input: Write a python function to extract all URLs from a given string using regular expressions.
  assert extract_urls("Check out this link: http://example.com and this one: https://example.org") == ["http://example.com", "https://example.org"]
  assert extract_urls("No URLs here!") == []
  assert extract_urls("Visit http://test.com and http://another-test.com") == ["http://test.com", "http://another-test.com"]
output: def extract_urls(text):
      import re
      pattern = r'https?://\S+'
      return re.findall(pattern, text)