haproxy, remove /app after selecting backend

haproxy, remove 'app' after selecting backend

Code to change a request from / to /app1/
  reqirep  ^([^\ :]*)\ /(.*) \1\ /app1/\2

If urls in the response contain absolute urls it might be required to use this:
  acl    no_redir url_beg   /app1/
  reqirep  ^([^\ :]*)\ /(.*) \1\ /app1/\2 if !no_redir

The code makes sure that the method and url-path behind the / stays the same. Which method you need exactly might depend on the application thats running.
For readability of the above how change a request from /app1/ to /app1/app1redir/
  reqirep  ^([^\ :]*)\ /app1/(.*) \1\ /app1/app1redir/\2

If those above dont work you might still be able to get a acceptable workaround by using a redirect:
  acl    no_redir url_beg   /app1/
  http-request redirect location http://%[req.hdr(Host)]/app1/ if !no_redir

What version(s) of haproxy will this work with?

Anything less than 2.1, because reqirep is deprecated in 2.1 onward