input: Write a function to remove all HTML tags from a given string. If the string is empty, return an empty string.
  def clean_html_tags(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("")==""
  assert clean_html_tags("<div><h1>Title</h1><p>Paragraph</p></div>")=="TitleParagraph"
output: def clean_html_tags(str):
      import re
      clean_str = re.sub("<.*?>", "", str)
      return clean_str