string = 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_6_8) AppleWebKit/536.5 (KHTML, like Gecko) Chrome/19.0.1084.56 Safari/536.5'
user_agent = UserAgent.parse(string)
user_agent.browser
# => 'Chrome'
user_agent.version
# => '19.0.1084.56'
user_agent.platform
# => 'Macintosh'

Browser = Struct.new(:browser, :version)

SupportedBrowsers = [
  Browser.new("Safari", "3.1.1"),
  Browser.new("Firefox", "2.0.0.14"),
  Browser.new("Internet Explorer", "7.0")
]

user_agent = UserAgent.parse(request.user_agent)
SupportedBrowsers.detect { |browser| user_agent >= browser }