input: Write a python function to extract all the href links from a given HTML string.
  assert extract_links('<a href="https://example.com">Example</a>') == ['https://example.com']
  assert extract_links('<a href="https://example1.com">Example1</a><a href="https://example2.com">Example2</a>') == ['https://example1.com', 'https://example2.com']
  assert extract_links('<div><a href="https://example3.com">Example3</a></div>') == ['https://example3.com']
output: def extract_links(html):
      import re
      return re.findall(r'href="(https?://[^"]+)"', html)