[Valid_CreditCard] checks the format of an credit card number. If the parameter appears to be a valid credit card number then True is returned. 

Credit card numbers entered into the tag should not have any spaces or punctuation.

This tag uses the ROT-13 algorithm to check the basic validity of a credit card number. It does not communicate with the bank network to check whether a given number is active or has credit available. Use your banking software to authorize credit card transactions.

This tag should return True for all valid credit card numbers from Visa/Mastercard/AmEx/Discover and major banks. It is intended as a first check for data entry and should not necessarily be used to deny purchases.