Rewriting url like in nginx

Help!
1

Hi,
I am currently trying to replace our nginx, that we only use for proxying.
I am currently working on our first application and got it almost working. The only problem is the path of the url:

The setup is as follows:

all our services run on subdomains
the applications themselves run on different servers with a path to them e.g.

appserver1.example.com/app1

should be mapped to

app1.example.com

I managed to get it working with the given configuration to work completely with

app1.example.com/app1/

but i struggle to remove the /app1 from the client after a call.

in nginx the config was simply a proxy_pass https://backend/app1
but i do not seem to get the rewrite right for haproxy

btw the version that i am runnning is 2.6 and i removed the letsencrypt part from here since that seems to be working fine and does not interfere with the problem i am trying to solve

the config looks as follows:

global
        log /dev/log    local0
        log /dev/log    local1 debug
        chroot /var/lib/haproxy
        stats socket /run/haproxy/admin.sock mode 660 level admin expose-fd listeners
        stats timeout 30s
        user haproxy
        group haproxy
        daemon

        ca-base /etc/ssl/certs
        crt-base /etc/ssl/private

defaults http_defaults
        log     global
        mode    http
        option  httplog
        option  dontlognull

        option  forwardfor

        timeout connect 50000
        timeout client  50000
        timeout server  50000

        errorfile 400 /etc/haproxy/errors/400.http
        errorfile 403 /etc/haproxy/errors/403.http
        errorfile 408 /etc/haproxy/errors/408.http
        errorfile 500 /etc/haproxy/errors/500.http
        errorfile 502 /etc/haproxy/errors/502.http
        errorfile 503 /etc/haproxy/errors/503.http
        errorfile 504 /etc/haproxy/errors/504.http

frontend app1 from http_defaults
        bind :443 ssl crt /etc/ssl/letsencrypt

        http-request set-header X-Forwarded-Port %[dst_port]
        http-request add-header X-Forwarded-Proto https if { ssl_fc }
        http-request add-header X-Forwarded-For %[src]
        http-request add-header X-Real-Ip %[src]

        acl cond_is_app1 hdr(Host) app1.example.com

        # set headers
        http-request set-header X-Forwarded-Port %[dst_port]
        http-request add-header X-Forwarded-Proto https if { ssl_fc }
        http-request add-header X-Forwarded-For %[src]
        http-request add-header X-Real-Ip %[src]

        acl cond_is_favicon path -i /favicon.ico
        acl cond_is_robot path -i /robots.txt
        acl cond_is_resources path_beg /resources
        use_backend hosted if cond_is_favicon || cond_is_robot || cond_is_resources

        # i tried this one but it did not help
        #        acl is_path_app1_set path_beg /app1
        #        http-request set-path %[path,regsub(^/app1/,/)] if is_path_app1_set

        use_backend %[req.hdr(Host),lower] if cond_is_app1


backend app1.example.com from http_defaults
        option accept-invalid-http-response
        balance roundrobin
        cookie SERVER insert indirect nocache
        http-request set-header Host app1.example.com
        acl is_path_app1_set path_beg /app1
        http-request replace-uri ([^/:]*://[^/]*)?(.*) \1/app1\2 if !is_path_app1_set
        server app-1 1.1.1.1:443 ssl check verify none cookie app-1

backend hosted from http_defaults
        option forwardfor
        http-request set-header Host hosteddata.otherservice.org
        http-request replace-uri ([^/:]*://[^/]*)?(.*) \1/stuff\2
        server resources hosteddata.otherservice.org ssl verify none