input: Write a function to clean a string of HTML tags. The function should remove all HTML tags and return the cleaned string.
  
  ### Function:
  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("<p>Another <i>example</i> with <br> tags</p>")=="Another example with  tags"
  assert clean_html_tags("<h1>Heading level 1</h1>")=="Heading level 1"
  assert clean_html_tags("<a href=\"www.example.com\">Link</a>")=="Link"
  assert clean_html_tags("")==""
output: def clean_html_tags(str):
      clean_str = re.sub("<.*?>", "", str)
      return clean_str