diff -r 29ea3827cfaa Lib/sre_parse.py
--- a/Lib/sre_parse.py	Mon Dec 21 14:17:12 2015 -0600
+++ b/Lib/sre_parse.py	Fri Dec 25 21:31:16 2015 +0100
@@ -853,6 +853,8 @@
     literal = []
     lappend = literal.append
     def addgroup(index):
+        if index > pattern.groups:
+            raise IndexError("unknown index %r" % index)
         if literal:
             literals.append(''.join(literal))
             del literal[:]