Tier 3 · Warlockcapture

The Clashing Digit

A Sudoku row is nine characters. Capture the digit that appears twice — and match nothing at all when the row is legal.

One character per cell, left to right. An empty cell is a full stop. Group 1 is the offending digit. Empty cells are not digits, so a row with two blanks in it has not broken any rule.

sudoku backreferences capture

Your pattern0 chars · par 8

Flags

Start typing — tests run as you go.

Test cases

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.

123456788→ ["8"]
123456789must not match
1.34.6781→ ["1"]
the pair is at opposite ends
1.34.6789must not match
two blanks are not a clash

+ 9 hidden tests, checked when you submit. They are what stops a pattern that only fits the examples above.