CP Users: username validation rules

Hello,
When creating or editing a CP user (System → CP Users), the following error message may appear:

Could you please clarify the exact rules applied?
—What is the maximum allowed length for a CP username?
—Which characters are allowed (and, if applicable, forbidden)?
—Are these rules identical for all ‘Username’ (username_txt) fields in ISPConfig?
If applicable, are these constraints defined:
—at the database schema level,
—or in the application code (and where)?
Thank you.

 

In
/usr/local/ispconfig/interface/lib/classes/functions.inc.php,
the function is_allowed_user() is used to validate system users (system_user):

Code:

public function is_allowed_user($username, $restrict_names = false) {
    $name_blacklist = array('root','ispconfig','vmail','getmail');
    if(in_array($username,$name_blacklist)) return false;
    if(preg_match('/^[a-zA-Z0-9\.\-_]{1,32}$/', $username) == false) return false;
    if($restrict_names == true && preg_match('/^web\d+$/', $username) == false) return false;
    return true;
}

Is this same function also used to validate the Username field of CP Users (sys_user.username)?
If not, where is the validation specific to this field implemented (file / function)?