DouKaku?

parens = [('(',')'), ('[',']'), ('{','}')]

isOpener c = elem c $ map fst parens
isCloser c = elem c $ map snd parens

closer p = case lookup p parens of
             Just q -> q
             Nothing -> error "huh?"

revs [] rs zs = ([],rs++zs)
revs (x:xs) rs zs
  | isOpener x = case revs xs [] [x] of
                   (xs',rs') -> revs xs' (rs'++rs) zs
  | isCloser x = case zs of
                   [z] | x == closer z -> (xs, [z]++rs++[x])
                   _                   -> (xs, [x]++rs++zs)
  | otherwise  = revs xs (x:rs) zs

reverseString2 s = snd $ revs s [] []