what he wants wont be doable with arrays because he wants to swap values, and that isnt how array-replacing works.
What he would get out is 2 lefts.
Why? Because it would be an iterative replacement.
Replace all lefts with rights.
Replace all rights with lefts.
Result: 2 lefts.
Now, you could do it with two preg_replace's; one with an array pattern replacement; the second which undoes the go-between step;