Credit Card Number Verification: users may be typing credit card numbers in, so we need to verify it's a good number just based on the digits and the math before moving ahead. People type things in wrong. Check to make sure the card number begins with 3, 4, 5, 6. Check to make sure it's 15 or 16 digits long. Do a Luhn check (a mathematical formula to verify the card number). Check the expiration date. Make sure it's in the future. Make sure if it's this year that the month is greater than or equal to today's month.