<style type="text/css">
#pun-register .button {
  display: none;
}

#inform_mess {
  position: absolute; 
  z-index: 1000; 
  padding: 5px;
  width: 400px; 
  margin-left: 200px; 
  margin-top: -22px;
}
</style>

<script type="text/javascript">
var fld_1 = document.getElementById("fld1"); 

function check_login() {
 var f1_value; var rule; var fld_1;
 var censored_login = new Array("Login_1", "Login_2", "Login_3");
 fld_1 = document.getElementById("fld1");
 f1_value = fld_1.value;
 if (f1_value != "") {
  rule = new RegExp(censored_login.join('|'), "i");
  if (rule.test(f1_value)) create_inform_mess("bad");
  else if (f1_value.length < 2) create_inform_mess("small");
  else create_inform_mess("good");
 }
}

function create_inform_mess(y) {
 var div = document.getElementById('inform_mess');
 var registration = document.getElementById("register").getElementsByTagName("input")[8];
 if (!div) {
  div = document.createElement("div");
  div.id = "inform_mess";
  document.getElementById("fld1").parentNode.appendChild(div);
 }
 if (y == "bad") { 
  div.innerHTML = "Такое имя пользователя недопустимо, выберите другой вариант.";
  div.style.backgroundColor = "#ff6a6a";
  document.getElementById("fld1").style.background = "#ff6a6a none";
  registration.style.display = "none";
 }
 else if (y == "good") {
  div.innerHTML = "Такое имя пользователя разрешено к регистрации.";
  div.style.backgroundColor = "#66cdaa";
  document.getElementById("fld1").style.background = "#66cdaa none";
  registration.style.display = "block";
 }
 else if (y == "small") {
  div.innerHTML = "Имя пользователя не должно быть короче 2 символов.";
  div.style.backgroundColor = "#ff6a6a";
  document.getElementById("fld1").style.background = "#ff6a6a none";
  registration.style.display = "none";
 }
}

if (document.URL.indexOf("register.php") != -1) {
 fld_1.onkeyup = fld_1.onkeydown = fld_1.onfocus = fld_1.onblur = check_login;
}
</script>

 var censored_login = new Array("Login_1", "Login_2", "Login_3");