input: Write a function to extract all the words starting with a specific letter from a given string. The function should take two parameters: the string of words and the letter to match. It should return a list of words that start with the given letter.
  
  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', 's']
  assert extract_words('Python programming is fun', 'p') == ['Python', 'programming', 'programming']
  assert extract_words('Hello world', 'H') == ['Hello']
  assert extract_words('', 'a') == []
  assert extract_words('No matches here', 'x') == []
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