code: def username_validator(self, form, field):
          
          username = field.data
          if len(username) < 3:
              raise ValidationError(
                  _('Username must be at least 3 characters long'))
          valid_chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-._'
          chars = list(username)
          for char in chars:
              if char not in valid_chars:
                  raise ValidationError(
                      _("Username may only contain letters, numbers, '-', '.' and '_'"))
docs: Ensure that Usernames contains at least 3 alphanumeric characters.
  
          Override this method to customize the username validator.