diff -r c2f3b7c56dff Lib/sre_parse.py
--- a/Lib/sre_parse.py	Sat Oct 15 15:41:05 2016 +0900
+++ b/Lib/sre_parse.py	Sun Oct 16 11:04:39 2016 +0200
@@ -884,6 +884,8 @@
     literal = []
     lappend = literal.append
     def addgroup(index):
+        if index > pattern.groups:
+            raise error("invalid group reference %r" % index)
         if literal:
             literals.append(''.join(literal))
             del literal[:]
diff -r c2f3b7c56dff Lib/test/test_re.py
--- a/Lib/test/test_re.py	Sat Oct 15 15:41:05 2016 +0900
+++ b/Lib/test/test_re.py	Sun Oct 16 11:04:39 2016 +0200
@@ -186,18 +186,18 @@
                                 r'octal escape value \777 outside of '
                                 r'range 0-0o377', 0)
 
-        self.checkTemplateError('x', r'\1', 'x', 'invalid group reference')
-        self.checkTemplateError('x', r'\8', 'x', 'invalid group reference')
-        self.checkTemplateError('x', r'\9', 'x', 'invalid group reference')
-        self.checkTemplateError('x', r'\11', 'x', 'invalid group reference')
-        self.checkTemplateError('x', r'\18', 'x', 'invalid group reference')
-        self.checkTemplateError('x', r'\1a', 'x', 'invalid group reference')
-        self.checkTemplateError('x', r'\90', 'x', 'invalid group reference')
-        self.checkTemplateError('x', r'\99', 'x', 'invalid group reference')
-        self.checkTemplateError('x', r'\118', 'x', 'invalid group reference') # r'\11' + '8'
-        self.checkTemplateError('x', r'\11a', 'x', 'invalid group reference')
-        self.checkTemplateError('x', r'\181', 'x', 'invalid group reference') # r'\18' + '1'
-        self.checkTemplateError('x', r'\800', 'x', 'invalid group reference') # r'\80' + '0'
+        self.checkTemplateError('x', r'\1', 'x', 'invalid group reference 1')
+        self.checkTemplateError('x', r'\8', 'x', 'invalid group reference 8')
+        self.checkTemplateError('x', r'\9', 'x', 'invalid group reference 9')
+        self.checkTemplateError('x', r'\11', 'x', 'invalid group reference 11')
+        self.checkTemplateError('x', r'\18', 'x', 'invalid group reference 18')
+        self.checkTemplateError('x', r'\1a', 'x', 'invalid group reference 1')
+        self.checkTemplateError('x', r'\90', 'x', 'invalid group reference 90')
+        self.checkTemplateError('x', r'\99', 'x', 'invalid group reference 99')
+        self.checkTemplateError('x', r'\118', 'x', 'invalid group reference 11') # r'\11' + '8'
+        self.checkTemplateError('x', r'\11a', 'x', 'invalid group reference 11')
+        self.checkTemplateError('x', r'\181', 'x', 'invalid group reference 18') # r'\18' + '1'
+        self.checkTemplateError('x', r'\800', 'x', 'invalid group reference 80') # r'\80' + '0'
 
         # in python2.3 (etc), these loop endlessly in sre_parser.py
         self.assertEqual(re.sub('(((((((((((x)))))))))))', r'\11', 'x'), 'x')
@@ -271,9 +271,9 @@
         self.checkTemplateError('(?P<a>x)', r'\g<1a1>', 'xx',
                                 "bad character in group name '1a1'", 3)
         self.checkTemplateError('(?P<a>x)', r'\g<2>', 'xx',
-                                'invalid group reference')
+                                'invalid group reference 2')
         self.checkTemplateError('(?P<a>x)', r'\2', 'xx',
-                                'invalid group reference')
+                                'invalid group reference 2')
         with self.assertRaisesRegex(IndexError, "unknown group name 'ab'"):
             re.sub('(?P<a>x)', r'\g<ab>', 'xx')
         self.assertEqual(re.sub('(?P<a>x)|(?P<b>y)', r'\g<b>', 'xx'), '')