Luhn Algorithm For Credit Card Validation

That is what I was thinking, the issue I think, is that I don't know how to read it so I don't know how to adjust it.  It is much more advanced that anything I've attempted so far...

if (event.value) {
	var result = validatePersonnummer(event.value);
	if (result==false) {
		app.alert("Invalid number!");
		event.rc = false;
	}
}

function validatePersonnummer(input) {
    // Check valid length & form
    if (!input) return false;

    if (input.indexOf('-') == -1) {
        if (input.length === 10) {
            input = input.slice(0, 6) + "-" + input.slice(6);
        } else {
            input = input.slice(0, 😎 + "-" + input.slice(8);
        }
    }
    if (!input.match(/^(\d{2})(\d{2})(\d{2})\-(\d{4})|(\d{4})(\d{2})(\d{2})\-(\d{4})$/)) return false;

    // Clean input
    input = input.replace('-', '');
    if (input.length == 12) {
        input = input.substring(2);
    }

    // Declare variables
    var d = new Date(((!!RegExp.$1) ? RegExp.$1 : RegExp.$5), (((!!RegExp.$2) ? RegExp.$2 : RegExp.$6)-1), ((!!RegExp.$3) ? RegExp.$3 : RegExp.$7)),
            sum = 0,
            numdigits = input.length,
            parity = numdigits % 2,
            i,
            digit;

    // Check valid date
    if (Object.prototype.toString.call(d) !== "[object Date]" || isNaN(d.getTime())) return false;

    // Check luhn algorithm
    for (i = 0; i < numdigits; i = i + 1) {
        digit = parseInt(input.charAt(i))
        if (i % 2 == parity) digit *= 2;
        if (digit > 9) digit -= 9;
        sum += digit;
    }
    return (sum % 10) == 0;
};

I know that I need to adjust the number of numbers it is reading...but I have no idea which numbers to adjust....