Is it possible to remove part of a path in HAProxy in a redirect?
You can do URL rewriting and HTTP headers manipulation (add, modify, and remove headers).
However, the haproxy documentation states that this is not encouraged and your setup should work (without reconfiguration) with and without haproxy running between your clients and your real servers. Following this recommendation, it will be better to use apache mod_rewrite.
You can do this with the 'reqrep' or 'reqirep' commands:
reqrep ^([^\ ]*)\ (.*)/dev \1\ /\2 if acl_dev