input: Write a function to remove all HTML tags from a given string. If the input string is empty, return the empty string.
  def clean_html_tags(str):
      if not str:
          return 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><span>Hello</span> World</div>")=="Hello World"
  assert clean_html_tags("No HTML here")=="No HTML here"
  assert clean_html_tags("")==""
  assert clean_html_tags("<body>The <h1>title</h1> is important</body>")=="The title is important"
output: def clean_html_tags(str):
      if not str:
          return str
      clean_str = re.sub("<.*?>", "", str)
      return clean_str