Used assertRaisesMessage() to test Django's error messages.

tests/validators/tests.py

@@ -334,7 +334,8 @@ class TestSimpleValidators(SimpleTestCase):
         self.assertEqual(repr(v), "ValidationError({'first': ['First Problem']})")
 
     def test_regex_validator_flags(self):
-        with self.assertRaises(TypeError):
+        msg = 'If the flags are set, regex must be a regular expression string.'
+        with self.assertRaisesMessage(TypeError, msg):
             RegexValidator(re.compile('a'), flags=re.IGNORECASE)
 
     def test_max_length_validator_message(self):