input: Write a python function to check if a sentence follows a specific grammar rule (a noun phrase). 
  assert check_grammar("The quick brown fox") == False
  assert check_grammar("A cat") == True
  assert check_grammar("An apple") == True
output: def check_grammar(sentence):
      import re
      pattern = r'^(A|The|An) [a-z]+$'
      return bool(re.match(pattern, sentence))