diff -r ab66423c3581 Doc/library/re.rst
--- a/Doc/library/re.rst	Mon Dec 05 22:31:12 2016 -0800
+++ b/Doc/library/re.rst	Tue Dec 06 12:16:42 2016 +0200
@@ -758,7 +758,12 @@ form.
       Unmatched groups are replaced with an empty string.
 
    .. versionchanged:: 3.6
-      Unknown escapes consisting of ``'\'`` and an ASCII letter now are errors.
+      Unknown escapes in *pattern* consisting of ``'\'`` and an ASCII letter
+      now are errors.
+
+   .. deprecated-removed:: 3.5 3.7
+      Unknown escapes in *repl* consist of ``'\'`` and ASCII letter now raise
+      a deprecation warning and will be forbidden in Python 3.7.
 
 
 .. function:: subn(pattern, repl, string, count=0, flags=0)
diff -r ab66423c3581 Lib/sre_parse.py
--- a/Lib/sre_parse.py	Mon Dec 05 22:31:12 2016 -0800
+++ b/Lib/sre_parse.py	Tue Dec 06 12:16:42 2016 +0200
@@ -947,7 +947,9 @@ def parse_template(source, pattern):
                     this = chr(ESCAPES[this][1])
                 except KeyError:
                     if c in ASCIILETTERS:
-                        raise s.error('bad escape %s' % this, len(this))
+                        import warnings
+                        warnings.warn('bad escape %s' % this,
+                                      DeprecationWarning, stacklevel=4)
                 lappend(this)
         else:
             lappend(this)
diff -r ab66423c3581 Lib/test/test_re.py
--- a/Lib/test/test_re.py	Mon Dec 05 22:31:12 2016 -0800
+++ b/Lib/test/test_re.py	Tue Dec 06 12:16:42 2016 +0200
@@ -126,7 +126,7 @@ class ReTests(unittest.TestCase):
                          (chr(9)+chr(10)+chr(11)+chr(13)+chr(12)+chr(7)+chr(8)))
         for c in 'cdehijklmopqsuwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ':
             with self.subTest(c):
-                with self.assertRaises(re.error):
+                with self.assertWarns(DeprecationWarning):
                     self.assertEqual(re.sub('a', '\\' + c, 'a'), '\\' + c)
 
         self.assertEqual(re.sub(r'^\s*', 'X', 'test'), 'Xtest')
diff -r ab66423c3581 Misc/NEWS
--- a/Misc/NEWS	Mon Dec 05 22:31:12 2016 -0800
+++ b/Misc/NEWS	Tue Dec 06 12:16:42 2016 +0200
@@ -26,6 +26,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #28450: Unknown escapes in re.sub() replacement template template are
+  allowed again.  But they still are deprecated and will be disabled in 3.7.
+
 - Issue #27172: To assist with upgrades from 2.7, the previously documented
   deprecation of ``inspect.getfullargspec()`` has been reversed. This decision
   may be revisited again after the Python 2.7 branch is no longer officially