testing input for values between approved alphacharacters

Hello all,

I have a question about  the script below.

I want to test input in a text field for allowing approved letter groups in a text field. The field works for the approved letters as providing options for entering A+, A, A-, B+,B,B-, C+,C,C-,D+,D,D-,.

But I need to change the code so that the user is limited to entering the following values as pairs:

EX, VG, G, S, NI and na (all of these are to be case-sensitive.

What changes should I make to the change event code?  I've tried separating the values with quotes, but that isn't the solution.

Here is the code on the change event in JavaScript:

if (xfa.event.newText.match (/[^ABCDEIRna\+\-]/))  {

     xfa.event.change ="";

     xfa.host.beep("3");

}

On the exit event I have this code in FormCalc:

if($ eq "EX" or $ eq "VG" or $ eq "G" or $ eq "S" or $ eq "NI" or $ eq "na" or  $ eq null)

then $ = $

else

     xfa.host.messageBox("Please insert a mark of EX, VG, G, S, NI, or use na.","Data Error - Incorrect Mark", 1, 0)

$ = null

endif

Any help would be appreciated.