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cc_validate.js
Set of JavaScript functions used to validate credit card numbers
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isValidCreditCardNumber()
This function checks the validity of a credit card number.  It
does this by getting the card number and type and running a series of checks
Arguments:
cardNum - String = the credit card number
cardType - String = the credit card type (mc,vs,ax)
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function isValidCreditCardNumber(cardNum, cardType) {
  var isValid = false;
  //check that cardNum contains numbers only
  var re1 = /^[\d]+$/;
  isValid = re1.test(cardNum);
  //check that cardNum has the right number of digits 
  //and right prefix for the given card type
  if(isValid) {
    var lengthIsValid = false;
    var prefixIsValid = false;
    var prefixRegExp;
    switch(cardType) {
      case "mc":
        lengthIsValid = (cardNum.length == 16);
        prefixRegExp = /^5[1-5]/;
        break;
      case "vs":
        lengthIsValid = ((cardNum.length == 16) || (cardNum.length == 13));
        prefixRegExp = /^4/;
        break;
      case "ax":
        lengthIsValid = (cardNum.length == 15);
        prefixRegExp = /^3(4|7)/;
        break;
      case "ds":
        lengthIsValid = (cardNum.length == 16);
        prefixRegExp = /^6011/;
        break;
      default:
        prefixRegExp = /^$/;
        alert("Card type not found");
    }
    prefixIsValid = prefixRegExp.test(cardNum);
    isValid = prefixIsValid && lengthIsValid;
  }
  //Use the Luhn formula to check the validity of the entered credit card number
  if(isValid) {
    var currDigitInt
    var total = 0;
   
    for (i = cardNum.length-1; i>=0; i--) {
      currDigitInt = parseInt(cardNum.charAt(i),10);
	total = total + currDigitInt;
	i--;
	if(i >= 0) {
	  currDigitInt = parseInt(cardNum.charAt(i),10) * 2;
	  if(currDigitInt > 9) total = total + (currDigitInt-10)+1
	  else total = total + currDigitInt
	}
    }
    isValid = (total % 10 == 0);
  }
  return isValid;
}