Navigating to http://whatsmyuseragent.com/ shows me my stock Android browser on my Galaxy Nexus running 4.2.1 has the user agent
Mozilla/5.0 (X11; Linux x86_
You must remove SamsungBrowser from your regexp.
For SamSungBrowser, you can use SamsungBrowser/([0-9]*.[0-9]*) or Android.*SAMSUNG.*Version/([0-9]*.[0-9]*) for old browser
http://developer.samsung.com/technical-doc/view.do?v=T000000203