Visa: ^4[0-9]{12}(?:[0-9]{3})?$ - Visa card will have 16 digits. Always starts
with 4.
MasterCard: ^5[1-5][0-9]{14}$ - Master Card will have 16 digits. Always starts
with 51 through 55.
American Express: ^3[47][0-9]{13}$ - American Express will have 15 digits. Always starts with 34 or 37.
