JS to run a validation in a form

Assuming this function is correct (I did not verify it and when I used a sample "correct" number from the Wiki page for Luhn algorithm it returned false, so use it at your own risk...), you can use this code as the custom validation script of your field:

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;
};