Regex for Discover credit card

Question

I have read through this question, but for Discover card, the starting digits are 6011, 622126-622925, 644-649, 65 instead of just 6011, 65. (Sourc

Answer 1

Here's your regex (demo):

^6(?:011\d{12}|5\d{14}|4[4-9]\d{13}|22(?:1(?:2[6-9]|[3-9]\d)|[2-8]\d{2}|9(?:[01]\d|2[0-5]))\d{10})$

Needless to say, I won't exactly call this pretty or easy to maintain. I would recommend parsing the number as an integer and using your programming language to do the checks.

You should also use Luhn algorithm to check if the credit card number is valid, and while you could theoretically do this with regex, it would many times worse than this.

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Allow me to show you how I arrived at this monstrosity, step by step. First, here is how you match each of those ranges:

6011        # matches 6011
65          # matches 65
64[4-9]     # matches 644-649
622(1(2[6-9]|[3-9]\d)|[2-8]\d{2}|9([01]\d|2[0-5]))  
            # matches 622126-622925

Now, you want to match the rest of the digits:

6011\d{12}        # matches 6011 + 12 digits
65\d{14}          # matches 65 + 14 digits
64[4-9]\d{13}     # matches 644-649 + 13 digits
622(1(2[6-9]|[3-9]\d)|[2-8]\d{2}|9([01]\d|2[0-5]))\d{10}
                  # matches 622126-622925 + 10 digits

Now you can combine all four, and add start and end of line anchors:

^(                  # match start of string and open group
 6011\d{12}|        # matches 6011 + 12 digits
 65\d{14}|          # matches 65 + 14 digits
 64[4-9]\d{13}|     # matches 644-649 + 13 digits
 622(1(2[6-9]|[3-9]\d)|[2-8]\d{2}|9([01]\d|2[0-5]))\d{10}
                    # matches 622126-622925 + 10 digits
)$                  # close group and match end of string

The final product above is a slightly compacted version of the previous regex, and I also made groups non-capturing (that's what those ?: are for).