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 next four or five numbers in the series identify which of the some 13,000 financial institutions in the U.S. issued the card, such as Bank of America, JPMorgan Chase or Citibank. When consumers swipe their cards, a terminal reads the magnetic stripe that stores the complete card number.