input: Write a function to clean a string of HTML tags using regular expressions. The function should strip all HTML tags from the input string and return the cleaned string. 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("<a href='link'>Click here</a>")=="Click here"
  assert clean_html_tags("")==""
  assert clean_html_tags("<p></p>")==""
  assert clean_html_tags("No tags here")=="No tags here"
output: import re
  
  def clean_html_tags(s):
      if not s:
          return ""
      return re.sub(r'<[^>]+>', '', s)