Exercise 1: A Real Slug Converter — Possible Solution
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ADDING THE CONVERTER
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    CONVERTERS['slug'] = (r'[a-z0-9]+(?:-[a-z0-9]+)*', str)

    router = Router()
    router.add_route(
        'GET', '/posts/<slug:title>',
        lambda **p: f"Post: {p['title']}", name='post_detail',
    )

The regex requires the whole segment to be one or more runs of
lowercase letters/digits, each run after the first separated by a
single hyphen - it rejects a leading/trailing hyphen, a double
hyphen, and any uppercase letter, space, or punctuation outright.

VERIFIED AGAINST BOTH REAL PATHS
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    /posts/hello-world-42  -> MATCH  Post: hello-world-42
    /posts/Hello World!    -> 404    (no route's pattern matched)

WHY THIS WORKS AS AN ANSWER
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It reuses the exact (regex, cast) shape every other CONVERTERS entry
already follows, so Router._compile() needs no changes at all to pick
it up - and it verifies both a real accept and a real reject on the
finished route, not just that the regex compiles, confirming the
slug converter actually behaves the way a URL slug is supposed to.