68 challenges across eight tiers. Tier 8 is a warm-up; tier 1 will take an evening. Nothing is locked — play them in any order.
Eighty-one cells. Match the grid only when no digit repeats in any row, any column, or any 3×3 box.
A Sudoku box, flattened. Extract every digit that appears again later in the box.
A Sudoku row is nine characters. Capture the digit that appears twice — and match nothing at all when the row is legal.