gone_too_far.rb

  def subdomain
    @subdomain ||= begin
      parts = begin
        if @request.host.ends_with?('.au')
          @request.subdomains(2)
        else
          @request.subdomains(1)
        end
      end
      parts.first == 'www' ? parts[1] : parts[0]
    end
  end
  

(yes, it has unit tests)

use a return-if construct, it's not as bad as it sounds

def subdomain
  return @subdomain if @subdomain
  parts = @request.subdomains(@request.host[-3, 3] == '.au' ? 2 : 1)
  @subdomain = parts.first == 'www' ? parts[1] : parts[0]
end

or use 'public_suffix_service' gem