How to validate textbox using Javascript Validator?

Here's a jQuery solution if you need to use inputs instead of checkboxes:

$(document).ready(function() {
    jQuery('#restricted-input').on('keydown', function(event) {
        // reset input
        var input = jQuery(this).val('');

        // code 88 = x
        // code 79 = o
        // code 8  = Backspace
        if (event.keyCode == 88 || event.keyCode == 79 || event.keyCode == 8) {
            // let input happen                 
        } else {
            event.preventDefault();
        }
    });
});

Hi if you like this solution

        function val(e) {
        tecla = (document.all) ? e.keyCode : e.which;
        if (tecla == 8) return true;
        patron = /[O-Xox]/;
        te = String.fromCharCode(tecla);
        return patron.test(te);
        }

Thank you everyone for your helpful suggestion to my problem. But here is how I have solved it just now.

This is my Textbox inside the gridview:

 <asp:TextBox ID="txtAttend" runat="server" BackColor="Control" MaxLength="1" 
EnableViewState="true" Width="15px" onkeyup="ValidateText(this);"></asp:TextBox>

Here, I am calling the ValidateText(this) javascript function.

And here is the function:

 <script type="text/javascript" language="javascript">
function ValidateText(i) 
{
    if(i.value != "X" || i.value != "O") 
    {
        alert("Please Enter 'X' for Present or 'O' for Absent");
    }
}
</script>

I got the hint for this solution from this link: TextBox inside GridView validation

Thanks.

Please see following code that help you to allow only two characters to input. This code will restrict other key strokes and allow only 'X' and 'O'. For demo please see : http://jsfiddle.net/BhaveshKachhadiya/uNfP2/7/

function validate(evt)
{
    evt = (evt) ? evt : window.event;
    var charCode = (evt.which) ? evt.which : evt.keyCode;
    // alert(charCode);
    if (charCode == 120 || charCode== 88 || charCode==111 || charCode==79 || charCode == 8) {
        return true;
    }
    else
    {
        alert("Please Enter 'X' for Present or 'O' for Absent");
        return false;    
    }
}