fluentd DSL config. I want to extract number from hostname.

fluentd.rb

source {
  type "forward"
  port 24224
}
match('test.**') {
  type "forward"

  num = ::Kernel.open("|hostname"){|o| o.read.chop}.gsub('hoge','').to_i / 20 + 1 # host01 => 1
  (num..num+1).each do |n|
    server {
      host "proxy%02d" % n
      port 24224
    }
  end
}