FEN tag

The Forsyth-Edwards Notation or shortly FEN tag defines a position on the board. In the PDN 2.0 draft standard the following syntax is proposed:

[TURN]:[COLOUR1][[K][SQUARE_NUM][,]...]:[COLOUR2][[K][SQUARE_NUM][,]...]

We extend this syntax with ranges of numeric fields, such that the start position of international draughts can be defined as [FEN "W:W31-50:B1-20"].

Another extension is that we allow ? to denote an unknown color.

The following grammar describes the formal syntax of the value of a FEN tag.

// Productions
Fen                        : COLOR (NumericSquares | AlphaNumericSquares) DOT?
NumericSquares             : (COLON COLOR NumericSquareSequence)+
NumericSquareSequence      : NumericSquareRange (COMMA NumericSquareRange)*
NumericSquareRange         : KING? NUMSQUARE (HYPHEN NUMSQUARE)?
AlphaNumericSquares        : (COLON COLOR AlphaNumericSquareSequence)+
AlphaNumericSquareSequence : KING? ALPHASQUARE (COMMA KING? ALPHASQUARE)*

// Tokens
COLOR                      : "[WB?]"
KING                       : "K"
ALPHASQUARE                : "[a-h][1-8]"
NUMSQUARE                  : "([1-9][0-9]*)|(0[1-9][0-9]*)|0"
HYPHEN                     : "\-"
COMMA                      : "\,"
COLON                      : "\:"
DOT                        : "\."

Important

Some corrections have been made.

  • The COMMA in NumericSquareSequence and AlphaNumericSquareSequence is mandatory.

  • A missing KING? in AlphaNumericSquareSequence has been added.

  • NUMSQUARE may contain more than two digits (it is up to the implementer to do range checks).

Restrictions

The following restrictions apply when writing a FEN tag:

  • No embedded spaces are allowed inside the value of a FEN tag

  • No dot (‘.’) is allowed at the end of the value of a FEN tag

Examples

[FEN "B:W18,24,27,28,K10,K15:B12,16,20,K22,K25,K29"]
[FEN "B:W18,19,21,23,24,26,29,30,31,32:B1,2,3,4,6,7,9,10,11,12"]
[FEN "W:W31-50:B1-20"]

Extensions

The above grammar does not allow positions with no pieces for one of the players. It should therefore be extended to accept empty ranges of pieces.