input: Write a function to clean a string of HTML tags using regular expressions. The function should remove all HTML tags and return the cleaned string. If the input string does not contain any HTML tags, the function should return the original string.
  assert clean_html_tags("<p>This is a <b>test</b> string</p>")=="This is a test string"
  assert clean_html_tags("This is a simple string")=="This is a simple string"
  assert clean_html_tags("")==""
output: import re
  
  def clean_html_tags(s):
      return re.sub(r'<[^>]+>', '', s)