想在xml中实现个字符串截取相加功能

[biqingjun]

想在xml中实现个字符串截取相加功能，在JAVASCRIPT中我已经实现这个功能了，现在想放进xml里不知道该怎么写了，望高手指教
网页javascript中代码如下
<script language="javascript" type="text/JavaScript">
function DoFileList()
{
var pelist="{PE.Field id="cone" fieldname="FileLists" /}";
var FileList ;
var top;
top = pelist.substring(1,4);
if(top=="http"){FileList = pelist;return FileList;}
else{
FileList = pelist.substring(6);
FileList="vcastr_file=/UploadFiles/"+FileList+"&LogoText=www.lnnews.net&BufferTime=3";
return FileList;
}
}
</script>


这段如果换成xml该怎么写啊

[Qiaorui]

截取：substring( string , number , number? )
合并：concat(string, string, string*)