Algebraic notation is relative to the current board position. So, you have to parse it backwards.

The last character may or may not be a '+', indicating check. (I'm going to assume that there is no analysis on the game, so I will ignore !, !!, ?, ??, ?!, and !?.) So, we have to see if that exists. If it does, we may or may not want to verify that, after the move, a state of check exists.

After we look for the '+', the last two characters are guaranteed to be the destination square in the form /[a-h][1-8]/.

Any remaining characters are the specification for the piece and/or starting-square coordinates. (Remember, we're still parsing the string in reverse.)

• If there is nothing there, look for a pawn that could make it to this square.
• If there is an 'x', that means that there is a capture. (This notation, while legal, isn't common.)
• If there is a K, Q, R, N, or B, look for the appropriate piece that could make it to this square. There should be only one of that piece type that can legally make this move.
• If there is [a-h1-8]+, that means that there is either more than one officer of that piece type that can make this move or it is a pawn capture (ed5 is a legal notation). Remember, Qa2b3 means that there are three queens, potentially at a2, a4, and b4. All of them can move to b3.

The key is parsing it backwards.

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