Fun kata, i think "proper spacing" should be clarified something like "space between* per word" for clearity. (Actually that was clarified i didn't read it awsome)

What to do when "quack" is part of a word?

Punctuation always makes things tricky, because it makes it difficult to define what is a word, and what is a proper spacing. For example, for input "Hello quack world quack!" it is not immediately clear how the final exclamation mark should be handled, because without proper spec, there is many possibilities:

• "quack!" is not a "quack", so it should be preserved,
• "quack" is removed and ! stays as what's left of the word, leaving us with "... world !",
• it's not a part of a word and should be handled separately from words,
• it's not even clear what punctuation marks are.

Is "Hello, quack-quack! world!" a valid input? What should be the answer?

You should clarify whether a sentence can begin with "quack" and whether it's capitalized or not.