Omiting a portion of a redirect

[root@tulip:Active] config  b virtual bar list
virtual bar {
   destination 172.28.17.66:http
   ip protocol tcp
   rules myrule
   profiles
      http
      tcp
}
[root@tulip:Active] config  b rule myrule list
rule myrule {
   when HTTP_REQUEST {
   if {[string tolower [HTTP::host]] ends_with "oursite.com" and [string tolower [HTTP::uri]] starts_with "/theirsite"} {
      HTTP::redirect "http://www.theirsite.com[string range [HTTP::uri] 10 end]"
   }
}
}

[root@tulip:Active] config  curl -I http://www.oursite.com/theirsite/test/about.shtml
HTTP/1.0 302 Found
Location: http://www.theirsite.com/test/about.shtml
Server: BigIP
Connection: Keep-Alive
Content-Length: 0