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