"^a$z" は数万回テスト回したんですが見つけられませんでした…犬も歩けば棒にあたる式テストで反例を探しています。正規表現に関する知識は一切実装していません。

gistfile1.hs

-- a lightweight pxeger with QuickCheck2 in Haskell
-- usage:
-- 1) find an example which satisfies the regex "a":
--   ghci> check (mkRegex "a")
--   *** Failed! Falsifiable (after 50 tests and 5 shrinks):
--   "a"
-- check tries at most 100 tests. here, we found within 50 tests.
-- 2) search more instances with the regex "a.b"
--   ghci> checkDeep 1000 (mkRegex "a.b")
--   *** Failed! Falsifiable (after 316 tests and 7 shrinks):
--   "aab"
-- see also the original:
-- https://github.com/leque/Gauche-pxeger

import Test.QuickCheck
import Text.Regex

-- the test which may not be satisfied with
test :: Regex -> String -> Bool
test r = ¥s -> matchRegex r s == Nothing

-- poor man's tester for matching with at most 100 tests
check :: Regex -> IO ()
check = quickCheck . test

-- the default of n in QuickCheck equals 100.
-- use checkDeep at your own lisk! (Hint: type ^C to terminate)
checkDeep :: Int -> Regex -> IO ()
checkDeep n = quickCheckWith args . test
  where -- tuned arguments for QuickCheck2
        args :: Args
        args = stdArgs { maxSuccess = n
                       , maxSize = maxSuccess args }

debug :: Regex -> IO Result
debug = quickCheckResult . test