Exercise 1 solution

    import re

    time_re = re.compile(r"(\d{2}):(\d{2})")

    def valid_time(s):
        m = time_re.fullmatch(s)
        return bool(m) and int(m[1]) <= 23 and int(m[2]) <= 59

    for s in ["09:30", "23:59", "24:00", "12:60", "9:30"]:
        print(s, valid_time(s))
    # 09:30 True
    # 23:59 True
    # 24:00 False
    # 12:60 False
    # 9:30 False

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

Explanation: the pattern checks the shape (two digits, a colon, two digits)
and fullmatch makes it cover the whole string. The ranges are then checked
with ordinary comparisons, which is far simpler than encoding 00-23 and
00-59 in the pattern. "bool(m) and ..." stops early when there is no match,
so m[1] is never used on None.