input: Write a function to clean a string of HTML tags and return the cleaned string. Use regular expression to remove all HTML tags. If the input string is empty, 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("Hello <a href='link'>world</a>!")=="Hello world!"
  assert clean_html_tags("This string has no tags.")=="This string has no tags."
  assert clean_html_tags("")==""
  assert clean_html_tags("<div><span>Nested tags</span></div>")=="Nested tags"
output: import re
  
  def clean_html_tags(s):
      if not s:
          return ""
      return re.sub(r'<[^>]+>', '', s)