diff --git a/Lib/unittest/case.py b/Lib/unittest/case.py
--- a/Lib/unittest/case.py
+++ b/Lib/unittest/case.py
@@ -1259,31 +1259,30 @@ class TestCase(object):
         return context.handle('assertWarnsRegex', callable_obj, args, kwargs)
 
     def assertRegex(self, text, expected_regex, msg=None):
         """Fail the test unless the text matches the regular expression."""
         if isinstance(expected_regex, (str, bytes)):
             assert expected_regex, "expected_regex must not be empty."
             expected_regex = re.compile(expected_regex)
         if not expected_regex.search(text):
-            msg = msg or "Regex didn't match"
-            msg = '%s: %r not found in %r' % (msg, expected_regex.pattern, text)
+            standardMsg = 'Regex didn\'t match: %r not found in %r' % (
+                expected_regex.pattern, text)
+            msg = self._formatMessage(msg, standardMsg)
             raise self.failureException(msg)
 
     def assertNotRegex(self, text, unexpected_regex, msg=None):
         """Fail the test if the text matches the regular expression."""
         if isinstance(unexpected_regex, (str, bytes)):
             unexpected_regex = re.compile(unexpected_regex)
         match = unexpected_regex.search(text)
         if match:
-            msg = msg or "Regex matched"
-            msg = '%s: %r matches %r in %r' % (msg,
-                                               text[match.start():match.end()],
-                                               unexpected_regex.pattern,
-                                               text)
+            standardMsg = 'Regex matched: %r matches %r in %r' % (
+                text[match.start():match.end()], unexpected_regex.pattern, text)
+            msg = self._formatMessage(msg, standardMsg)
             raise self.failureException(msg)
 
 
     def _deprecate(original_func):
         def deprecated_func(*args, **kwargs):
             warnings.warn(
                 'Please use {0} instead.'.format(original_func.__name__),
                 DeprecationWarning, 2)
diff --git a/Lib/unittest/test/test_assertions.py b/Lib/unittest/test/test_assertions.py
--- a/Lib/unittest/test/test_assertions.py
+++ b/Lib/unittest/test/test_assertions.py
@@ -388,11 +388,25 @@ class TestLongMessage(unittest.TestCase)
         def raise_wrong_message():
             warnings.warn('foo')
         self.assertMessagesCM('assertWarnsRegex', (UserWarning, 'regex'),
                               raise_wrong_message,
                               ['^"regex" does not match "foo"$', '^oops$',
                                '^"regex" does not match "foo"$',
                                '^"regex" does not match "foo" : oops$'])
 
+    def testAssertRegex(self):
+        self.assertMessages('assertRegex', ('foo', 'bar'),
+                            ["^Regex didn't match:",
+                             "^oops$",
+                             "^Regex didn't match:",
+                             "^Regex didn't match: (.*) : oops$"])
+
+    def testAssertNotRegex(self):
+        self.assertMessages('assertNotRegex', ('foo', 'foo'),
+                            ["^Regex matched:",
+                             "^oops$",
+                             "^Regex matched:",
+                             "^Regex matched: (.*) : oops$"])
+
 
 if __name__ == "__main__":
     unittest.main()