Nginx redirect only root domain but not subdomain to www

Currently I’m using setting below to redirect non-www domain to www domain and it’s working fine:

server { listen 80; server_name example.com; return 301 http://www.example.com$request_uri; }

server { listen 80; server_name www.example.com; location / { proxy_pass http://www.example.com:8888; proxy_http_version 1.1; proxy_set_header Upgrade $http_upgrade; proxy_set_header Connection ‘upgrade’; proxy_set_header Host $host; proxy_cache_bypass $http_upgrade; } }

However, now I would like to allow wildcard subdomain but it seems like all the subdomains were being redirect to www.domain.com. So my question is how can I make it only redirect the root domain to www only and excluding all other subdomain? Thanks.