请问用正则表达式去除数字和Xx外的字符

请教用正则表达式去除数字和Xx外的字符
var   sdf   =   "     2345234523 ";
var   re   =   new   RegExp( "[^0-9xX] ", " ");
sSFZH   =   re.exec(sdf   );

这么写出错
请教正确方法

------解决方案--------------------
用replace

<script language= "javascript ">
var sdf = " 2345234523 ";
re = /[^0-9xX]/g;
sSFZH = sdf.replace(re, " ");
alert(sSFZH);
</script>