# File lib/jpmobile/mobile/docomo.rb, line 38
    def serial_number
      case @env["HTTP_USER_AGENT"]
      when /ser([0-9a-zA-Z]{11})$/ # mova
        return $1
      when /ser([0-9a-zA-Z]{15});/ # FOMA
        return $1
      else
        return nil
      end
    end