I simply changed the type-in to "^[A-Z]" so any letter will work. This is so because the answer only means next to nothing; the explanation with the answer will give you the win!
I also suppose it's not that easy. Before forcibly forging for the answer by much effort, don't feel forlorn! You can afford to calm the infuriated mind. The answer is forthcoming, perforce.
4th is hidden all around the title, description, etc... every fourth word of the clues reads: GUESS THE SEVERAL PIECES TO WIN. Name the pieces, as the puzzle read in full refers to chess.
One of my favourite challenges so far, by the way.