jQuery.fn.ForceNumericOnly = function(){
    return this.each(function(){
        $(this).keydown(function(e){
            var key = e.charCode || e.keyCode || 0;
            // allow backspace, tab, delete, arrows, numbers and keypad numbers ONLY
            return (key == 8 ||
            key == 9 ||
            key == 46 ||
            (key >= 37 && key <= 40) ||
            (key >= 48 && key <= 57) ||
            (key >= 96 && key <= 105));
        })
    })
};

$(document).ready(function(){
    $('input[name=cardnr]').ForceNumericOnly();
});