input: Write a function to extract URLs from a given string using regular expressions.
  assert extract_urls("There is a website: https://example.com") == ["https://example.com"]
  assert extract_urls("Check these links: http://test.com, https://example.org") == ["http://test.com", "https://example.org"]
  assert extract_urls("No URLs here!") == []
output: def extract_urls(text):
      import re
      url_pattern = re.compile(r'https?://\S+')
      return url_pattern.findall(text)