diff -r 965109e81ffa Lib/sre_parse.py
--- a/Lib/sre_parse.py	Wed Nov 04 22:38:40 2015 +0200
+++ b/Lib/sre_parse.py	Thu Nov 05 09:00:00 2015 +0100
@@ -13,6 +13,7 @@
 # XXX: show string offset and offending character for all errors
 
 from sre_constants import *
+import weakref
 
 SPECIAL_CHARS = ".\\[{()*+?^$|"
 REPEAT_CHARS = "*+?{"
@@ -195,7 +196,9 @@
                 lo = lo + 1
                 hi = hi + 1
             elif op is GROUPREF:
-                i, j = self.pattern.subpatterns[av].getwidth()
+                # subpatterns[av] is a weakref object.
+                # get the original object
+                i, j = self.pattern.subpatterns[av]().getwidth()
                 lo = lo + i
                 hi = hi + j
             elif op is GROUPREF_EXISTS:
@@ -780,7 +783,7 @@
                 raise source.error("missing ), unterminated subpattern",
                                    source.tell() - start)
             if group is not None:
-                state.closegroup(group, p)
+                state.closegroup(group, weakref.ref(p)) # Parse weakref object
             subpatternappend((SUBPATTERN, (group, p)))
 
         elif this == "^":