Let visitors add codes on their comments

I’ve seen a few sites where visitors use < code> and even < pre>.

I tested it in my site and I’m able to do so in comments, but when visitors are not. I added code and pre into $allowedtags together with lang attribute and now they are able to, but some stuff are still fcked up, as => turns into =>

How can that be accomplished without removing the security of filtering comments code?