You can modify your regex to only match when it's not a shortened url - then the short one will redirect to the real www.reddit.com address, before the redirect matches.
(Don't have the correct regex on hand right now, as I changed browsers and decided to use Old reddit redirect extension instead of scripting, but it worked in my previous browser)
to do that you have to open the link in new reddit first to expand it, then change it to old reddit. if you use a tool that automatically replaces www.reddit.com with old.reddit.com the shortened links break.
I got permanently banned for the "Christianity is just worshipping a Jewish zombie who is his own father who will save you if you invite him into your head, symbolically drink his blood, and eat his flesh" copypasta, so not everyone can log in :)
> All the redditors on board: https://old.reddit.com/r/unitedairlines/comments/1tse6mq/ua_...
> A passenger with a hilariously illtimed username: https://old.reddit.com/r/unitedairlines/comments/1tse6mq/ua_...