input: Write a function to clean a string of HTML tags. The function should use regular expressions to remove all HTML tags from the input string.
  
  def clean_html_tags(str):
      clean_str = re.sub("<.*?>", "", str)
      return clean_str
  
  assert clean_html_tags("<p>This is a <b>test</b> string</p>") == "This is a test string"
  assert clean_html_tags("<div><h1>Title</h1><p>Paragraph</p></div>") == "TitleParagraph"
  assert clean_html_tags("") == ""
  
output: def clean_html_tags(str):
      clean_str = re.sub("<.*?>", "", str)
      return clean_str