V2 Help with path_regexp and rewrite

Oh dear… I have no idea how I came up with this, but this is the config which works.

{
    debug
}

sub.domain.com {
    tls /etc/ssl/cert.pem /etc/ssl/key.pem
    encode gzip

    root * /www

    @ipfilter {
        not remote_ip 192.168.100.0/24
    }
    respond @ipfilter 403

    @zxp path_regexp myregex ^/([zx]p?)/(.*)/(.+\.(?:gif|jpe?g|png|txt|html?|css|js))$
    rewrite @zxp /{re.myregex.1}/{re.myregex.2}/img/{re.myregex.3}
    file_server @zxp {
        root @zxp /mnt
    }
    file_server *

    log {
        output stdout
        format single_field common_log
    }

    header Strict-Transport-Security max-age=31536000;
}

If someone could explain my why, I’d appreciate.