input: Write a function to clean a string of HTML tags using regular expressions. The function should remove all HTML tags from the input string and return the cleaned string.
  
  def clean_html_tags(str):
      clean_str = re.sub("<.*?>", "", str)
      return clean_str
  
  assert clean_html_tags("<p>This is a <b>test</b> string</p>")=="This is a test string"
  assert clean_html_tags("<div>Example with <a href='#'>link</a></div>")=="Example with link"
  assert clean_html_tags("<br>")==""
  assert clean_html_tags("No tags here")=="No tags here"
output: import re
  
  def clean_html_tags(str):
      clean_str = re.sub("<.*?>", "", str)
      return clean_str