input: Write a function to clean a string of HTML tags by removing all HTML tags using regular expressions. Return the cleaned string with only the plain text content.
  
  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("<html><body><h1>Title</h1></body></html>") == "Title"
  assert clean_html_tags("<a href='example.com'>Example</a>") == "Example"
  assert clean_html_tags("No tags here") == "No tags here"
  assert clean_html_tags("") == ""
output: def clean_html_tags(str):
      clean_str = re.sub("<.*?>", "", str)
      return clean_str