allow regex options on username site settings

lib/validators/username_setting_validator.rb

@@ -1,13 +1,21 @@
 class UsernameSettingValidator
+
+  include RegexSettingValidation
+
   def initialize(opts = {})
     @opts = opts
+    initialize_regex_opts(opts)
   end
 
   def valid_value?(val)
-    !val.present? || User.where(username: val).exists?
+    !val.present? || (User.where(username: val).exists? && regex_match?(val))
   end
 
   def error_message
-    I18n.t('site_settings.errors.invalid_username')
+    if @regex_fail
+      I18n.t(@regex_error)
+    else
+      I18n.t('site_settings.errors.invalid_username')
+    end
   end
 end