auto-registering child classes with instance variables

sample.rb

# This is all mostly working except for the "on_message" class method.
# It needs to set the regex and block as a variable that can be accessed later from an instance of the class.

class MyExtension < ExtensionBase

  on_message '/join (.+)' do |username|
    # this will be a callback function used later
  end

end

class ExtensionBase

  def self.inherited(child)
    MainAppModule.registered_extensions << child.new
  end

  def self.on_message(string, &block)
    # these need to be set on child instance
    @regex = Regexp.new(string)
    @on_message_callback = block
  end

  def exec(message)
    args = @regex.match(message).captures
    @on_message_callback.call(args)
  end

end

# in the main app code, I loop through the registered extensions and test the regex...

MainAppModule.registered_extensions.each do |extension|
    puts extension.regex.inspect # this is nil and I dont want it to be
    if message =~ extension.regex
      return extension.exec(message)
    end
end