Nginx location regex with proxy_pass

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 this document 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 this document for details.