Judging from the diff you did the following:

• add a blank line in the sample tests
• remove export from solution setup
• remove Rust specific code block from description
• rework the authors solution by removing return types, comments, and inverting the conditionals

Where exactly did you fix the sample test cases?
I will reject this fork as I see no improvements.

Fixed sample test case. Please review.

yes, same here :)

[1,1,1,1,2,2,2,3,3,3,4,4,4] is array and [month] selects value by position in array. same as
const values = [1,1,1,1,2,2,2,3,3,3,4,4,4];
return values[month];

because regex will check for match of single character from inside brackets. Check https://regex101.com/ for explanation of this one.

Nice! If string contains 3 or 4 aren't those replaced wiht ' '?