function Validate() { };

Validate.prototype.isValidCreditCardNumber = function(cardNumber, cardType)	{
  var isValid = false;
  var ccCheckRegExp = /[^\d ]/;
  isValid = !ccCheckRegExp.test(cardNumber);

  if (isValid)	{
    var cardNumbersOnly = cardNumber.replace(/ /g,"");
    var cardNumberLength = cardNumbersOnly.length;
    var lengthIsValid = false;
    var prefixIsValid = false;
    var prefixRegExp;

    switch(cardType)	{
      case "Master Card":
        lengthIsValid = (cardNumberLength == 16);
        prefixRegExp = /^5[1-5]/;
        break;

      case "Visa":
        lengthIsValid = (cardNumberLength == 16 || cardNumberLength == 13);
        prefixRegExp = /^4/;
        break;

      case "American Express":
        lengthIsValid = (cardNumberLength == 15);
        prefixRegExp = /^3(4|7)/;
        break;

      default:
      	lengthIsValid = (cardNumberLength == 16);
        prefixRegExp = /^$/;
    }

    prefixIsValid = prefixRegExp.test(cardNumbersOnly);
    isValid = prefixIsValid && lengthIsValid;
  }

  if(isValid) {
    var numberProduct;
    var numberProductDigitIndex;
    var checkSumTotal = 0;

    for (digitCounter = cardNumberLength - 1; digitCounter >= 0; digitCounter--) {
      checkSumTotal += parseInt (cardNumbersOnly.charAt(digitCounter));
      digitCounter--;
      numberProduct = String((cardNumbersOnly.charAt(digitCounter) * 2));
      for (var productDigitCounter = 0; productDigitCounter < numberProduct.length; productDigitCounter++) {
        checkSumTotal += 
          parseInt(numberProduct.charAt(productDigitCounter));
      }
    }

    isValid = (checkSumTotal % 10 == 0);
  }

  return isValid;
}

function changeColor(elementId) {
    var elem = document.getElementById(elementId);
    elem.style.color = (elem.style.color == "black") ? "white" : "black";
    setTimeout("changeColor('" + elementId + "')", 1000);
}