diff -r 5873cfb42ebe Lib/http/cookies.py
--- a/Lib/http/cookies.py	Tue Feb 02 17:04:56 2016 -0600
+++ b/Lib/http/cookies.py	Sat Feb 06 23:35:24 2016 +0530
@@ -174,7 +174,7 @@
     ord('\\'): '\\\\',
 })
 
-_is_legal_key = re.compile('[%s]+' % _LegalChars).fullmatch
+_is_legal_key = re.compile('[%s]+' % re.escape(_LegalChars)).fullmatch
 
 def _quote(str):
     r"""Quote a string for use in a cookie header.
diff -r 5873cfb42ebe Lib/test/test_http_cookies.py
--- a/Lib/test/test_http_cookies.py	Tue Feb 02 17:04:56 2016 -0600
+++ b/Lib/test/test_http_cookies.py	Sat Feb 06 23:35:24 2016 +0530
@@ -210,6 +210,10 @@
                 C1 = pickle.loads(pickle.dumps(C, protocol=proto))
                 self.assertEqual(C1.output(), expected_output)
 
+    def test_legal_chars(self):
+        self.assertTrue(bool(cookies._is_legal_key('a')))
+        self.assertTrue(not bool(cookies._is_legal_key(',')))
+
 
 class MorselTests(unittest.TestCase):
     """Tests for the Morsel object."""