mod_rewrite - how to create a rule based on first letter of a directory?

Hello,

I am a mod_rewrite newbie.  Can someone help me create a rule, perhaps using a map, to change urls based upon the first letter of the first directory after the server root?

For example,
http://www.server.com/apple/index.html gets mapped to /contenta/apple/index.html
and
http://www.server.com/apple/slice/index.php is mapped to /contenta/apple/slice/index.php

similarly,
http://www.server.com/bananna/slice/peel/index.php maps to /contentb/bananna/slice/peel/index.php
and
http://www.server.com/bananna/index.php  to /contentb/bananna/index.php

I hope that is clear ... files off the root such as http://www.site.com/index.html would also get their own mapping to /content_index or something like that.

Thank you for the help!