ActionScript 1/2 :: Restrict Characters In A Dynamic Text Field?

The following code does what I want (i.e. any key I press on the keyboard displays the character in uppercase format in the dynamic field on the stage) except that it does not properly restrict the characters to A-Z and 0-9.  I can still press the <,.>?!/ keys and each of these chars shows up in the dynamic text field  My dynamic text field is txt_BoxWhat is wrong with the following code?

var varKeyListener:Object = new Object();varKeyListener.onKeyDown = function(){  txt_Box.restrict = "A-Z 0-9";  txt_Box.text = String.fromCharCode(Key.getAscii());  txt_Box.text = txt_Box.text.toUpperCase();}

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