diff --git a/Lib/email/quoprimime.py b/Lib/email/quoprimime.py
index b9dbfa9..0c18a9e 100644
--- a/Lib/email/quoprimime.py
+++ b/Lib/email/quoprimime.py
@@ -333,4 +333,4 @@
     the high level email.header class for that functionality.
     """
     s = s.replace('_', ' ')
-    return re.sub(r'=\w{2}', _unquote_match, s)
+    return re.sub(r'=[a-fA-F0-9]{2}', _unquote_match, s)
diff --git a/Lib/email/test/test_email.py b/Lib/email/test/test_email.py
index 300fc21..c3610bf 100644
--- a/Lib/email/test/test_email.py
+++ b/Lib/email/test/test_email.py
@@ -1621,6 +1621,12 @@
             dh = decode_header(s % q)
             self.assertEqual(dh, [(a, 'iso-8859-1')])
 
+    def test_rfc2047_Q_invalid_digits(self):
+        # issue 10004.
+        s = '=?iso-8659-1?Q?andr=e9=zz?='
+        self.assertEqual(decode_header(s),
+                        [(b'andr\xe9=zz', 'iso-8659-1')])
+
 
 # Test the MIMEMessage class
 class TestMIMEMessage(TestEmailBase):