AJAX消息清除

Question

I am finishing up a form for user registration, and I wanted to know how to clear messages generated dynamically by AJAX, when the user clears the input (backspace). For example, let's say they enter a valid input, but then clear it with all backspaces - my code currently shows the valid input message, but I want that to switch to an empty string / no message when that happens:

if ($nameCheckCount < 1) {
    if (preg_match("/^\w+@[a-zA-Z_]+?\.[a-zA-Z]{2,3}$/", $email)) {
        echo 'This email is available.';
        exit();
    } else {
        echo 'You entered an email with an invalid format.';
        exit();
    }   
} else {
    echo 'This email is taken.';
    exit();
}

Is there some way I can change the above code I currently have to clear the message if there is no input after a backspace?

Secondly, once all the input is validated (I have one main HTML file for the form, and then three PHP files to check username, password, and email validity through the use of MySQL, or in the case of the passwords, a simple string match), can I set some sort of flag to then allow the user to submit? For example, the code that checks the email is as such:

function checkEmail() {
        "use strict";

        var status = document.getElementById("email_status");
        var email = document.getElementById("email").value;

        if (!(email == "")) {
            status.innerHTML = "Checking...";
            var request = new XMLHttpRequest();
            request.open("POST", "email_check.php", true);

            request.setRequestHeader("Content-type", "application/x-www-form-urlencoded");

            request.onreadystatechange = function() {
                if (request.readyState == 4 && request.status == 200) {
                    status.innerHTML = request.responseText;
                }
            }

            var verify = "emailToCheck=" + email;
            request.send(verify);
        }
    }

This is within my HTML file with the form. Once all fields are validated, is there a way to then allow the user to submit? One user may only have a unique combination of the username and email fields which are stored in the user table along with other data, and no username can be associated with multiple usernames, and vice versa. Thank you in advance for any tips!