Nginx - how to limit requests by User Agent

gistfile1.txt

 http {

     map $http_user_agent $limit_bots {
         default '';
         ~*(google|bing|yandex|msnbot) $binary_remote_addr;
     }

     limit_req_zone $limit_bots zone=bots:10m rate=1r/m;

     server {
         location / {
             limit_req zone=bots burst=5 nodelay;
         }
     }
 }