The Whole Grid
Eighty-one cells. Match the grid only when no digit repeats in any row, any column, or any 3×3 box.
Boards flatten row-major, top-left first, with (width − 1) hashes between rows: nine cells, eight hashes, nine cells, and so on — 145 characters in all. An empty cell is a full stop. The padding is the design decision, and it is doing real work. It makes a row exactly 17 characters, which is the stride you will need for columns; more importantly it means two cells are in the same row exactly when there is no hash between them. Strip it out and a row check has no edge to stop at. Twenty-seven constraints, but not twenty-seven pieces of pattern — rows collapse into one idea, columns into another, and boxes need the grid anchored from the start so a band boundary cannot be crossed. This is legality, not solvability. A grid that breaks no rule yet has no solution still matches.
Wrapped into rows of 9 so you can see the board. The string itself is one line with no line breaks in it — the 8 # between rows are ordinary characters, and a dot will happily match straight past them.
a finished grid
legal so far
the same grid with a 6 added: two 6s in the top row
a 1 instead: two 1s down the second column
a 2 instead: legal in its row and its column, twice in its box
+ 8 hidden tests, checked when you submit. They are what stops a pattern that only fits the examples above.