input: Write a function to convert an HTML string to plain text without using any libraries. The function should strip all tags and only return the plain text content.
  
  assert html_to_text("<p>Hello, <b>world</b>!</p>") == "Hello, world!"
  assert html_to_text("<h1>Title</h1><p>This is a paragraph.</p>") == "TitleThis is a paragraph."
  assert html_to_text("<html><body><p>Hello</p></body></html>") == "Hello"
  assert html_to_text("<a href='example.com'>Link</a>") == "Link"
  assert html_to_text("") == ""
output: def html_to_text(html):
      import re
      return re.sub(r'<[^>]+>', '', html)