input: Write a function to clean a string of HTML tags. The function should remove all tags from the input string and return the cleaned string.
  
  
  import re
  
  # Clean a string of HTML tags 
  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>Hello, <span>world</span>!</div>")=="Hello, world!"
  assert clean_html_tags("<h1>Title</h1><p>Content</p>")=="TitleContent"
  assert clean_html_tags("<a href='#'>Link</a>")=="Link"
  assert clean_html_tags("")==""
  
output: def clean_html_tags(str):
      clean_str = re.sub("<.*?>", "", str)
      return clean_str