these are some example formats of the most common credit cards:

American Express3400 0000 0000 009
Carte Blanche3000 0000 0000 04
Discover6011 0000 0000 0004
Diner's Club3000 0000 0000 04
enRoute2014 0000 0000 009
JCB2131 0000 0000 0008
MasterCard5500 0000 0000 0004
Solo6334 0000 0000 0004
Switch4903 0100 0000 0009
Visa4111 1111 1111 1111

this is very hepful when you hare creating a script and you want to validate that the user entered a correct format