It would be nice to show the white pawn’s direction in your examples, maybe something like:

8|                
7| p p p p       p
6|           P     
5|           ↓     
4|           ↓     
3|           ↓    
2|           ↓     
1|           ↓     
  ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
   a b c d e f g h 

The direction wasn't completely clear from the description.

Needs better variation (or just more) random tests. Suboptimal solutions can pass from time to time. I passed a couple of times with an incomplete "pawn" formula. I think it's best to mix pure random tests, with randomised tests generated from templates.

But in the examples in the description, P is moving up. This is mighty confusing.

clarified the description to have the direction the white pawn is moving in the array example

Good suggestion, implemented it.

Aren't ranks described in the description example array? Maybe something like Rank 1 (white) and Rank 8 (black) would make it clearer?

basically: you should clarify that the P pawn is moving "down", considering the input board (visually, in the array shown in the description)

What do you mean, "from blacks perspective"? board[0] is the first rank (with the white king), board[1] the second (with white pawns), etc.?

It should be specified more clearly that the board is being provided from blacks perspective.

Make empty squares spaces rather than empty strings for ease of translation into languages which have a dedicated char type (all of these that I know disallow the empty character).

This will also make the formatting much nicer on the test cases and description for free.

done

fixed

Yeah copy paste error, fixed now.

My bad. I completely missed that fact in the description.

Lowercase characters are black pieces, and uppercase characters are white pieces.