Regex to match something based on what was matched

问题:

I'm trying to write a regular expression to validate moves in algebraic chess notation. Here's what I have so far:

/
    O-O(-O)?                 # Castling
    |[KQRBN]x?[a-h][1-8]\+?  # Most normal moves and captures
/

Where I am lost is pawn promotion.

A knight, bishop, or centre pawn may only promote on the file from which it starts or the file on either side via a capture. A rook pawn may promote by moving straight or capturing to one side, depending on whether it is on the a- or h- files. So something like

/[a-h](x[a-h])?[18]=[QRBN]\+?/

doesn't work, because fxh8 is not a valid move (only fxe8 and fxg8 are). I could go the long route with

/(a(xb)?|h(xg)?|b(x[ac])?.../ # insert 5 more files in place of the ...

but that's not very efficient. I want to use grouping, so that I can do rook-pawns, and everything else. I have something like this in mind:

/([b-g])(x(\1±1))?/

To indicate "the letters b to g may be followed by the letter that comes before or after them".

回答1:

Matching the adjacent letter is not hard using alternations, but you won't find anything compact as there is no character arithmetic in regex.

Someone just posted a similar question out of sympathy for your plight.

The long way is the only way.