input: Write a function to extract all the words starting with a specific letter from a given string. The function should return a list of words that match the criteria.
  
  
  def extract_words(word, letter):
      words_list = []
      for w in word.split():
          if w[0] == letter:
              words_list.append(w)
      return words_list
  
  assert extract_words("This is a sample sentence to test the program", 's') == ["sample", "sentence", "to", "the"]
  assert extract_words("Python programming is fun", 'p') == ["Python", "programming"]
  assert extract_words("All you need is love", 'a') == ["All"]
  assert extract_words("Nothing to extract here", 'z') == []
  assert extract_words("", 'a') == []
  
output: def extract_words(word, letter):
      words_list = []
      for w in word.split():
          if w[0].lower() == letter.lower():
              words_list.append(w)
      return words_list