How to exclude packages in yum.conf ?

■ Requirement : How to exclude packages in yum.conf 
■ OS Environment : Linux[RHEL, Centos]
■ Application: yum
■ Implementation Steps : 

vi /etc/yum.conf and append following line :

exclude=pkgname*

Then save it and execute : #yum update

Example :

exclude=php* kernel*
exclude=gdm grub