How to enable CORS in nginx with origin matching

nginx.conf

server {
    listen 80 default_server;
    root /var/www;

    location / {
        set $cors '';
        set $cors_allowed_methods 'OPTIONS, HEAD, GET';

        if ($http_origin ~ '^https?://(www\.)?example.com$') {
            set $cors 'origin_matched';
        }

        # Preflight requests
        if ($request_method = OPTIONS) {
            set $cors '${cors} & preflight';
        }

        if ($cors = 'origin_matched') {
            add_header Access-Control-Allow-Origin $http_origin;
            add_header Access-Control-Allow-Methods $cors_allowed_methods;
        }

        if ($cors = 'origin_matched & preflight') {
            add_header Access-Control-Allow-Origin $http_origin always;
            add_header Access-Control-Allow-Methods $cors_allowed_methods;
            add_header Content-Type text/plain;
            add_header Content-Length 0;
            return 204;
        }
    }
}