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

Matches username that is between 5 and 32 characters.
    Starts with lowercase character.
    Can contain lowercase characters, numbers and underscores.
    Can't contain spaces, underscores and hyphens.
    Can't start with "telegram", "admin" or "support".
    Ends with lowercase character or a number.