The example redirect regx formula to replace reddit.com to old.reddit.com doesn't actually work at all.
To make it work correctly, the code should be:
^https://(www\.)?reddit\.com(.*)|https://old.reddit.com$2
I'm not sure if the code is at issue, or if the issue lies with the interpreter. In any case, the example in the redirect settings page doesn't do anything for me, and never has done.
I expected it to work 😉