Every last digit of any credit card is a check sum digit that is determined by all digits in front of it. Ruby can use this number in order to validate if Credit Card number was entered correctly into the Credit Card field.

There are several known Ruby Gems that can help you with this task. One such gem is called CreditCard Library. This gem is installed via creditcard gem. You can use this  gem in the following ways

gem ‘creditcard’
require ‘creditcard’

‘4111 1111 1111 1111’.creditcard? //returns true if valid or false if not.

You can use creditcard gem in order to figure out type of the card being used as well.

4111111111111111.creditcard_type //returns type of the card such as MasterCard etc…

CreditCard library relies on the checksum digit of the credit card which is always last digit on the card.