input: Write a python function to extract numbers from a string using regular expressions.
  assert extract_numbers("The 3 monkeys had 2 apples")==['3', '2']
  assert extract_numbers("There are 15 cats, 23 dogs, and 42 birds")==['15', '23', '42']
  assert extract_numbers("No numbers here!")==[]
output: def extract_numbers(s):
      import re
      return re.findall(r'\d+', s)