$("#register-form").submit(function(){
    var isFormValid = true;

    $(".required input").each(function(){
        if ($.trim($(this).val()).length == 0){
            $(this).addClass("highlight");
            isFormValid = false;
        }
        else{
            $(this).removeClass("highlight");
        }
    });

    if (!isFormValid) alert("Please fill in all the required fields (indicated by *)");

    return isFormValid;
});