I use mod_rewrite a a fair amount, and for something like this, I'd do:

RewriteRule ^/(news|ftp|http)/([^/]+)/([\d]+)$ \ 
            /index.pl?type=$1&feed=$1://$2&limit=$3
[download]

I could be wrong about this, but the reason in my mind for using this little bit of voodoo is that .* will match from the second character to the end of the uri, leaving $2 and $3 undefined. ([^/]+)/([^/]+) is analogous to using (\w+)\W+(\w+) to pick out 2 series of non-spaces bracketing a length of whitespace.