kod php/html;

<input id="<?=$prefix?>" style="text-align:center" size="1" type="text" name="<?=$id?>[prefix]" onkeyup="UCF(this);" value="<?=$prefix?>">

  <script language="javascript">
        <!--
        var prefix_array=new Array(<?foreach ($prefix_array as $v) echo $v.", ";?>0);

        function UCF(AField)
        {
                var check=1;

                if (AField.id!=AField.value)
                {
                        if (AField.value=="") check=-1; else check=AField.value;
                        if (prefix_array.in_array(check)) AField.style.color="red";
                        else AField.style.color="black";
                }

        }



        Array.prototype.in_array = function(p_val)
        {
                for(var i = 0, l = this.length; i < l; i++) {
                        if(this[i] == p_val) {
                        return i;
                }
        }
                return false;
        }