benc:

I've defined a helper function to let me do regexps in functional style:

...

sed exp str = unsafePerformIO $ do regexp <- regcomp exp 0 regexec regexp str

Is this always safe? or where is it not?

(I'm using any one regexp more than once so it doesn't bother me that it compiles each time)

Hopefully regex(3) is referentially transparent, so this should be fine :) -- Don