function checkCreditCard(cardnumber) {
	/* custom */
	var cardname = "";
	if (cardnumber.charAt(0) == "3") { cardname = "AmEx"; } else
	if (cardnumber.charAt(0) == "4") { cardname = "Visa"; } else
	if (cardnumber.charAt(0) == "5") { cardname = "MasterCard"; } else
	if (cardnumber.charAt(0) == "6") { cardname = "Discover"; } else {
		return false; 
	}
	/* custom */
	var cards = new Array();
	cards [0] = {name: "Visa", length: "13,16", prefixes: "4", checkdigit: true};
	cards [1] = {name: "MasterCard", length: "16", prefixes: "51,52,53,54,55", checkdigit: true};
	cards [2] = {name: "Discover", length: "16", prefixes: "6011,650", checkdigit: true};
	cards [3] = {name: "AmEx", length: "15", prefixes: "34,37", checkdigit: true};
	// cards [] = {name: "DinersClub", length: "14,16", prefixes: "300,301,302,303,304,305,36,38,55", checkdigit: true};
	// cards [] = {name: "CarteBlanche", length: "14", prefixes: "300,301,302,303,304,305,36,38", checkdigit: true};
	// cards [] = {name: "JCB", length: "15,16", prefixes: "3,1800,2131", checkdigit: true};
	// cards [] = {name: "enRoute", length: "15", prefixes: "2014,2149", checkdigit: true};
	// cards [] = {name: "Solo", length: "16,18,19", prefixes: "6334, 6767", checkdigit: true};
	// cards [] = {name: "Switch", length: "16,18,19", prefixes: "4903,4905,4911,4936,564182,633110,6333,6759", checkdigit: true};
	// cards [] = {name: "Maestro", length: "16", prefixes: "5020,6", checkdigit: true};
	// cards [] = {name: "VisaElectron", length: "16", prefixes: "417500,4917,4913", checkdigit: true};
	var cardType = -1;
	for (var i=0; i<cards.length; i++) {
		if (cardname.toLowerCase () == cards[i].name.toLowerCase()) {
			cardType = i;
			break;
		}
	}
	if (cardType == -1) {
		return false; 
	}
	if (cardnumber.length == 0)  {
		return false; 
	}
	cardnumber = cardnumber.replace (/\s/g, "");
	cardnumber = cardnumber.replace (".", "");
	cardnumber = cardnumber.replace ("-", "");
	if((cardnumber == "4111111111111111") || (cardnumber == "4111-1111-1111-1111")) {
		/* this is the test card number */
		return true;
	}
	var cardNo = cardnumber;
	var cardexp = /^[0-9]{13,19}$/;
	if (!cardexp.exec(cardNo))  {
		return false; 
	}
	if (cards[cardType].checkdigit) {
		var checksum = 0;
		var mychar = "";
		var j = 1;
		var calc;
		for (i = cardNo.length - 1; i >= 0; i--) {
			calc = Number(cardNo.charAt(i)) * j;
			if (calc > 9) {
				checksum = checksum + 1;
				calc = calc - 10;
			}
			checksum = checksum + calc;
			if (j == 1) {j = 2} else {j = 1};
		} 
		if (checksum % 10 != 0)  {
			return false; 
		}
	}  
	var LengthValid = false;
	var PrefixValid = false; 
	var undefined; 
	var prefix = new Array ();
	var lengths = new Array ();
	prefix = cards[cardType].prefixes.split(",");
	for (i=0; i<prefix.length; i++) {
		var exp = new RegExp ("^" + prefix[i]);
		if (exp.test (cardNo)) PrefixValid = true;
	}
	if (!PrefixValid) {
		return false; 
	}
	lengths = cards[cardType].length.split(",");
	for (j=0; j<lengths.length; j++) {
		if (cardNo.length == lengths[j]) LengthValid = true;
	}
	if (!LengthValid) {
		return false; 
	}
	return true;
}