#27364: fix "incorrect" uses of escape character in the stdlib.

And most of the tools.

Patch by Emanual Barry, reviewed by me, Serhiy Storchaka, and
Martin Panter.
diff --git a/Lib/xml/etree/ElementTree.py b/Lib/xml/etree/ElementTree.py
index 67b7f2b..4b0c661 100644
--- a/Lib/xml/etree/ElementTree.py
+++ b/Lib/xml/etree/ElementTree.py
@@ -1030,7 +1030,7 @@
     ValueError is raised if prefix is reserved or is invalid.
 
     """
-    if re.match("ns\d+$", prefix):
+    if re.match(r"ns\d+$", prefix):
         raise ValueError("Prefix format reserved for internal use")
     for k, v in list(_namespace_map.items()):
         if k == uri or v == prefix: