It sets the 'case-insensitive' flag for the remaining pattern

what's (?i) for?

/** @exception NullPointerException if <code>input</code> is null. */

One thing is missing. Null input check.

You're (hehe) actually right about this one, but I think this concerns almost all other solutions as well, since it was not specified in the kata description that contractions should be respected. Also, regarding the kata rank, I suppose that it was not intended either. Translating "you're" to "your sister is" would require a much more complex regex/algorithm.

I think you're will error handling into your sister're

Very sophisticated solution! I was not aware of "\b" yet, so I definitely learned something new. thx!