input: Write a function to clean a string of HTML tags by removing all HTML tags from the given string. 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("<a href='#'>Link</a>")=="Link"
  assert clean_html_tags("No tags to clean")=="No tags to clean"
  assert clean_html_tags("<div><span><b>Nested</b> tags</span></div>")=="Nested tags"
  assert clean_html_tags("")==""
output: def clean_html_tags(str):
      clean_str = re.sub("<.*?>", "", str)
      return clean_str