Merge pull request pypa#1491 from Darsstar/patch-1

Replace regex which allows dubble hyphens by one that does not.

source/specifications/core-metadata.rst

@@ -631,7 +631,7 @@ of lowercase ASCII letters, ASCII numbers, and hyphen. It must start and end
 with a letter or number. Hyphens cannot be followed by another hyphen. Names are
 limited to those which match the following regex (which guarantees unambiguity)::
 
-    ^([a-z0-9]|[a-z0-9]([a-z0-9-](?!--))*[a-z0-9])$
+    ^[a-z0-9]+(-[a-z0-9]+)*$
 
 
 The specified name may be used to make a dependency conditional on whether the