This is the regex pattern. As you can see, @ works, but is by itself, I’d like to allow for the # symbol along with a custom h/ symbol if possible. h/ would work similar to reddit insofar as r/askreddit becomes a hyper-link.
there are 3 groups in the expression, you need to modify your replacement accordingly (you want $3 somewhere). The quantity modifier at the end is for exactly 3 characters, you can add a coma after the 3 to make it from 3 chars to the end of the word