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

(?=.{5,32}$) checks if the length of a username is greater than 5 and less than 32.
(?!.*__) checks if the username does not contain two consecutive underscores.
(?!^(telegram|admin|support)) checks if the username does not start with "telegram", "admin", or "support".
[a-z][a-z0-9_]*[a-z0-9]$ checks if the username only contains lowercase alphanumeric characters and underscores and must begin and end with alphanumeric characters.