function subscribeToNewsletter(){
    var email = document.getElementById("email_newsletter").value;
    
    var mail_filter  = /^([a-zA-Z0-9_\.\-])+\@(([a-zA-Z0-9\-])+\.)+([a-zA-Z0-9]{2,4})+$/;
    if( mail_filter.test(email) ){
        if( ajax("ajax/isAlreadyIn.php", "email="+email) ){
            ajax("ajax/saveNewsletterEmail.php", "email="+email);
            document.getElementById("subscribe_newsletter").innerHTML = "<p>Thank you, you will receive the newsletter.</p>";
        } else alert("This e-mail address is already registered in the database.");
    } else alert("Invalid e-mail address.");
}