llama.cpp/grammars/chess.gbnf

# Specifies chess moves as a list in algebraic notation, using PGN conventions

# Force first move to "1. ", then any 1-2 digit number after, relying on model to follow the pattern
root    ::= "1. " move " " move "\n" ([1-9] [0-9]? ". " move " " move "\n")+
move    ::= (pawn | nonpawn | castle) [+#]?

# piece type, optional file/rank, optional capture, dest file & rank
nonpawn ::= [NBKQR] [a-h]? [1-8]? "x"? [a-h] [1-8]

# optional file & capture, dest file & rank, optional promotion
pawn    ::= ([a-h] "x")? [a-h] [1-8] ("=" [NBKQR])?

castle  ::= "O-O" "-O"?