btnCheck.addEventListener(MouseEvent.CLICK, checkPostalCode);
txtinPostalCode.addEventListener(MouseEvent.CLICK, clearLabels);
function checkPostalCode(e:MouseEvent):void
{
    var code: String;
    var letter1: String;
    var number1: String;
    var letter2: String;
    var space: String;
    var number2: String;
    var letter3: String;
    var number3: String;
    var UpperCase: String;

    code= txtinPostalCode.text;

    letter1= code.charAt(0);
    letter1= letter1.toUpperCase();

    letter2= code.charAt(code.indexOf("") + 1);
    letter2= letter2.toUpperCase();

    space= code.charAt(code.indexOf("") + 1);

    letter3= code.charAt(code.indexOf("") + 1);
    letter3= letter3.toUpperCase();

    lblOutput.text = letter1 + number1 + letter2 + space + number2 + letter3 + number3;


if
    (letter1 == UpperCase)
{
    lblOutput.text= "Valid postal code";
}

else if (letter1 != UpperCase)
{
lblOutput.text= "Not a valid postal code. The first character is incorrect";
}
    else if (letter2 == UpperCase)
    {
        lblOutput.text= "Valid postal code";
    }
    else if (letter2 != UpperCase)
    {
        lblOutput.text= "Not a valid postal code. The third character is incorrect";

    }
    else if (letter3 == UpperCase)
    {
        lblOutput.text= "Valid postal code";
    }
    else if (letter3 != UpperCase)
    {
        lblOutput.text= "Not a valid postal code. The third character is incorrect";
    }
}

function clearLabels(e:MouseEvent):void
{
lblOutput.text = "";
txtinPostalCode.text = "";
}