How to properly rewrite in Nginx?

Question

Nginx

Hello super community.
Tell me please.
I want to make it so that users accessing the site from a smartphone
see AMP pages, but at the same time, if they go to the main page, then it is given without the /amp prefix.
That is, you need this:
examle.ru -> examle.ru
but
examle.ru /post-1/ -> examle.ru/post-1/amp
examle.ru/page-3/ -> examle.ru/page-3/amp
Now in nginx like this:

if ($mobile_rewrite = perform) {
        rewrite ^(.*(?<!/amp))/$ https://$server_name$1/amp/ break;
}

and everyone who sees from smartphones like this:
examle.ru -> examle.ru/amp
examle.ru/post-1/ -> examle.ru/post-1/amp
examle.ru/page-3/ -> examle.ru /page-3/amp
How to fix?

Answer 1

Make two locations - with exact match and prefix, get rid of regular expressions.