Exercise 3 solution

    import re
    pat = re.compile(r"""
        ^\s*
        (?P<h>\d{1,2})      # hours
        :
        (?P<m>\d{2})        # minutes
        (?:                 # optional seconds
            :(?P<s>\d{2})
        )?
        \s*(?P<ap>am|pm)?   # optional am/pm
        \s*$
    """, re.X | re.I)

    for t in ["9:05", "14:30:59", "9:05 PM", "9:5", "25:61x"]:
        m = pat.match(t)
        print(repr(t), m.groupdict() if m else None)

    # '9:05' {'h': '9', 'm': '05', 's': None, 'ap': None}
    # '14:30:59' {'h': '14', 'm': '30', 's': '59', 'ap': None}
    # '9:05 PM' {'h': '9', 'm': '05', 's': None, 'ap': 'PM'}
    # '9:5' None
    # '25:61x' None

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

Explanation: "9:5" fails because minutes need exactly two digits. "25:61x"
fails because of the trailing "x": after the optional parts the pattern
allows only spaces and then requires the end of the string. (The pattern checks
shape only; it would accept 25:61 if the x were absent, which is why real
validation needs a range check as well, covered in Regular Expressions in
Practice 7.)