wordpress internal redirect using nginx

I have a WordPress site with permalinks set to Post name.

Using Nginx rewrite, I am trying to have an internal redirect (with no change in browser URL), but so far, I am failing.

This works, but the URL changes

location ~ ^/u/(.*) {
    rewrite ^/u/(.*) /p/?username=$1 redirect; 
}

I don't understand why this does not work:

location ~ ^/u/(.*) {
    # this returns 404
    rewrite ^/u/(.*) /p/?username=$1 last;
}

The whole config

server {
    listen 80;
    listen [::]:80;
    server_name example.com;

    root /srv/www/html;
    error_log /var/log/nginx/error.log;
    index index.php;

    location = /favicon.ico {
        log_not_found off;
        access_log off;
    }

    location = /robots.txt {
        allow all;
        log_not_found off;
        access_log off;
    }

    location ~ ^/u/(.*) {
        rewrite ^/u/(.*) /p/?username=$1 last;
        # try_files $uri /p/?username=$1;
    }

    location / {
        try_files $uri $uri/ /index.php?$args;
    }

    location ~ \.php$ {
        client_max_body_size 8M;
        include fastcgi-php.conf;

        # Mitigate https://httpoxy.org/ vulnerabilities
        fastcgi_param HTTP_PROXY "";
        fastcgi_intercept_errors on;
        fastcgi_pass 0.0.0.0:9000;
    }

    location ~* \.(js|css|png|jpg|jpeg|gif|ico)$ {
        expires max;
        log_not_found off;
    }
}

I am writing a WordPress plugin, and I want users to enter URLs such as https://example.com/u/john71 and internally redirect to https://example.com/p/?username=john71

Both u and p are WordPress pages, and p contains a shortcode that allows me to retrieve the username.

I could have https://example.com/p/?username=john71, but something like: https://example.com/u/john71 looks better.

Any help on the rewrite rule or approach is welcome.

The URI is ultimately rewritten to /index.php - so the extra internal rewrite is never seen by WordPress.

WordPress parses the original request, which is passed to it in the REQUEST_URI parameter (which will be defined in your fastcgi-php.conf file, which probably includes it from another file called fastcgi_params).

You may be able to make this work by setting REQUEST_URI and SCRIPT_FILENAME directly before calling fastcgi_pass - for example:

location ~ ^/u/(.*) {
    include fastcgi_params;

    fastcgi_param SCRIPT_FILENAME $document_root/index.php;
    fastcgi_param REQUEST_URI /p/?username=$1;

    fastcgi_param HTTP_PROXY "";
    fastcgi_intercept_errors on;
    fastcgi_pass 0.0.0.0:9000;
}

I haven't included fastcgi-php.conf as it may contain a try_files statement that will not work in this location.