There are two factors to consider when deciding how to handle the problem of a work sample: sample size and population characteristics. Sample size is determined by a formula based on the power of the sample or its ability to represent the population with a predictable margin of error.
The resulting complexity makes card type detection code difficult to write, read, and maintain. As an example of this complexity, consider this JavaScript regular expression used to detect Discover card numbers starting with 6011, 622126-622925, 644-649, and 65:
The number of a bank card contains a certain number of digits, which usually varies between 13, 16 and 19. Indeed, each of those numbers is not randomly conferred, but bears a certain meaning. For instance, Visa credit & debit cards have card numbers with either 13 or 16 digits.