[.@](?=.{5,32}$)(?!.*__)(?!^(telegram|admin|support))[a-z][a-z0-9_]*[a-z0-9]$

The regex matches all words that start with "." or "@" and contain at least one lowercase letter. The word should be exactly 5-32 characters long.
Well, the word can't contain "__" substring, and its first two characters can't be "telegram", "admin" or "support".