No you can't catch darkrai in ruby unless you hack (witch i do not recommend)
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You can't catch a sentret in Ruby version, but you can catch one in Emerald version.
Darkrai is a dark type of Pokemon. You can catch one in Pokemon Diamond version on Newmoon Island but you need a membership card.
use poke balls on it
You cannot catch darkrai in any generation three game even WITH the use of a cheating device. This is because they were not created yet. Darkrai is only obtainable in Generation 4 games through a Nintendo event or a cheating device.
no you have to migrate it from either Sapphire or ruby version hope i helped