input: def clean_html_tags(html_str: str) -> str:
      """ Remove all HTML tags from a given string.
      >>> clean_html_tags("<p>This is a <b>test</b> string</p>")
      'This is a test string'
      >>> clean_html_tags("<div><h1>Title</h1><p>This is a paragraph.</p></div>")
      'TitleThis is a paragraph.'
      """
output:     import re
      return re.sub(r'<[^>]+>', '', html_str)