How to limit browser support to only recent versions of Chrome, Firefox, Internet Explorer and Safari in Nginx

browsers.conf

#
# Supported browsers
#

modern_browser gecko 1.9;

# note that Safari related directives match
# Chrome, Mobile Safari, Palm Pre and other WebKit-based browsers here, too, thanks
# to all of them using almost identical User-Agent strings
modern_browser safari 3.0;
modern_browser safari 3.1;
modern_browser safari 3.2;
modern_browser safari 4.0;

modern_browser msie 7.0;
modern_browser msie 8.0;

modern_browser  unlisted;

#
# Non-supported browsers
#

ancient_browser msie 4.0;
ancient_browser msie 5.0;
ancient_browser msie 5.5;
ancient_browser msie 6.0;

# Here is how one can disable support of a specific browser
ancient_browser opera 7;
ancient_browser opera 8;
ancient_browser opera 9;
ancient_browser opera 10;

ancient_browser konqueror 3;
ancient_browser konqueror 4;

ancient_browser Links Lynx Netscape4;

#
# Now, to the fun part. If we perform a rewrite on every
# request here, images won't be displayed, so we have to
# do some extra work besides just
#
# if ($ancient_browser){
#   rewrite  ^  /unsupported_browser.html;
#   break;
# }

set $unsupported_browser_rewrite     do_not_perform;

if ($ancient_browser){
  set $unsupported_browser_rewrite   perform;
}

if ($uri !~* /\.(jpeg|jpg|png|ico|gif|js|css)$/) {
  set $unsupported_browser_rewrite   do_not_perform;
}

if ($unsupported_browser_rewrite = perform){
  rewrite  ^  /unsupported_browser.html;
  break;
}