example.rb

# Usage:

browser.ie6?
browser.gecko?
browser.chrome? and browser.linux?

# In the wild:

<% if browser.ios? %>
  <script src="/js/iphone.js"></script>
<% end %>

# Or maybe:

get '/' do
  redirect '/touch'  if browser.ios?
end

sinatra_helper.rb

class Main
  helpers do
    def browser
      UserAgent.new(env['HTTP_USER_AGENT'])
    end
  end
end

user_agent.rb

class UserAgent
  def initialize(ua)
    @ua_string = ua
    @ua = ua.scan(%r{([^ /]+)/([^ ]+)(?: \(([^)]+)\))?}).map { |r| { :product => r[0], :version => r[1], :details => (r[2] ? r[2].split(';') : nil) } }
  end

  # Browsers
  def webkit?()    product?('AppleWebKit'); end
  def chrome?()    product?('Chrome'); end
  def safari?()    product?('Safari'); end
  def ios?()       product?('Safari') && product?('Mobile') end
  def gecko?()     product?('Gecko'); end
  def firefox?()   product?('Firefox'); end
  def opera?()     product?('Opera'); end
  def ie?()        detail?(/^MSIE/, 'Mozilla'); end

  # OS's
  def linux?()     detail?(/^Linux/, 'Mozilla'); end
  def windows?()   detail?(/^Windows/, 'Mozilla'); end
  def osx?()       detail?(/^Intel Mac OS X/, 'Mozilla'); end
  def mac?()       detail?(/^Macintosh/, 'Mozilla') || osx?; end

  # IE
  def ie9?()       detail?('MSIE 9.0', 'Mozilla'); end
  def ie8?()       detail?('MSIE 8.0', 'Mozilla'); end
  def ie7?()       detail?(/^MSIE 7.0/, 'Mozilla'); end
  def ie6?()       detail?(/^MSIE 6/, 'Mozilla'); end

  def to_s()       @ua_string; end
  def inspect()    @ua.inspect; end

  # Checks for, say, 'Gecko' in 'Gecko/3.9.82'
  def product?(str)
    !! @ua.detect { |p| p[:product] == str }
  end

  # Checks for, say, 'MSIE' in 'Mozilla/5.0 (MSIE; x; x)'
  def detail?(detail, product=nil)
    !! @ua.detect do |p|
      (product.nil? || product == p[:product]) &&
      !p[:details].nil? &&
      p[:details].detect { |d| d.match(detail) }
    end
  end
end