function check_form(obj) {
    pattern = /^([a-zA-Z0-9_\.\-])+\@(([a-zA-Z0-9\-])+\.)+([a-zA-Z0-9]{2,4})+$/;
    if (!pattern.test(obj.elements['email'].value)) {
        output_error('Please, type your email address');
        obj.elements['email'].focus();
        return false;
    }
    if (obj.elements['comments'].value.length == 0) {
        output_error('Please, fill out a field COMMENTS');
        obj.elements['comments'].focus();
        return false;
    }
    return true;
}

function output_error(error_msg) {
    alert(error_msg);
}