Maxperson
Morkus from Orkus
Since you are able to ignore a trigger for another trigger that may or may not follow, it's usually easier just to say, "I ready my bow to shoot something coming through that doorway!" Then, every instance of that trigger is open for you to decide if you wish to use your Reaction to shoot. An ally runs in? Wait. A wimpy goblin chasing him? Maybe not. I'll save it in case something scarier comes in. Oh, look, an ogre lumbering in next? Now we're in business, FIRE!
For my games, triggers have to be specific. That's what "particular" means. If you can just use vague triggers, it reduces having a trigger to uselessness. I wouldn't allow the above trigger.
I ready an action to do X if anything happens anywhere!!!! That trigger will happen a lot and allow the PC to just pick and choose to a greater degree than I think ready action was intended for.