redirect only every 2nd or 3rd request

[laszlo]

I'm trying to set up a rule which is going to redirect only every second or
third request only if the URL contains a specific string.
I did already the redirect based on a string in the URL but I can't find how
to redirect only every second on third request:

  if ($request_uri ~ .*.WHATEVER_STRING.*) {
    if (this is the 2nd request than) {
                        rewrite ^/(.*) WHATEVER_URL;
                }
}

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,253011,253011#msg-253011