how to validate a textbox in asp.net using javascript.

txtBox.attributes.add("onkeypress", "javascript:return isAlphaNumeric(event);");

function isAlphaNumeric(e)
 {
//    alert(e.which);
    if ((e.which < 48 || e.which > 57))
    {
        if ((e.which < 123 && e.which > 96))
        {
            return true;
        }
        if ((e.which < 91 && e.which > 64))
        {
            return true;
        }
        if (e.which == 8 || e.which == 0 || e.which == 32)
        {
            return true;
        }
        else
        {
            return false;
        }
    }
}