<script>
var reg=/<si>(.*?)<\/si>/img;
var str="<si>as</si><si>der</si><si>bs</si><si>sb</si><si>bseb</si>";
document.write(str.replace(reg,"<si><\/si>\n"));
</script>
<script>
var reg=/<si>(.*?)<\/si>/img;
var str="<si>as</si><si>der</si><si>bs</si><si>sb</si><si>bseb</si>";
alert(str.replace(reg,"<si><\/si>\n"));
</script>