Nginx proxy_pass exclude url with regex

Question

I'm trying to proxy pass a location excluding only a casistic with a regex.

I have this url /autocomplete/<var1>/<var2> and I want to proxy pass only if var1 is different from a, so b/foo, c/foo ecc will be proxied.

In apache I used this:

ProxyPassMatch ^/autocomplete/a/(.*)$ !
ProxyPass /autocomplete/ http://localhost:1234/autocomplete/

In NGINX I tried with this:

location ~ /autocomplete/(?<var1>.*)/(?<var2>.*) {
    if ($var1 != "a") {
        proxy_pass http://127.0.0.1:1234/autocomplete/$var1/$var2;
   }
}

It works for other values of var1, but for a it returns 404.

Answer 1

Solved with:

location ~ ^/autocomplete/(?!(a)) {
    proxy_pass http://localhost:1234;
}