NGINX config pitfall

nginx.conf

http {
    server {
        server_name example.net;
        listen 80;
    
        types {}
        default_type text/plain;
    
        set $12 "Hello";

        location /ng {
            # responds "2" instead of "Hello"!
            # NGINX interprets '$' immediately followed by a number as the
            # reference to regex captures regardless of following characters.
            # https://github.com/nginx/nginx/blob/828fb94e1dbe1c433edd39147ba085c4622c99ed/src/http/ngx_http_script.c#L480-L505
            return 200 "$12";
        }

        location /ok {
            return 200 "${12}";
        }
    }
}