Problem 61144. FEN Chess Notation

Note: This problem is the reverse, or complement, of Problem 61155 - FEN to Chess Board.
Given an 8-by-8 character matrix representing a chess board, convert it to FEN (Forsyth-Edwards Notation) board position format.
The input matrix uses:
  • Dots (".") for empty squares
  • Letters for pieces: "rnbqkp" (black pieces) and "RNBQKP" (white pieces)
FEN notation encodes each row from top to bottom, separated by slashes ("/"). Consecutive empty squares are replaced by their count (1-8).
Example 1
Starting position:
input = [ ...
'rnbqkbnr'
'pppppppp'
'........'
'........'
'........'
'........'
'PPPPPPPP'
'RNBQKBNR']
output =
'rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR'
Example 2
After white plays e4:
input = [ ...
'rnbqkbnr'
'pppppppp'
'........'
'........'
'....P...'
'........'
'PPPP.PPP'
'RNBQKBNR']
output =
'rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR'
Note: Empty squares before the P are "4", the P itself, then "3" empty squares after.
Solve

Solution Stats

87.5% Correct | 12.5% Incorrect
Last Solution submitted on Jan 08, 2026

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chess encoding

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