input: Write a python function to find the most common word in a given string.
  assert most_common_word("This is a test. This test is only a test.") == "test"
  assert most_common_word("apple orange apple banana orange orange") == "orange"
  assert most_common_word("Python is great, and I love Python.") == "python"
output: def most_common_word(s):
      import re
      from collections import Counter
  
      words = re.findall(r'\b\w+\b', s.lower())
      word_counts = Counter(words)
      most_common = word_counts.most_common(1)
  
      return most_common[0][0] if most_common else None