gistfile1.pp

node /hub[0-9]+\.example/ {
    $hub_service_node = 'hub1.example'
    class { 'base': }
    class { 'service_a': active_node => $hub_service_node }
    class { 'service_b': active_node => $hub_service_node }
    class { 'service_c': active_node => 'hub3.example' }
    class { 'service_d': active_node => 'hub5.example' }
}