input: Write a function to remove all HTML tags from a given string. The function should return the cleaned string without any HTML tags.
  
  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("<a href='#'>Link</a>")=="Link"
  assert clean_html_tags("<br>")==""
  assert clean_html_tags("")==""
output: def clean_html_tags(str):
      clean_str = re.sub("<.*?>", "", str)
      return clean_str