Exercise 2 solution

    import re
    from collections import Counter

    WORD = re.compile(r"[a-z]+(?:'[a-z]+)?")

    def top_words(text, n=3):
        return Counter(WORD.findall(text.lower())).most_common(n)

    text = "The cat and the hat. The CAT didn't sit; the hat didn't mind."
    print(top_words(text))
    print(top_words(text, 5))
    # [('the', 4), ('cat', 2), ('hat', 2)]
    # [('the', 4), ('cat', 2), ('hat', 2), ("didn't", 2), ('and', 1)]

(All lines were run and printed exactly what is shown.)

Explanation: lower-casing first makes The and the count as one word, and the optional apostrophe part keeps didn't whole instead of splitting it.