String operation in ruby for credit card number

Question

Working on a rails project where there's an order confirmation string with a credit card number with all but the last four digits starred out. What's the proper way to do a string substitution?

What's the operation to get this

credit_card_number = "1111111111111111"

to this?

credit_card_number = "************1111"

Thanks,

Kenji

Answer 1

There isn't a 'proper' way, but I quite like regex so I might do this:

x.gsub!(/.(?=....)/, '*')

A more conventional way would be this:

x = '*' * (x.size - 4) + x[-4, 4]

Answer 2

If you're using ActiveMerchant, ActiveMerchant::Billing::CreditCard has an instance method called display_number which does this e.g. XXXX-XXXX-XXXX-4338

If you're not, copy activemerchant:

def last_digits(number)    
  number.to_s.length <= 4 ? number : number.to_s.slice(-4..-1) 
end

def mask(number)
 "XXXX-XXXX-XXXX-#{last_digits(number)}"
end

credit_card_number = "1111111111111111"

display_number = mask credit_card_number