sed: find and replace string but not super-string -

i have file env looks like

....

 legacy_database_server=10.0.0.1 server=10.1.1.1 

and here sed command:

sed -e "s/server=.*/server=$instance_ip/g;n" $env_file > $env_file.tmp && mv $env_file.tmp $env_file

the problem sed replacing legacy_database_server not want. want server replaced.

(legacy_database_server super string of server , want replace server)

what missing?

presumably, want make sure sed knows "server" @ beginning of line:

sed -e "s/^server=.*/server=$instance_ip/g;n" $env_file > $env_file.tmp && mv $env_file.tmp $env_file