Exercise 3 solution

    import re

    lines = [
        "10:00 user=ann ip=10.0.0.5 action=login",
        "10:01 user=bob ip=192.168.1.20 action=logout",
        "10:02 action=ping",
    ]
    for l in lines:
        print(re.sub(r"(ip=)\d{1,3}(?:\.\d{1,3}){3}", r"\1x.x.x.x", l))
    # 10:00 user=ann ip=x.x.x.x action=login
    # 10:01 user=bob ip=x.x.x.x action=logout
    # 10:02 action=ping

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

Explanation: group 1 captures the literal "ip=", and \1 in the replacement
puts it back, so only the address is replaced. The address itself is matched
by a non-capturing group repeated three times after the first number. A line
with no "ip=" has no match, so re.sub returns it unchanged.