input: Write a function to clean a string of HTML tags. If the given string is empty then return an empty string.  
  def clean_html_tags(str):  
      if not str:  
          return ""  
      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("<html><head><title>Test</title></head><body>Hello World</body></html>") == "TestHello World"  
  assert clean_html_tags("<p>    </p>") == "    "  
  assert clean_html_tags("No HTML tags here") == "No HTML tags here"  
  assert clean_html_tags("") == ""
output: def clean_html_tags(str):  
      if not str:  
          return ""  
      clean_str = re.sub("<.*?>", "", str)  
      return clean_str