Tier 2 · Archmageextract

Clashes In The Box

A Sudoku box, flattened. Extract every digit that appears again later in the box.

Boards flatten row-major, top-left first, with (width − 1) hashes between rows — so a 3×3 box is three cells, ##, three cells, ##, three cells. An empty cell is a full stop. You want each occurrence that has a copy after it, which is why a digit appearing three times comes out twice. Finding the clash is easy; not consuming the rest of the box while you look is the challenge.

sudoku lookahead backreferences global

Your pattern1 chars · par 13

//g

Flags

Start typing — tests run as you go.

Test cases

Wrapped into rows of 3 so you can see the board. The string itself is one line with no line breaks in it — the 2 # between rows are ordinary characters, and a dot will happily match straight past them.

123## 456## 789→ []
a legal box
123## 451## 789→ ["1"]
123## 423## 789→ ["2", "3"]
111## ...## ...→ ["1", "1"]
three of a kind, two of them repeat
1.3## .5.## 7.9→ []

+ 8 hidden tests, checked when you submit. They are what stops a pattern that only fits the examples above.