input: Write a function to clean a string of HTML tags using regular expressions. The function should replace all HTML tags with an empty string, removing them from the input.
  import re
  
  # Clean a string of HTML tags 
  def clean_html_tags(str):
      clean_str = re.sub("<.*?>", "", str)
      return clean_str
  
  # Test cases
  assert clean_html_tags("<p>This is a <b>test</b> string</p>")=="This is a test string"
  assert clean_html_tags("<div>Example <span>with nested</span> tags</div>")=="Example with nested tags"
  assert clean_html_tags("No tags to remove")=="No tags to remove"
  assert clean_html_tags("")==""
  assert clean_html_tags("<a href=\"https://example.com\">Link</a>")=="Link"
output: def clean_html_tags(str):
      clean_str = re.sub("<.*?>", "", str)
      return clean_str