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

The regex matches words that are between 5 and 32 characters long.
    The regex rejects words that contain two consecutive underscores.
    The regex rejects words that start with "admin", "telegram", or "support".
    The regex matches words that begin with a letter.
    The regex matches words that end with a letter or number.
    The regex matches words that contain only letters, numbers, or underscores.
    The regex matches no uppercase letters.