Merge commit from fork

This should not change the set of words matched by `USERNAME_RE` but does
change the one matched by `MENTION_RE`. Indeed, the previous regexp allowed
a domain part to start with `.` or `-`, which the new regexp does not allow.

Author: ClearlyClaire

app/models/account.rb

@@ -68,8 +68,8 @@ class Account < ApplicationRecord
   DEFAULT_FIELDS_SIZE = 4
   INSTANCE_ACTOR_ID = -99
 
-  USERNAME_RE   = /[a-z0-9_]+([a-z0-9_.-]+[a-z0-9_]+)?/i
-  MENTION_RE    = %r{(?<![=/[:word:]])@((#{USERNAME_RE})(?:@[[:word:].-]+[[:word:]]+)?)}
+  USERNAME_RE   = /[a-z0-9_]+([.-]+[a-z0-9_]+)*/i
+  MENTION_RE    = %r{(?<![=/[:word:]])@((#{USERNAME_RE})(?:@[[:word:]]+([.-]+[[:word:]]+)*)?)}
   URL_PREFIX_RE = %r{\Ahttp(s?)://[^/]+}
   USERNAME_ONLY_RE = /\A#{USERNAME_RE}\z/i
   USERNAME_LENGTH_LIMIT = 30