Issue #25953: re.sub() now raises an error for invalid numerical group

reference in replacement template even if the pattern is not found in
the string.  Error message for invalid group reference now includes the
group index and the position of the reference.
Based on patch by SilentGhost.

diff --cc Misc/NEWS
index cec6306f68eb87b47ec4ef34fbbbec7d07176e0c,0021b19ac418ca11acb982fb1f9f468890ec69ee..fb14c004007296a225ae5aa93acf5074268b78e8
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -16,6 -16,68 +16,12 @@@ Core and Builtin
  - Issue #23782: Fixed possible memory leak in _PyTraceback_Add() and exception
    loss in PyTraceBack_Here().
  
 -- Issue #28471: Fix "Python memory allocator called without holding the GIL"
 -  crash in socket.setblocking.
 -
 -
 -Library
 --------
 -
+ - Issue #25953: re.sub() now raises an error for invalid numerical group
+   reference in replacement template even if the pattern is not found in
+   the string.  Error message for invalid group reference now includes the
+   group index and the position of the reference.
+   Based on patch by SilentGhost.
+ 
 -- Issue #18219: Optimize csv.DictWriter for large number of columns.
 -  Patch by Mariatta Wijaya.
 -
 -- Issue #28448: Fix C implemented asyncio.Future didn't work on Windows.
 -
 -- Issue #28480: Fix error building socket module when multithreading is
 -  disabled.
 -
 -- Issue #24452: Make webbrowser support Chrome on Mac OS X.
 -
 -- Issue #20766: Fix references leaked by pdb in the handling of SIGINT
 -  handlers.
 -
 -- Issue #28492: Fix how StopIteration exception is raised in _asyncio.Future.
 -
 -- Issue #28500: Fix asyncio to handle async gens GC from another thread.
 -
 -- Issue #26923: Fix asyncio.Gather to refuse being cancelled once all
 -  children are done.
 -  Patch by Johannes Ebke.
 -
 -- Issue #26796: Don't configure the number of workers for default
 -  threadpool executor.
 -  Initial patch by Hans Lawrenz.
 -
 -Build
 ------
 -
 -- Issue #28208: Update Windows build to use SQLite 3.14.2.0.
 -
 -- Issue #28248: Update Windows build to use OpenSSL 1.0.2j.
 -
 -Tests
 ------
 -
 -- Issue #26944: Fix test_posix for Android where 'id -G' is entirely wrong or
 -  missing the effective gid.
 -
 -- Issue #28409: regrtest: fix the parser of command line arguments.
 -
 -
 -What's New in Python 3.6.0 beta 2
 -=================================
 -
 -*Release date: 2016-10-10*
 -
 -Core and Builtins
 ------------------
 -
  - Issue #28183: Optimize and cleanup dict iteration.
  
  - Issue #26081: Added C implementation of asyncio.Future.