mod_rewrite: redirect to another directory with additional parameters

Mod_rewrite is one of the hardest pieces of apache to get right: the documentation is awful, there is no way to test it and the syntax is arcane. All I wanted to do was redirect URLs from one directory to another, copying the existing GET parameters and adding some more. But there are a number of tricks to get it right. So my original URLs look like this:

http://me.org/mycms/olddir?param1=foo

And I wanted to redirect it to:

http://me.org/mycms/newdir?param1=foo&param2=poo&param3=roo

An additional requirement was that I wanted to add the rule to .htaccess in the CMS-directory, not root, since that's where the rules applied. The following rule worked in the "mycms" directory:

  RewriteEngine on
  RewriteRule ^olddir$ /mycms/newdir?param2=poo&param3=roo  [L,R,QSA]
  ... (other rules)

Since I was using Drupal, the <IfModule... was already present in the existing .htaccess file. So all I needed to do was add the correct RewriteRule. To explain what I understand by this: "^" means a URL starting with "mydir" since we are already in mycms (that's where the .htaccess file is), and ending ($) there. So only parameters follow. Now redirect to /mycms/newdir. If you don't start with "/" it will prepend the entire server path. Next, add your desired parameters, and finally append the existing ones via the QSA flag, which means "query string append". The "R" flag is needed because this is a redirect. The "L" flag is needed because otherwise other rules might be applied and this is the "last" rule we need here. I'm not sure all this is correct, but it is simple and it works. So I'm sticking with it.