commit 53c53ea4c5440593a527bf3d106b5f7feebeed40
author Serhiy Storchaka <storchaka@gmail.com> Tue Dec 06 19:15:29 2016 +0200
committer Serhiy Storchaka <storchaka@gmail.com> Tue Dec 06 19:15:29 2016 +0200
tree f02e42b90c60a91a4eabbc08d748704785288dd8
parent b0f75c520ee882295b0d2884469056ebf0d0568b

Issue #27030: Unknown escapes in re.sub() replacement template are allowed
again.  But they still are deprecated and will be disabled in 3.7.

Lib/sre_parse.py

diff --git a/Lib/sre_parse.py b/Lib/sre_parse.py
index ab37fd3..6aa49c3 100644
--- a/Lib/sre_parse.py
+++ b/Lib/sre_parse.py
@@ -947,7 +947,9 @@
                     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)