Some regex have sub-categories, for example:

• Mastercard numbers -- the first 4 digits represent what company that card belongs to.
• Phone numbers. If they start with +44 or 07, it's the UK!

As such, I propose we change our code into:

    {
       "Name": "MasterCard Number",
       "Regex": "^(?:5[1-5][0-9]{2}|222[1-9]|22[3-9][0-9]|2[3-6][0-9]{2}|27[01][0-9]|2720)[0-9]{12}$",
       "plural_name": false,
       "Description": null,
       "Rarity": 0.5,
       "Tags": [
          "Credit Card",
          "Finance"
       ],
        "Children": [
            {
                "Name": "BarclayCard",
                "Regex:" "5117.*"
                Same value as the parent above
            }
        ]
    },

We want to keep the same object for each child in case we have anything special to add like a website or a cool description etc. It doesn't cost us much to do this, but it is helpful!

Also, our regex for the children can be much broader. Read it as:

1. When I pass the mastercard regex
2. When I pass the barclaycard regex (which just checks to see if the first few numbers are equal to 5117)
3. Return
   Because the children only run when the parents do, the child regex can be lighter.

As an aside, Mastercard have very helpfully provided a CSV table of all the companies and what their Mastercard subrange is:
https://www.mastercard.us/en-us/business/issuers/get-support/simplified-bin-account-range-table.html