I'm learning to document everything I do and explain it in a way that a wide range of audiences can understand, even those who don't understand anything about programming, so it may seem like I write like an AI. By the way, I've already seen it, everything is correct. Thanks for your contribution.

Here are a couple of points to consider:

Misleading Test Description in Random Tests for Invalid Inputs: In the random tests for invalid inputs, the description currently reads "Should return true for random invalid inputs." This is misleading because for invalid inputs, the test should clearly indicate that false is expected. Adjusting this description will help clarify the intended behavior.

Stylistic Aspects: Although using string.find is perfectly acceptable for verifying the input format, some might prefer string.match as it can enhance readability when simply checking for a pattern match. This is a matter of style and does not affect the correctness of the solution.

These points are relatively minor, but addressing them can improve the clarity and overall quality of your test suite.

Hello,

I wanted to let you know that I have completely remodeled the test suite structure based in my own design and your valuable feedback. I stand by my approach:

Fixed edge cases: I still include tests for specific valid DNIs (like "00000001R" and "99999999R") so that the expected behavior is crystal clear.

500 Random Tests: I intentionally kept 500 random tests (for both valid and invalid cases) because it dramatically lowers the risk of a solution passing by pure chance. In many similar katas, fewer random tests allow some candidates to pass accidentally by playing around with random inputs, which is something I wanted to avoid.

Clear error messages: I’ve refined the assertion messages so that every failure clearly states what the result was versus what was expected.

If you’re curious, feel free to take a look at the complete test suite.

approved, thank you!