The code also works with flash cs3

import fl.controls.TextInput;
import fl.controls.Label;

var myLabel:Label = new Label();
myLabel.text = "Caret Index Word";
myLabel.x = 5;
myLabel.y = 0;
addChild(myLabel);

var textInput:TextInput = new TextInput();
textInput.x = 100;
textInput.y = 10;
addChild(textInput);

var t:Timer = new Timer(100);
t.addEventListener(TimerEvent.TIMER, caretindex);

function caretindex(e:TimerEvent):void {
	var caretIndex:Number = textfield.getCharIndexAtPoint(textfield.mouseX, textfield.mouseY)
	if(caretIndex == -1) {
		return;
	}
	var str:String = textfield.text;
	var words:Array = str.split(' ');
	var n:Number = 0;
	while(n<=words.length){
		var subset:Array = words.slice(0, n);
		var sliceString:String = subset.join(' ');
		if(sliceString.length>caretIndex) {
			textInput.text = String(subset[n-1]);
			break
		}
		n++;
	}
}
t.start();