input: Write a function to clean a string of HTML tags. Use regular expressions to remove all HTML tags from the input string. If the input string is empty, return the 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><p>Hello</p></div>")=="Hello"
  assert clean_html_tags("<a href=''>Link</a>")=="Link"
  assert clean_html_tags("")==""
  assert clean_html_tags("<br>")==""
output: import re
  
  def clean_html_tags(s):
      return re.sub(r'<[^>]+>', '', s)