Exercise 3 solution

    import re
    s = "a [one] b [two words] c [] d"

    print(re.findall(r"(?<=\[).*?(?=\])", s))
    # ['one', 'two words', '']

    print(re.findall(r"(?<=\[).+?(?=\])", s))
    # ['one', 'two words']

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

Explanation: (?<=\[) requires an opening bracket just before the match and
(?=\]) a closing bracket just after, and neither is part of the result. The
lazy .*? stops at the first closing bracket; because * allows zero
characters, the empty brackets produce an empty string. Changing * to +
demands at least one character, so empty brackets are ignored.