input: Write a function to clean a string of HTML tags. The function should remove all HTML tags and return the cleaned string.
  
  import re
  
  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><p>Another example</p></div>")=="Another example"
  assert clean_html_tags("<h1>Title</h1>")=="Title"
  assert clean_html_tags("<span>Span text</span>")=="Span text"
  assert clean_html_tags("")==""
output: def clean_html_tags(str):
      clean_str = re.sub("<.*?>", "", str)
      return clean_str