diff -r a0372781eafb Doc/library/re.rst
--- a/Doc/library/re.rst	Thu Sep 18 06:05:37 2014 +0300
+++ b/Doc/library/re.rst	Thu Sep 18 13:54:11 2014 +0300
@@ -701,6 +701,9 @@ form.
    .. versionchanged:: 3.1
       Added the optional flags argument.
 
+   .. versionchanged:: 3.5
+      Unmatched groups are replaced by empty string.
+
 
 .. function:: subn(pattern, repl, string, count=0, flags=0)
 
@@ -710,6 +713,9 @@ form.
    .. versionchanged:: 3.1
       Added the optional flags argument.
 
+   .. versionchanged:: 3.5
+      Unmatched groups are replaced by empty string.
+
 
 .. function:: escape(string)
 
@@ -885,6 +891,8 @@ Match objects support the following meth
    (``\g<1>``, ``\g<name>``) are replaced by the contents of the
    corresponding group.
 
+   .. versionchanged:: 3.5
+      Unmatched groups are replaced by empty string.
 
 .. method:: match.group([group1, ...])
 
diff -r a0372781eafb Lib/sre_parse.py
--- a/Lib/sre_parse.py	Thu Sep 18 06:05:37 2014 +0300
+++ b/Lib/sre_parse.py	Thu Sep 18 13:54:11 2014 +0300
@@ -846,14 +846,12 @@ def parse_template(source, pattern):
 
 def expand_template(template, match):
     g = match.group
-    sep = match.string[:0]
+    empty = match.string[:0]
     groups, literals = template
     literals = literals[:]
     try:
         for index, group in groups:
-            literals[index] = s = g(group)
-            if s is None:
-                raise error("unmatched group")
+            literals[index] = g(group) or empty
     except IndexError:
         raise error("invalid group reference")
-    return sep.join(literals)
+    return empty.join(literals)
diff -r a0372781eafb Lib/test/test_re.py
--- a/Lib/test/test_re.py	Thu Sep 18 06:05:37 2014 +0300
+++ b/Lib/test/test_re.py	Thu Sep 18 13:54:11 2014 +0300
@@ -220,9 +220,11 @@ class ReTests(unittest.TestCase):
         self.assertRaises(re.error, re.sub, '(?P<a>x)', '\g<a a>', 'xx')
         self.assertRaises(re.error, re.sub, '(?P<a>x)', '\g<>', 'xx')
         self.assertRaises(re.error, re.sub, '(?P<a>x)', '\g<1a1>', 'xx')
+        self.assertRaises(re.error, re.sub, '(?P<a>x)', r'\g<2>', 'xx')
+        self.assertRaises(re.error, re.sub, '(?P<a>x)', r'\2', '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')
-        self.assertRaises(re.error, re.sub, '(?P<a>x)|(?P<b>y)', '\\2', 'xx')
+        self.assertEqual(re.sub('(?P<a>x)|(?P<b>y)', r'\g<b>', 'xx'), '')
+        self.assertEqual(re.sub('(?P<a>x)|(?P<b>y)', r'\2', 'xx'), '')
         self.assertRaises(re.error, re.sub, '(?P<a>x)', '\g<-1>', 'xx')
         # New valid/invalid identifiers in Python 3
         self.assertEqual(re.sub('(?P<µ>x)', r'\g<µ>', 'xx'), 'xx')
@@ -427,6 +429,10 @@ class ReTests(unittest.TestCase):
                                   "first second")
                                   .expand(r"\2 \1 \g<second> \g<first>"),
                          "second first second first")
+        self.assertEqual(re.match("(?P<first>first)|(?P<second>second)",
+                                  "first")
+                                  .expand(r"\2 \g<second>"),
+                         " ")
 
     def test_repeat_minmax(self):
         self.assertIsNone(re.match("^(\w){1}$", "abc"))