input: Write a function to remove all HTML tags from a given string. If the input string is empty, the function should 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("<h1>Heading</h1>")=="Heading"
  assert clean_html_tags("")==""
  assert clean_html_tags("<a href='example.com'>Link</a>")=="Link"
  assert clean_html_tags("<div><p>Nested</p><span>Tags</span></div>")=="NestedTags"
output: def clean_html_tags(str):
      if not str:
          return ""
      clean_str = re.sub("<.*?>", "", str)
      return clean_str