input: def html_to_text(html_string: str) -> str:
      """ Converts HTML to plain text without using any libraries.
      >>> html_to_text('<p>Hello, <span>world!</span></p>')
      'Hello, world!'
      >>> html_to_text('<title>This is a title</title><body>Content here</body>')
      'This is a titleContent here'
      >>> html_to_text('<div><h1>Header</h1><p>Some text</p></div>')
      'HeaderSome text'
      """
output:     
      import re
  
      def remove_tags(match):
          return match.group(1)
  
      text = re.sub(r'<[^>]+>([^<]+)</[^>]+>', remove_tags, html_string)
      text = re.sub(r'<[^>]+>', '', text)
      return text
  