I'm having trouble with this reg expression which i belive is correct, but it is not working. What im trying to do is redirect bunch of urls containing a specific string like this:

http://www.example.com/**undesired-string**_another-string to http://www.example.com/**new-string**_another-string
and
http://www.example.com/folder/**undesired-string**/another-string to http://www.example.com/folder/**new-string**/another-string

So i have this code in the .htaccess:

<IfModule mod_rewrite.c>
  RewriteEngine On
  RewriteBase /
  RewriteRule (.+)+(undesired-string)+(.+) $1new-string$2 [R=301,L]
</IfModule>

This should replace ANY undesired-string in any url to new-string, but it is not working, any idea why ?

Thank you