checking for keyboard input with exceptions

I'm making a hangman type of game, and basically in the CheckIfCorrect() function i call it when a user enters a letter. However it still is called when the user hits tab or backspace. How can i make these two buttons an exception? After further tests, it seems that backspace and tab are exceptions only when no letters have been guessed. Not sure why though. Code:

function OnGUI() {

 if(endGame == false)
 {
     TimeHandler();
     
     var keyEvent : Event = Event.current;    
     
     var textPosX = 45;
     
     var textPosY = Screen.height - 50;
     
     lastLetterSet = new String[typeAreas.Length];
     
     for(i = 0; i < typeAreas.Length; i++)
     {
         typeAreas[i] = GUI.TextField(Rect(textPosX, textPosY, 50, 50), typeAreas[i], 1, typeAreaStyle);
         
         textPosX += 50;
         
         if(typeAreas[i] == "")        //reset to underscore if area is empty
         {
             typeAreas[i] = "_";
         }
     
     }

 
     if(keyEvent.isKey)
     {
         if(keyEvent.keyCode != KeyCode.Tab && keyEvent.keyCode != KeyCode.Backspace)
         {
             Debug.Log("key event pressed");
             checkWord = true;
             FinalAnswer();
         }
     }
 }
 

}

function CheckIfCorrect() {

 if(checkWord == true) {
     for(i = 0; i < typeAreas.Length; i++)
     {
         if(typeAreas[i] != "_")
         {
             if(typeAreas[i] == word[i])
             {
                 isCorrect = 1;
                 feedback = true;
                             
             }
             
             else {
                 isCorrect = 2;
                 feedback = true;
             }
         }    
     }
     
     checkWord = false;
 }

}