php正則匹配gb2312和utf-8中文

<?php教程
$action = trim($_get['action']);
if($action == "sub")
{
$str = $_post['dir'];
//if(!preg_match("/^[".chr(0xa1)."-".chr(0xff)."a-za-z0-9_]+$/",$str)) //gb2312漢字字母數字底線Regex
if(!preg_match("/^[x{4e00}-x{9fa5}a-za-z0-9_]+$/u",$str)) //utf-8漢字字母數字底線Regex
{
      echo "<font color=red>您輸入的[".$str."]含有違法字元</font>";
}
else
{
       echo "<font color=green>您輸入的[".$str."]完全合法,通過!</font>";
}
}
?>