input: Write a function to convert an HTML string to plain text without using any libraries. The function should ignore all HTML tags and only extract the text content between them. Return the extracted plain text as a string.
  assert html_to_text("<p>Hello, <b>world</b>!</p>") == "Hello, world!"
  assert html_to_text("<h1>Heading 1</h1>") == "Heading 1"
  assert html_to_text("<a href='https://example.com'>Link</a>") == "Link"
  assert html_to_text("<div>Content<span>inner</span></div>") == "Contentinner"
  assert html_to_text("") == ""
output: def html_to_text(html):
      import re
      return re.sub(r'<[^>]+>', '', html)