input: Write a function that takes a string containing HTML tags as input and removes all HTML tags from it, returning the cleaned string. If the input string is empty, it should return an empty string. Use regular expressions to match and remove the HTML tags.
  
  def clean_html_tags(html_str):
      if not html_str:
          return ""
      cleaned_str = re.sub(r"<.*?>", "", html_str)
      return cleaned_str
  
  assert clean_html_tags("<p>This is a <b>test</b> string</p>") == "This is a test string"
  assert clean_html_tags("<div>Another example <span>with nested tags</span></div>") == "Another example with nested tags"
  assert clean_html_tags("<a href='example.com'>Link</a>") == "Link"
  assert clean_html_tags("") == ""
output: def clean_html_tags(html_str):
      if not html_str:
          return ""
      cleaned_str = re.sub(r"<.*?>", "", html_str)
      return cleaned_str