Find out domain in URI in Nginx Conf

The question of url is domain.com/proxy/seconddomain.tld/images/file.jpg and we want to find what seconddomain.tld is so far we are trying nginx map module

map $request_uri $topdir {
        ~(?<captured_topdir>^/proxy/[a-zA-Z0-9\.-_]+)[/] $captured_topdir;
}

but this is returning empty, can say its okay to get /proxy/seconddomain.tld but need it without the folders.

Try this:

if ($host ~* ^(.+)\.([a-z0-9-]+)$) {
   set $domainpart $1;
   set $tld $2;
}

@drookie

This is not working please, it is causing a 502 error

if ($host ~* ^(.+)\.([a-z0-9-]+)$) {
   set $domainpart $1;
   set $tld $2;
}

                add_header X-domain "$domainpart";
                add_header X-tld "$tld";
                subs_filter_types *;
                subs_filter 'action="/'  action="/proxy/$domainpart.$tld/;
                subs_filter 'href="/'  href="/proxy/$domainpart.$tld/;

It works if I take the if part out and set the domainpart and tld to fixed variables.

Thanks