Sorry, that won't work. The User-Agent header can be faked like any other HTTP header value.

If you need that for generating browser-specific web pages, you are barking at the wrong tree: Generate ONE page for ALL browsers, use CSS (plus CSS hacks) to style the page. If you really can't avoid Javascript, use feature detection, not browser detection (i.e. if (document.getElementById) { var o=document.getElementById("foo"); ... } instead of if (navigator.userAgent.indexOf("MSIE")>0) { ... } or the even more scary if (document.all) { ... }). This is the state of the art, and all other approaches will end in maintainance nightmares.

If you want to guess what browsers are used to visit a certain web page, well, guess. Count all different instances of the HTTP user agent in the web server's access log for a certain period of time, identify and group the user agents you know, and stuff the remaining ones into the "Others" group. It's better than throwing dices, but not the absolute truth. Several page visitors will use faked user agents, either intentionally or because some "security" software manipulated the HTTP requests on-the-fly.

By the way: Did you know that there are more browsers than just Firefox, Internet Exploder and Safari? Do you know lynx, links, Opera, Konqueror, Arachne? The old Netscape? Far more browsers than you ever want to know are collected at http://browsers.evolt.org/ ...

Alexander

--
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)

In reply to Re^3: HTTP_USER_AGENT detection by afoken
in thread HTTP_USER_AGENT detection by rootcho

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