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 first six digits of the credit card number (including the initial MII digit) make up the issuer identifier, which denotes the credit card network to which the number belongs. Some of the better-known issuer identifiers include: 1  American Express -- 34xxxx and 37xxxx. 2  Visa -- 4xxxxx. 3  MasterCard -- 51xxxx-55xxxx.