understand it. Regards, Kashyap