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

On Fri, Aug 12, 2011 at 4:34 PM, Sebastian Fischer wrote:

...

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

Thanks Sebastian ... this is what I was asking for. I'll try and digest it. Regards, Kashyap