input: Write a function to clean a string of HTML tags using a Python program. The function should use regular expressions to remove any HTML tags and return the cleaned string. If the input string is empty, it should return an empty 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><h1>Header</h1><p>Paragraph</p></div>")=="HeaderParagraph"
  assert clean_html_tags("<a href='example.com'>Link</a>")=="Link"
  assert clean_html_tags("")==""
output: def clean_html_tags(str):
      import re
      clean_str = re.sub("<.*?>", "", str)
      return clean_str