正規表現でいうと /(A|B)*B/ みたいなケース

abb.hs

module Main where

import Control.Applicative hiding ((<|>))
import Text.ParserCombinators.Parsec

data Result = Init String Result
            | Last String
            deriving (Show)

a = string "a"
b = string "b"
f = try (Init <$> (a <|> b) <*> f)
  <|> (Last <$> b)

main = print (parse f "test" "ababb")