[Mod] Use a better converter for Hackban & Unban (#3524)

closes #3523

redbot/cogs/mod/converters.py

@@ -1,16 +1,20 @@
+import re
 from redbot.core.commands import Converter, BadArgument
 from redbot.core.i18n import Translator
 
 _ = Translator("Mod", __file__)
 
+_id_regex = re.compile(r"([0-9]{15,21})$")
+_mention_regex = re.compile(r"<@!?([0-9]{15,21})>$")
+
 
 class RawUserIds(Converter):
     async def convert(self, ctx, argument):
-        # This is for the hackban command, where we receive IDs that
+        # This is for the hackban and unban commands, where we receive IDs that
         # are most likely not in the guild.
-        # As long as it's numeric and long enough, it makes a good candidate
-        # to attempt a ban on
-        if argument.isnumeric() and len(argument) >= 17:
-            return int(argument)
+        # Mentions are supported, but most likely won't ever be in cache.
+
+        if match := _id_regex.match(argument) or _mention_regex.match(argument):
+            return int(match.group(1))
 
         raise BadArgument(_("{} doesn't look like a valid user ID.").format(argument))