<script>
line = ">2　>3　>4　>５　>６　>7-8 >90-100　＞100-200";
sp1 = "<a href=\"http:\/\/server.2ch.net/test/+read.cgi?bbs=board&key=123456789&st=";
sp2 = "&to=";
sp3 = "&nofirst=true\" target=\"_blank\">";

line = line.replace(/(>|>>|＞|＞＞|≫)([0-9０-９]+)(-([0-9０-９]+))?/g,
sp1 + "$2" + sp2
+ ("$4" ? "$2" : "$4")
+ sp3 + "$1$2$3<\/a>");

document.write(line);
</script>

ではこれでどう？