Credit card check using REGEX

gistfile1.rb

  def card_number_format
    case card_type.downcase
      when 'visa'
        /^4\d{3}\d{4}\d{4}\d{4}$/
      when 'mastercard'
        /^5[1-5]\d{2}\d{4}\d{4}\d{4}$/
      when 'american express'
        /^3[4,7]\d{13}$/
    end
  end