input: Write a python function to match strings that start with exactly two letters followed by exactly two digits.
  assert match_string("ab12") == True
  assert match_string("a12") == False
  assert match_string("abc123") == False
  assert match_string("XY99") == True
output: def match_string(s):
      import re
      pattern = r'^[a-zA-Z]{2}\d{2}$'
      return bool(re.match(pattern, s))