Merge documentation for issue #27030 from 3.6.

Doc/library/re.rst

@@ -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.
+
+   .. versionchanged:: 3.7
+      Unknown escapes in *repl* consisting of ``'\'`` and an ASCII letter
+      now are errors.
 
 
 .. function:: subn(pattern, repl, string, count=0, flags=0)

Doc/whatsnew/3.6.rst

@@ -2021,8 +2021,9 @@ API and Feature Removals
 ------------------------
 
 * Unknown escapes consisting of ``'\'`` and an ASCII letter in
-  regular expressions will now cause an error.  The :const:`re.LOCALE`
-  flag can now only be used with binary patterns.
+  regular expressions will now cause an error.  In replacement templates for
+  :func:`re.sub` they are still allowed, but deprecated.
+  The :const:`re.LOCALE` flag can now only be used with binary patterns.
 
 * ``inspect.getmoduleinfo()`` was removed (was deprecated since CPython 3.3).
   :func:`inspect.getmodulename` should be used for obtaining the module

Doc/whatsnew/3.7.rst

@@ -116,6 +116,11 @@ Deprecated
 Removed
 =======
 
+API and Feature Removals
+------------------------
+
+* Unknown escapes consisting of ``'\'`` and an ASCII letter in replacement
+  templates for :func:`re.sub` will now cause an error.
 
 
 Porting to Python 3.7

Misc/NEWS

@@ -165,6 +165,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #27030: Unknown escapes consisting of ``'\'`` and an ASCII letter in
+  re.sub() replacement templates regular expressions now are errors.
+
 - Issue #28835: Fix a regression introduced in warnings.catch_warnings():
   call warnings.showwarning() if it was overriden inside the context manager.