function isValidCreditCard(type, ccnum) {     if (type == "Visa") {        // Visa: length 16, prefix 4, dashes optional.        var re = /^4\d{3}-?\d{4}-?\d{4}-?\d{4}$/;     } else if (type == "MC") {        // Mastercard: length 16, prefix 51-55, dashes optional.        var re = /^5[1-5]\d{2}-?\d{4}-?\d{4}-?\d{4}$/;     } else if (type == "Disc") {        // Discover: length 16, prefix 6011, dashes optional.        var re = /^6011-?\d{4}-?\d{4}-?\d{4}$/;     } else if (type == "AmEx") {        // American Express: length 15, prefix 34 or 37.        var re = /^3[4,7]\d{13}$/;     } else if (type == "Diners") {        // Diners: length 14, prefix 30, 36, or 38.        var re = /^3[0,6,8]\d{12}$/;     }     if (!re.test(ccnum)) return false;     // Remove all dashes for the checksum checks to eliminate negative numbers     ccnum = ccnum.split("-").join("");     // Checksum ("Mod 10")     // Add even digits in even length strings or odd digits in odd length strings.     var checksum = 0;     for (var i=(2-(ccnum.length % 2)); i<=ccnum.length; i+=2) {        checksum += parseInt(ccnum.charAt(i-1));     }     // Analyze odd digits in even length strings or even digits in odd length strings.     for (var i=(ccnum.length % 2) + 1; i<ccnum.length; i+=2) {        var digit = parseInt(ccnum.charAt(i-1)) * 2;        if (digit < 10) { checksum += digit; } else { checksum += (digit-9); }     }     if ((checksum % 10) == 0) return true; else return false;  }