input: Write a function to remove all HTML tags from a given string using the regular expression module re. The function should return the cleaned string, which contains no HTML tags. If the input is empty, the function should return an empty string.
  
  assert clean_html_tags("<p>This is a <b>test</b> string</p>")=="This is a test string"
  assert clean_html_tags("<h1>Heading</h1>")=="Heading"
  assert clean_html_tags("")==""
  assert clean_html_tags("This is an untagged string")=="This is an untagged string"
  assert clean_html_tags("<a href='example.com'>Link</a>")=="Link"
output: import re
  
  def clean_html_tags(text):
      if not text:
          return ""
      return re.sub(r'<[^>]+>', '', text)