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

I can easily understand how + can be built but am having trouble with building ? (zero or one).

If there is a regular expression e for the empty word, one can define ? as a? = e | a If there is a regular expression o that never matches one can define e as e = o* If there are character classes one can define o as o = [] Apart from that, I have no idea.. Sebastian