After quickly looking over many different mod10 validation scripts on the web, I decided to create my own. 🙂

The Luhn algorithm, sometimes also referred to as mod10 is actually very simple to do and I recommend first trying it by hand to make sure you fully understand it. The wiki article covers it pretty well.

Once you understand, you might be considering the best way to implement it in a realtime clientside validation scenario. If you’re lucky enough to be using .net, then the solution below should work perfect for you.

Pop the function call into your page load and pass in the id of the textbox which will contain the cc and you’re good to go. Keep in mind if you’re using AJAX you need to register the javascript function with
ScriptManager instead of ClientScript or it will stop working after postback.

(C#)

protected void Page_Load(object sender, EventArgs e)
        {
CreateLuhnJavascriptValidator(tbCC.ClientID);
}
private void CreateLuhnJavascriptValidator(string id)
        {
            WebControl validator = null;

            string js= "function LuhnValidate(sender, args) {var ctlcc = document.getElementById('ctlcard_accountnumber'); var m=0; var cc = ctlcc.value; for(i=0; i<cc.length; i=i+1) { var c = cc.charAt(i); if (i%2==0) { var a=(2*parseInt(c)); if (a>=10) {a=parseInt(a%10)+parseInt(a/10);} m=a+m;}else {m=m+parseInt(c);} /*alert(m);*/} if ((m%10)==0) {args.IsValid=true;} else {args.IsValid=false;} }";
            ClientScript.RegisterClientScriptBlock(this.GetType(), "LuhnValidate", js, true);

            validator = new CustomValidator();
            ((CustomValidator)validator).ControlToValidate = id; //id of control on clientside not serverside
            ((CustomValidator)validator).ErrorMessage = "*mod10";
            ((CustomValidator)validator).ClientValidationFunction = "LuhnValidate";
((CustomValidator)validator).ServerValidate += new ServerValidateEventHandler(ServerLuhnValidator);

            pnlSomePanel.Controls.Add(validator); //alternatively you can also simply place custom validator in your page next to the textbox. placement in a panel or on design side of aspx next to textbox only affects appearance of error message
        }

Pure algorithm (js)

//.net version reqires args and sender to wire up correctly
function dotnetwireup(sender, args) {
args.IsValid=LuhnValidate();
}

function LuhnValidate() {
var ctlcc = document.getElementById('ctlcard_accountnumber'); 
var m=0; 
var cc = ctlcc.value; 
for(i=0; i<cc.length; i=i+1) 
{ 
var c = cc.charAt(i); 
if (i%2==0) 
{ 
var a=(2*parseInt(c)); 
if (a>=10) 
{
a=parseInt(a%10)+parseInt(a/10);
} 
m=a+m;
}
else 
{
m=m+parseInt(c);
} /*alert(m);*/
} if ((m%10)==0) {
return true;
} else {
return false;
}
}

(C#)

 private void ServerLuhnValidator(object sender, ServerValidateEventArgs e)
        {
            try
            {
                e.IsValid = LuhnValidate(e.Value);
            }
            catch (Exception ex)
            {
                //handle error
            }
        }

        private bool LuhnValidate(string cc)
        {
            int m = 0;
            for (int i = 0; i < cc.Length; i++)
            {
                char c = cc.ToCharArray()[i];
                if (i % 2 == 0)
                {
                    int a = 2 * Convert.ToInt32(c.ToString());
                    if (a >= 10)
                    {
                        a = Convert.ToInt32(a % 10) + Convert.ToInt32(a / 10);
                    }
                    m = a + m;
                }
                else
                {
                    m = m + Convert.ToInt32(c.ToString());
                }
            }
            if (m % 10 == 0)
            {
                return true;
            }
            else
            {
                return false;
            }
        }

References
Wikipedia, http://en.wikipedia.org/wiki/Luhn_algorithm