docs: def is_email(string):
      """
      >>> is_email('username@example.com')
      True
      >>> is_email('example.com')
      False
      >>> is_email('firstname.lastname@domain.co.uk')
      True
      """
      email_regex = r'^[A-Za-z0-9\.\+_-]+@[A-Za-z0-9\._-]+\.[a-zA-Z]*$'
  
      if isinstance(string, str) and not re.match(email_regex, string):
          return False
      else:
          return True