Your regex would validate "////.html".
RewriteRule ^(.+?)/(.+?)/(.+?)/(.+?)/(.+?).html /products.php?addtocart=$1&iid=$2&iname=$3&iprice=$4&amount=$5
Which will validate anything.html that is in a fourth level directory (there have to be four directories above it--no more, no less).
The + means one or more of the previous character. The ? means one or zero normally, but here it stems the .+ so that it only catches one instance of it.
I'm not
too familiar with mod_rewrite, but I believe you may need the address there.
RewriteRule ^(.+?)/(.+?)/(.+?)/(.+?)/(.+?).html
http://youraddy.ext/products.php?addtoc ... &amount=$5