internal URL rewrite

set $cust_uri $request_uri;
    set $cust_host $host;
    if ($host ~* "rk_([0-9a-zA-Z]{3})\.reality\.cz") {
        set $cust_uri "/firma/$1$request_uri";
        set $cust_host "www.reality.cz";
#        rewrite ^(.*)$ https://www.reality.cz/firma/$LIC$request_uri last;
    }

      location / {
        proxy_pass  https://realitycz-main-ssl $cust_uri;
        proxy_set_header       Host            $cust_host;
}