diff -r 961145c548e2 Lib/sre_parse.py
--- a/Lib/sre_parse.py	Sun Nov 30 11:51:48 2014 -0500
+++ b/Lib/sre_parse.py	Sun Nov 30 19:48:33 2014 +0200
@@ -69,6 +69,8 @@ class Pattern:
         self.open = []
         self.groups = 1
         self.groupdict = {}
+        self.lookbehind = 0
+
     def opengroup(self, name=None):
         gid = self.groups
         self.groups = gid + 1
@@ -299,6 +301,9 @@ def _escape(source, escape, state):
             if group < state.groups:
                 if not state.checkgroup(group):
                     raise error, "cannot refer to open group"
+                if state.lookbehind:
+                    raise error('group references in lookbehind '
+                                'assertions are not supported')
                 return GROUPREF, group
             raise ValueError
         if len(escape) == 2:
@@ -578,6 +583,9 @@ def _parse(source, state):
                         if gid is None:
                             msg = "unknown group name: {0!r}".format(name)
                             raise error(msg)
+                        if state.lookbehind:
+                            raise error('group references in lookbehind '
+                                        'assertions are not supported')
                         subpatternappend((GROUPREF, gid))
                         continue
                     else:
@@ -606,7 +614,10 @@ def _parse(source, state):
                             raise error, "syntax error"
                         dir = -1 # lookbehind
                         char = sourceget()
+                        state.lookbehind += 1
                     p = _parse_sub(source, state)
+                    if dir < 0:
+                        state.lookbehind -= 1
                     if not sourcematch(")"):
                         raise error, "unbalanced parenthesis"
                     if char == "=":
@@ -637,6 +648,9 @@ def _parse(source, state):
                             condgroup = int(condname)
                         except ValueError:
                             raise error, "bad character in group name"
+                    if state.lookbehind:
+                        raise error('group references in lookbehind '
+                                    'assertions are not supported')
                 else:
                     # flags
                     if not source.next in FLAGS:
diff -r 961145c548e2 Lib/test/test_re.py
--- a/Lib/test/test_re.py	Sun Nov 30 11:51:48 2014 -0500
+++ b/Lib/test/test_re.py	Sun Nov 30 19:48:33 2014 +0200
@@ -449,7 +449,7 @@ class ReTests(unittest.TestCase):
         self.assertEqual(re.match("a.*b", "a\n\nb", re.DOTALL).group(0),
                          "a\n\nb")
 
-    def test_non_consuming(self):
+    def test_lookahead(self):
         self.assertEqual(re.match("(a(?=\s[^a]))", "a b").group(1), "a")
         self.assertEqual(re.match("(a(?=\s[^a]*))", "a b").group(1), "a")
         self.assertEqual(re.match("(a(?=\s[abc]))", "a b").group(1), "a")
@@ -463,6 +463,44 @@ class ReTests(unittest.TestCase):
         self.assertEqual(re.match(r"(a)(?!\s\1)", "a b").group(1), "a")
         self.assertEqual(re.match(r"(a)(?!\s(abc|a))", "a b").group(1), "a")
 
+        # Group reference.
+        self.assertTrue(re.match(r'(a)b(?=\1)a', 'aba'))
+        self.assertIsNone(re.match(r'(a)b(?=\1)c', 'abac'))
+        # Named group reference.
+        self.assertTrue(re.match(r'(?P<g>a)b(?=(?P=g))a', 'aba'))
+        self.assertIsNone(re.match(r'(?P<g>a)b(?=(?P=g))c', 'abac'))
+        # Conditional group reference.
+        self.assertTrue(re.match('(?:(a)|(x))b(?=(?(2)x|c))c', 'abc'))
+        self.assertIsNone(re.match('(?:(a)|(x))b(?=(?(2)c|x))c', 'abc'))
+        self.assertTrue(re.match('(?:(a)|(x))b(?=(?(2)x|c))c', 'abc'))
+        self.assertIsNone(re.match('(?:(a)|(x))b(?=(?(1)b|x))c', 'abc'))
+        self.assertTrue(re.match('(?:(a)|(x))b(?=(?(1)c|x))c', 'abc'))
+        # Group used before defined.
+        self.assertTrue(re.match('(a)b(?=(?(2)x|c))(c)', 'abc'))
+        self.assertIsNone(re.match('(a)b(?=(?(2)b|x))(c)', 'abc'))
+        self.assertTrue(re.match('(a)b(?=(?(1)c|x))(c)', 'abc'))
+
+    def test_lookbehind(self):
+        self.assertTrue(re.match('ab(?<=b)c', 'abc'))
+        self.assertIsNone(re.match('ab(?<=c)c', 'abc'))
+        self.assertIsNone(re.match('ab(?<!b)c', 'abc'))
+        self.assertTrue(re.match('ab(?<!c)c', 'abc'))
+        # Group reference.
+        self.assertRaises(re.error, re.compile, r'(a)a(?<=\1)c')
+        self.assertRaises(re.error, re.compile, r'(a)b(?<!\1)c')
+        # Named group reference.
+        self.assertRaises(re.error, re.compile, r'(?P<g>a)a(?<=(?P=g))c')
+        self.assertRaises(re.error, re.compile, r'(?P<g>a)b(?<!(?P=g))c')
+        # Conditional group reference.
+        self.assertRaises(re.error, re.compile, '(a)b(?<=(?(1)b|x))c')
+        self.assertRaises(re.error, re.compile, '(a)b(?<!(?(1)x|b))c')
+        # Group used before defined.
+        self.assertRaises(re.error, re.compile, '(a)b(?<=(?(2)b|x))(c)')
+        # Group defined in the same lookbehind pattern
+        self.assertRaises(re.error, re.compile, r'(a)b(?<=(.)\2)(c)')
+        self.assertRaises(re.error, re.compile, '(a)b(?<=(?P<a>.)(?P=a))(c)')
+        self.assertRaises(re.error, re.compile, '(a)b(?<=(a)(?(2)b|x))(c)')
+
     def test_ignore_case(self):
         self.assertEqual(re.match("abc", "ABC", re.I).group(0), "ABC")
         self.assertEqual(re.match("abc", u"ABC", re.I).group(0), "ABC")