input: Write a function to clean a string of HTML tags using regex. If the given string is empty then 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("<html><head><title>Title</title></head></html>")=="Title"
  assert clean_html_tags("")==""
  assert clean_html_tags("<p>Hello, <a href='example.com'>world</a>!</p>")=="Hello, world!"
output: def clean_html_tags(s):
      import re
      if not s:
          return ""
      return re.sub(r'<[^>]+>', '', s)