[commands] Update MessageConverter link regex

Remove redundant parts of the MessageConverter regexes and support www

discord/ext/commands/converter.py

@@ -254,12 +254,11 @@ class MessageConverter(Converter):
     .. versionchanged:: 1.5
          Raise :exc:`.ChannelNotFound`, `MessageNotFound` or `ChannelNotReadable` instead of generic :exc:`.BadArgument`
     """
-
     async def convert(self, ctx, argument):
-        id_regex = re.compile(r'^(?:(?P<channel_id>[0-9]{15,21})-)?(?P<message_id>[0-9]{15,21})$')
+        id_regex = re.compile(r'(?:(?P<channel_id>[0-9]{15,21})-)?(?P<message_id>[0-9]{15,21})$')
         link_regex = re.compile(
-            r'^https?://(?:(ptb|canary)\.)?discord(?:app)?\.com/channels/'
-            r'(?:([0-9]{15,21})|(@me))'
+            r'https?://(?:(ptb|canary|www)\.)?discord(?:app)?\.com/channels/'
+            r'(?:[0-9]{15,21}|@me)'
             r'/(?P<channel_id>[0-9]{15,21})/(?P<message_id>[0-9]{15,21})/?$'
         )
         match = id_regex.match(argument) or link_regex.match(argument)