ActionScript 2.0 :: Alphanumeric Keypad Input (T9) In TextArea

I have a project that involves taking user input from a keyboard/numeric keypad. The script I wrote detects when the user actually enters a numeral in the text area, and replaces it with step-by-step with a series of letters and numbers in a similar fashion to keypad input on a phone. Everyone else's solution assumes that the user is A.) On an actual mobile device, and B.) That the LAST character is the one that should be replaced in the series.

Whereas, in my case, the user can use standard keyboard/numeric keypad input. Moreover, the caret should be able to move to any arbitrary position within a given string of text, and not just to the end of that string. My script works for the most part, except for some reason, the caret advances on its own without me/the user telling it to, and from what I can see, there's nothing in the code to allow for this:

Code:
var _1:String = "1@._-";
var _2:String = "2abc";
var _3:String = "3def";
var _4:String = "4ghi";
var _5:String = "5jkl";
[Code] .....

Lastly; my code does not account for when a user presses a DIFFERENT number than the one that they are currently pressing--in which case the caret should naturally advance automatically to allow for a new numeral to be inserted. My example is attached.