Exercise 2 solution

    import re

    def parse_time(t):
        m = re.fullmatch(r"(?P<h>\d{1,2}):(?P<m>\d{2})(?::(?P<s>\d{2}))?", t)
        if not m:
            return None
        return {k: (int(v) if v else 0) for k, v in m.groupdict().items()}

    for t in ["9:05", "14:30:59", "9:5", "1:2:3"]:
        print(t, parse_time(t))
    # 9:05 {'h': 9, 'm': 5, 's': 0}
    # 14:30:59 {'h': 14, 'm': 30, 's': 59}
    # 9:5 None
    # 1:2:3 None

(The code was run and printed exactly the lines shown.)

Explanation: the seconds are wrapped in an optional group, (?::(?P<s>\d{2}))?.
The outer (?: ... ) is non-capturing and only groups the colon with the
digits so that the ? can apply to both. When the seconds are absent, the
named group "s" is None, so groupdict() holds None for it and the
"if v else 0" turns that into 0. fullmatch rejects "9:5" and "1:2:3"
because they do not fit the whole pattern.