input: Write a function to remove all HTML tags from a given string. The function should return the cleaned string. If the input string is empty, the function should return an empty string.
  assert clean_html_tags("<p>This is a <b>test</b> string</p>")=="This is a test string"
  assert clean_html_tags("<div><h1>Heading</h1><p>Paragraph</p></div>")=="HeadingParagraph"
  assert clean_html_tags("<a href='example.com'>Link</a>")=="Link"
  assert clean_html_tags("<br/>")==""
  assert clean_html_tags("")==""
output: def clean_html_tags(s):
      import re
      if not s:
          return ""
      return re.sub(r'<.*?>', '', s)