php正则匹配gb2312和utf-8中文

$action = trim($_get['action']);
if($action == "sub")
{
$str = $_post['dir'];
//if(!preg_match("/^[".chr(0xa1)."-".chr(0xff)."a-za-z0-9_]+$/",$str)) //gb2312汉字字母数字下划线正则表达式
if(!preg_match("/^[x{4e00}-x{9fa5}a-za-z0-9_]+$/u",$str)) //utf-8汉字字母数字下划线正则表达式
{
      echo "您输入的[".$str."]含有违法字符";
}
else
{
       echo "您输入的[".$str."]完全合法,通过!";
}
}
?>