"proxy_pass" cannot have URI part in location given by regular expression

Question

I've developed a URL shortening web application.

It consists of two separate docker containers: one containing the backend REST api and another containing the frontend static website.

These two containers are linked to an nginx container. The configuration for this nginx container is below:

worker_processes 1;

events { worker_connections 1024; }

http {
    upstream api {
        server short-url:8080;
    }

    upstream frontend {
        server short-url-frontend:8081;
    }

    gzip on;
    gzip_vary on;
    gzip_min_length 860;
    gzip_proxied expired no-cache no-store private auth;
    gzip_types text/plain text/css text/xml application/javascript application/x-javascript application/xml;
    gzip_disable "MSIE [1-6]\.";

    server {
        listen 80;
        root    /user/share/nginx/html;

        location /urlshortener/v1 {
            proxy_pass         http://api/urlshortener/v1;
            proxy_redirect     off;
            proxy_set_header   Host $host;
            proxy_set_header   X-Real-IP $remote_addr;
            proxy_set_header   X-Forwarded-For $proxy_add_x_forwarded_for;
            proxy_set_header   X-Forwarded-Host $server_name;
        }

        location ~ ^/([A-Za-z0-9]+) {
            rewrite ^/([A-Za-z0-9]+) /$1
            proxy_pass         http://api/urlshortener/v1;
        }

        location / {
            proxy_pass          http://frontend;
            proxy_set_header    Host              $host;
            proxy_set_header    X-Real-IP         $remote_addr;
            proxy_set_header    X-Forwarded-for   $remote_addr;
        }
    }

}

If a url ends with /urlshortening/v1, I'm proxying to the backend.

If a url starts with /, I'm proxying to the frontend.

Shortened urls e.g. /3xTy or /a0q need to be proxied to the backend so that the user can be navigated to the original url. In order to do this, I've defined a location with a regular expression.

location ~ ^/([A-Za-z0-9]+) {
    rewrite ^/([A-Za-z0-9]+) /$1
   proxy_pass         http://api/urlshortener/v1;
}

This block of code gives me the following error:

2018/11/17 16:47:03 [emerg] 1#1: "proxy_pass" cannot have URI part in location given by regular expression, or inside named location, or inside "if" statement, or inside "limit_except" block in /etc/nginx/nginx.conf:36

I've gone through several examples and reviewed a number of answers and I believe that the configuration I have should work. Can someone please explain why I'm getting this error?

Answer 1

If you use a URI with a proxy_pass statement within a regular expression location, you need to build the entire URI using one or more variables. See proxy_pass docs for details.

So the alternatives are to (1), capture the URI from the location expression and add it to the proxy_pass statement. For example:

location ~ ^/([A-Za-z0-9]+) {
    proxy_pass http://api/urlshortener/v1/$1;
}

Or (2), use proxy_pass without a URI part, and construct the desired URI using a rewrite...break. For example:

location ~ ^/([A-Za-z0-9]+) {
    rewrite ^/([A-Za-z0-9]+) /urlshortener/v1/$1 break; 
    proxy_pass http://api;
}

See rewrite docs for details.