I agree with Ralkage, it's really dangerous to let lambda users add bbcodes that way.
If the user/admin is able to understand what the regex does and understand the risks, I expect him to be able to fork a simple Flarum extension to add its custom bbcode. It's not that hard, Flarum makes it very easy.
At the moment it's also easy to find someone here on Discuss to update an existing extension to add such requirements ?