input: Write a function to extract all words that start with a specific letter from a given string. If no words start with the specified letter, return an empty list.
  
  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', 'test', 'the', 'program']
  assert extract_words('This is a test', 't')==['test']
  assert extract_words('No matching words', 'a')==[]
  assert extract_words('', 's')==[]
  assert extract_words('Python programming is fun', 'p')==['Python', 'programming']
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