input: def html_to_text(html_string: str) -> str:
      """
      Convert a given HTML string to plain text by removing all HTML tags.
  
      >>> html_to_text("<p>This is a test.</p>")
      'This is a test.'
      >>> html_to_text("<p><strong>Bold text</strong> within a paragraph.</p>")
      'Bold text within a paragraph.'
      >>> html_to_text("<p>No tags here.</p>")
      'No tags here.'
      >>> html_to_text("<h1>Header</h1><p>Paragraph</p>")
      'HeaderParagraph'
      >>> html_to_text("")
      ''
      """
output:     import re
      return re.sub(r'<[^>]+>', '', html_string)