input: Write a function to clean a string of HTML tags using Python regular expressions. The function should take a string as input and return a cleaned version of the string without any HTML tags.
  
  ### Function Signature:
  def clean_html_tags(str):
  
  ### Example 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>Hello <span>World</span></div>")=="Hello World"
  assert clean_html_tags("<h1>Title</h1><p>Paragraph</p>")=="TitleParagraph"
  assert clean_html_tags("")==""
  assert clean_html_tags("<br>")==""
  assert clean_html_tags("No tags here.")=="No tags here."
output: import re
  
  def clean_html_tags(str):
      return re.sub(r'<[^>]+>', '', str)