input: Write a function to validate phone numbers in the format xxx-xxx-xxxx.
  assert is_valid_phone_number("123-456-7890") == True
  assert is_valid_phone_number("123-45-7890") == False
  assert is_valid_phone_number("123-4567-890") == False
  assert is_valid_phone_number("123-456-789a") == False
  assert is_valid_phone_number("123-456-78901") == False
output: def is_valid_phone_number(phone_number):
      import re
      pattern = re.compile(r'^\d{3}-\d{3}-\d{4}$')
      return bool(pattern.match(phone_number))