How to encode special characters using mod_rewrite & Apache?

I would like to have pretty URLs for my tagging system along with all the special characters: +, &, #, %, and =. Is there a way to do this with mod_rewrite without having to double encode the links?

I notice that delicious.com and stackoverflow seem to be able to handle singly encoded special characters. What's the magic formula?

Here's an example of what I want to happen:

http://www.example.com/tag/c%2b%2b

Would trigger the following RewriteRule:

RewriteRule ^tag/(.*)   script.php?tag=$1

and the value of tag would be "c++"

The normal operation of apache/mod_rewrite doesn't work like this, as it seems to turn the plus signs into spaces. If I double encode the plus sign to '%252B' then I get the desired result - however it makes for messy URLS and seems pretty hack to me.