diff -r d6d15fcf5eb6 Lib/sre_parse.py
--- a/Lib/sre_parse.py	Mon Nov 21 18:04:30 2011 +0100
+++ b/Lib/sre_parse.py	Thu Nov 24 10:18:51 2011 -0500
@@ -222,6 +222,8 @@
 
 def isname(name):
     # check that group name is a valid string
+    if name == "":
+        return False
     if not isident(name[0]):
         return False
     for char in name[1:]:
diff -r d6d15fcf5eb6 Lib/test/test_re.py
--- a/Lib/test/test_re.py	Mon Nov 21 18:04:30 2011 +0100
+++ b/Lib/test/test_re.py	Thu Nov 24 10:18:51 2011 -0500
@@ -154,6 +154,8 @@
         self.assertRaises(re.error, re.sub, '(?P<a>x)', '\g<', 'xx')
         self.assertRaises(re.error, re.sub, '(?P<a>x)', '\g', 'xx')
         self.assertRaises(re.error, re.sub, '(?P<a>x)', '\g<a a>', 'xx')
+        self.assertRaises(re.error, re.sub, '(?P=)'   , '\g', 'xx')
+        self.assertRaises(re.error, re.sub, '(?P<>)'  , '\g', 'xx')
         self.assertRaises(re.error, re.sub, '(?P<a>x)', '\g<1a1>', 'xx')
         self.assertRaises(IndexError, re.sub, '(?P<a>x)', '\g<ab>', 'xx')
         self.assertRaises(re.error, re.sub, '(?P<a>x)|(?P<b>y)', '\g<b>', 'xx')