edit card is valid using the Luhn formula.
        
        @param cardNumber: The credit card number.
        @return Returns &lt;code>true&lt;/code> if string is a valid credit card number; otherwise &lt;code>false&lt;/code>.
    */
    public static function isCreditCard(cardNumber:String):Boolean {
        if (cardNumber.length &lt; 7 || cardNumber.length > 19 || Number(cardNumber) &lt; 1000000)
            return false;
        
        var pre:Number;
        var sum:Number  = 0;
        var alt:Boole