Doomsday and library searching

[TheShaun]

My opponent casts Doomsday and in response I vial in Leonin Arbiter. Doomsday oracle reads:

Search your library and graveyard for five cards and exile the rest. Put the chosen cards on top of your library in any order. You lose half your life, rounded up.

So my understanding is that he would select 5 cards from his graveyard (what if there are less than 5), exile his library and the rest of his graveyard, and then put the selected cards on top of the library in any order. So I'm wondering if this is correct assuming there are at least 5 cards in the graveyard, and also what happens if there are less than 5? The card doesn't say "up to 5", so I'm a bit confused.

Edit: This is assuming that he doesn't have the 2 mana to pay for the search, of course.

[cdr]

Per Doomsday, he must find five cards. If he can't search his library, they have to come from the graveyard. If there are fewer than five cards left in the zones he can search, he takes as many as there are available.

If Doomsday said "up to five" that would mean the player got choose the number to search for. It doesn't, so you have to take five if you can, but effects always do as much as possible when the full effect is impossible. That's the same reason you discard your three card hand when you get hit with an effect that says "discard five cards".