All of the format definitions adhere to these typical patterns:
Generate a random number or random digits.
Perform post-processing on the above-generated value to ensure that the final result is a valid, realistic value.
For example, a valid credit card number must pass Luhn's check. That is, the last digit of any credit card number is a checksum digit, which is always computed. Also, the first few digits indicate the card type (MasterCard, Amex, Visa, and so forth). Consequently, the format definition of a credit card would be as follows:
Generate random and unique 10-digit numbers.
Using a post-processing function, transform the values above to a proper credit card number by adding well known card type prefixes and computing the last digit.
This format is capable of generating 10 billion unique credit card numbers.