JavaScript function for validating subscription information - assumes certain form input IDs for name,adress, CVV, card number.

credit_card_validation.js

function cardval() {
	// validate presence of first name, last name, billing address and CVV.
	if ($("#subscription_first_name").val().length == 0 || $("#subscription_last_name").val().length == 0 || $("#subscription_address").val().length == 0 || $("#subscription_city").val().length == 0 || $("#subscription_state").val().length == 0 || $("#subscription_zipcode").val().length == 0 || $("#subscription_cvv2").val().length == 0) {
		alert("All fields on the form must be filled out. Please complete the form and try again.");
		return false;
	}
	
	// check CVV2 is 3 or 4 digits
	
	var re = new RegExp(/^\d{3,4}$/); 
	if ($("#subscription_cvv2").val().match(re)==null) { //if match failed
		alert("CVV must be a three or four digit number.");
		return false;
	}

	// remove non-numerics from card number
	var s = $("#subscription_card").val();
	var v = "0123456789";
	var w = "";
	for (i=0; i < s.length; i++) {
		x = s.charAt(i);
		if (v.indexOf(x,0) != -1)
		w += x;
	}
	// validate number
	j = w.length / 2;
	if (j < 6.5 || j > 8 || j == 7) {
		alert("The card number you've entered is not valid. Please check the number and try again.");
		return false;
	}
	k = Math.floor(j);
	m = Math.ceil(j) - k;
	c = 0;
	for (i=0; i<k; i++) {
		a = w.charAt(i*2+m) * 2;
		c += a > 9 ? Math.floor(a/10 + a%10) : a;
	}
	for (i=0; i<k+m; i++) c += w.charAt(i*2+1-m) * 1;
	if (c%10 == 0) {
		return true;
	} else {
		alert("The card number you've entered is not valid. Please check the number and try again.");
		return false;
	}
}