Run a custom fact based on core facts

So what if my previous example puppet custom fact easy as 1 2 3, needs to be run only on RedHat systems?

An additional line on the ruby file is added:

Facter.add('netuuid') do
   confine :operatingsystem => "RedHat"
   setcode do
      Facter::Core::Execution.exec("echo `grep UUID /etc/sysconfig/network-scripts/ifcfg-eth0 | cut -c 7-42`")
   end
end

Any core fact can be referenced with the colon and name of the fact
:operatingsystem
:osfamily
:kernel
etc.