Вам не нужно регулярное выражение для удаления запятых и нижнего регистра:
s = "She loves you, yeah, yeah, yeah She loves you, yeah, yeah, yeah, yeah You think you lost your love Well, I saw her yesterday It's you she's thinking of And she told me what to say She says she loves you And you know that can't be bad"
s = ''.join(c.lower() for c in s if c != ',')
print(s.split())
Вывод:
['she', 'loves', 'you', 'yeah', 'yeah', 'yeah', 'she', 'loves', 'you', 'yeah', 'yeah', 'yeah', 'yeah', 'you', 'think', 'you', 'lost', 'your', 'love', 'well', 'i', 'saw', 'her', 'yesterday', "it's", 'you', "she's", 'thinking', 'of', 'and', 'she', 'told', 'me', 'what', 'to', 'say', 'she', 'says', 'she', 'loves', 'you', 'and', 'you', 'know', 'that', "can't", 'be', 'bad']