input: Write a function to clean a string of HTML tags using regular expressions. The function should return the cleaned string without any HTML tags. If the input string is already clean, it should return the same string.
  assert clean_html_tags("<p>This is a <b>test</b> string</p>")=="This is a test string"
  assert clean_html_tags("<div>Another example with <a href='#'>link</a></div>")=="Another example with link"
  assert clean_html_tags("Plain text without tags")=="Plain text without tags"
  assert clean_html_tags("")==""
  assert clean_html_tags("<br>")==""
output: import re
  
  def clean_html_tags(text):
      return re.sub(r'<.*?>', '', text)