Rack full example

config.ru

Application = proc do |env|  
  names = env['names'] || %w[world]
  text = "Hello, #{names.join ', '}!"
  [200, {'Content-Type' => 'text/plain'}, [text]]
end

class NameMiddleware
  def initialize(app)
    @app = app
  end

  def call(env)
    names = env['REQUEST_PATH'].split('/').reject(&:empty?)
    env['names'] = names unless names.empty?

    @app.call env
  end
end

class Blackhole
  def initialize(app, regex)
    @app = app
    @regex = regex
  end

  def call(env)
    if @regex =~ env['REQUEST_PATH']
      [404, {'Content-Type' => 'text/plain'}, ['Not found']]
    else
      @app.call env
    end
  end
end

use Blackhole, %r{\A/favicon\.ico}
use NameMiddleware
run Application