php - Get nginx location wildcard inside location block?

I am trying to set up an API where I can access API "foobar" through the URL http://my-apis.com/foobar/route. This is what I have so far:

location ~ ^/foobar(/.*)$ {
    root /var/www/mysite/foobar/public;

    ... more fastcgi stuff ...

    fastcgi_param SCRIPT_FILENAME $document_root/index.php$1;
    fastcgi_param PATH_INFO $fastcgi_path_info;
}

The API is routing to a Slim framework application and currently it successfully routes to the correct index.php, showing an nginx 404/403 whenever the URL does not start with /foobar. However the route passed to Slim (which looks like it's represented by $1 on line 6) is still the full /foobar/route. This means I have to append all all my Slim routes with /foobar, which, although I can use a Slim group, is still a pain. I would like to be able to pass just the /route bit to Slim.

Is there a way I can extract just the wildcard-matched bit of the location directive? Since $1 gives the full route. Alternatively I might be able to do this with some kind of rewrite, but I don't know enough about Slim.

Any help would be much appreciated!

Thanks!