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;}