replace directory string in url

Hi folks,

i searched several hours google and other places to solve my problem
but till now i do not find any solution for that.

Scenario:

I use Apache as webserver and my files lie in the directory DIR1 and
above. I have also files and directories in the root directory of
apache webdocs.

My problem:

If i would get the following URL http://www.example.de/DIR1/page1.php
Apache should rewrite the URL in the Browser to
http://www.example.de/page1.php.

I can not change my DocumentRoot directive. If so i did not have a
problem because this way is very easy to realize. I try to solve my
problem with the Alias directive but in this case the URL in the
Browser did not change.

Is it possible to replace only the /DIR1/ string in the URL, put them
to the browser but inside apache is using the directory /var/www/DIR1?

Thanks.

Stefan