gistfile1.rb

require 'pp'

def substitute_command_tokens(check)
  unmatched_tokens = Array.new
  substituted = check[:command].gsub(/:::([^:]*?):::/) do
    token, default = $1.to_s.split('|', -1)
    matched = token.split('.').inject(@settings[:client]) do |client, attribute|
      if client[attribute].nil?
        default.nil? ? break : default
      else
        client[attribute]
      end
    end
    pp matched
    if matched.nil?
      unmatched_tokens << token
    end
    matched
  end
  [substituted, unmatched_tokens]
end

check = {
  command: "PATH=$PATH:/usr/local/nagios/libexec check_activemq -H :::amq.host|localhost-8161|||::: -q mailer.fast -w 5:5 -c 2:5 -C"
}

@settings = {
  client: {
    'amq' => {
      'host' => 'foobar'
    }
  }
}

pp substitute_command_tokens(check)