input: Write a function to clean a string of HTML tags using regular expressions. The function should remove all HTML tags from the given string and return the cleaned string. If the input string is empty, 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("Hello <a href='url'>world</a>!") == "Hello world!"
  assert clean_html_tags("<div>Some content</div>") == "Some content"
  assert clean_html_tags("") == ""
  assert clean_html_tags("<p>No tags here</p>") == "No tags here"
output: def clean_html_tags(str):
      if not str:
          return ""
      clean_str = re.sub("<.*?>", "", str)
      return clean_str