map argument to new variable with NGINX

nginx.conf

user  nginx;
worker_processes  auto;

error_log  /var/log/nginx/error.log notice;
pid        /var/run/nginx.pid;


events {
    worker_connections  1024;
}


http {
    include       /etc/nginx/mime.types;
    default_type  application/octet-stream;

    log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
                      '$status $body_bytes_sent "$http_referer" '
                      '"$http_user_agent" "$http_x_forwarded_for"';

    access_log  /var/log/nginx/access.log  main;

    sendfile        on;
    #tcp_nopush     on;

    keepalive_timeout  65;

    #gzip  on;

    map $arg_parameter1 $mapped_value {
        "~([^&]{1,})"   $arg_parameter1;
        default         '';
    }


    server {
        listen 80;
        location / {
            return 200 "query_string: $query_string\narg_parameter1: $arg_parameter1\nmapped_value: $mapped_value\n";
       }
    }


}