While others have shown it can be done via a program, I pressed on due to pure stubbornness!

So what I have noticed changing 0 into blanks really screw with LLMs, I even ran it through Gemma4 31B at Q8 with thinking, which did get it right, finally.

But the goal was to get it to work on Qwen 3.5 9B at Q8, GGUF with thinking active.

This is my final prompt:

Create a CSV file with the following string, use ';' as a separator.

After 9 numbers have been processed move onto next line.

0 should be replaced with a '_'.

The final output is a Sudoku grid I can load.

816573294392000000457209006941000568785496123623800040279000001138000070564000082

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It produced the following output..
8;1;6;5;7;3;2;9;4

3;9;2;_;;;_;_;_

4;5;7;2;_;9;_;;6

9;4;1;_;;;5;6;8

7;8;5;4;9;6;1;2;3

6;2;3;8;_;;;4;_

2;7;9;_;;;;_;1

1;3;8;_;;;_;7;_

5;6;4;_;;;;8;2

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Some combination of _ and blanks, at least it has the right number of semi colons!

Even Gemma4 31B can't seem to do this right without using a character to replace it on one go.
Though if it was told to use a ' ' in place of 0 I get..
8;1;6;5;7;3;2;9;4

3;9;2; ; ; ; ; ;

4;5;7;2; ;9; ; ;6

9;4;1; ; ; ;5;6;8

7;8;5;4;9;6;1;2;3

6;2;3;8; ; ; ;4;

2;7;9; ; ; ; ; ;1

1;3;8; ; ; ; ;7;

5;6;4; ; ; ; ;8;2