How to write a generic .htaccess file rule, regarding RewriteBase

Trying the same and on my search I found that symfony2 frameworks .htaccess file is doing that at least it is stated in the file that it would. I tried to use it in my own setup but didn't work. Maybe that will help you guys somehow.

# Determine the RewriteBase automatically and set it as environment variable.
# If you are using Apache aliases to do mass virtual hosting or installed the
# project in a subdirectory, the base path will be prepended to allow proper
# resolution of the app.php file and to redirect to the correct URI. It will
# work in environments without path prefix as well, providing a safe, one-size
# fits all solution. But as you do not need it in this case, you can comment
# the following 2 lines to eliminate the overhead.

RewriteCond %{REQUEST_URI}::$1 ^(/.+)/(.*)::\2$
RewriteRule ^(.*) - [E=BASE:%1]