input: Write a python function to determine the most common word in a given string.
  assert most_common_words('Hello world. This is a sentence. This is another sentence.') == 'This'
  assert most_common_words('test test test test test') == 'test'
  assert most_common_words('apple banana apple orange apple banana') == 'apple'
output: def most_common_words(text):
      from collections import Counter
      import re
  
      words = re.findall(r'\b\w+\b', text.lower())
      most_common = Counter(words).most_common(1)
      return most_common[0][0] if most_common else None