input: Write a function to clean a string of HTML tags using the regex method. The function should remove all HTML tags from the given string and return the cleaned 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>Heading</h1><p>Paragraph</p></div>")=="HeadingParagraph"
  assert clean_html_tags("<a href='#'>Link</a>")=="Link"
  assert clean_html_tags("No tags here")=="No tags here"
output: def clean_html_tags(str):
      clean_str = re.sub("<.*?>", "", str)
      return clean_str