Your readied action sounds fine. You could still make AoO as normal and do not lose your readied action if you make an AoO.
You don't need to name the opponent for your readied action; it is adequate to just say "the first foe..." or even "the first armed foe" or "the first spellcasting foe" or "the first for with a reach weapon" or whatever you like - the more specific you are, the more likely you are that it never happens and you wasted your readied action, so try to keep it simple.
So your readied action to trip the first foe to enter your designated 5x5 area is just fine.
AoOs never prevent your own actions, even readied ones.
You don't need to name the opponent for your readied action; it is adequate to just say "the first foe..." or even "the first armed foe" or "the first spellcasting foe" or "the first for with a reach weapon" or whatever you like - the more specific you are, the more likely you are that it never happens and you wasted your readied action, so try to keep it simple.
So your readied action to trip the first foe to enter your designated 5x5 area is just fine.
AoOs never prevent your own actions, even readied ones.
Blake is correct.
I'm curious, though.
Say enemy passed within the 5x5 area you designated triggering your readied action. You step up and strike them. Does the enemy have to attempt to continue moving towards his destination or can he stop his movement there and take the rest of his turn?
I think he can stop if my memory of the movement rules is correct, but this is a situation that does not often come up.
Hydromancer,
Your question is debated in many forms on this forum. There is no clear consensus at this time.
I thought all the debate was around if your readied action had a trigger where there's a transition from movement to another action (attack) for your foe and your action was to attack and take a step back if the foe could continue moving or his attack is wasted since you're not there.
In Pathfinder you don't have to declare where you're moving to during your turn. You can just start moving your mini and let what happens happen to my knowledge. I suppose the exception here could be if the foe declared a charge.
In the instance here (says he's just moving, so move action to move I'd assume) I think it's pretty clear the foe can continue his movement as he desires, if he's not completed one full move action he could try to attack assuming he can reach you.
Now obviously if you trip him he's boned since standing is a move action and at his point he'll have already used at least one move action to move.
Anyway, not trying to derail this thread.
Samas and DM Blake have covered the OP's question well.
Of course he can stop. There should be no debate.
It's not like he's a robot who received move orders in his program, code that forces him to move, mechanically, unthinkingly, and unresponsively from x to y with no intelligence to alter that program.
If he's moving, and something on the battlefield changes, he can change his mind and end his movement before he reaches his destination. He doesn't get his move action back; that was still spent, but he is in no way forced to finish his movement.
That would be silly.
My favorite readied action: to trip an opponent I threaten if they are no longer prone.
Of course he can stop. There should be no debate.
It's not like he's a robot who received move orders in his program, code that forces him to move, mechanically, unthinkingly, and unresponsively from x to y with no intelligence to alter that program.
If he's moving, and something on the battlefield changes, he can change his mind and end his movement before he reaches his destination. He doesn't get his move action back; that was still spent, but he is in no way forced to finish his movement.
That would be silly.
There is a large debate about it. You presented a very limited case. I would suggest debating it further elsewhere as it will quickly overpower the original question.