input: Write a python function to extract ZIP codes from a string using regular expressions.
  assert extract_zip_codes("My address is 123 Main St, ZIP 12345")==['12345']
  assert extract_zip_codes("ZIP 54321 is my ZIP code, ZIP 67890 is my office ZIP")==['54321', '67890']
  assert extract_zip_codes("No ZIP code here")==[]
output: def extract_zip_codes(s):
      import re
      return re.findall(r'\b\d{5}\b', s)