commentStrings should be escaped before concatenated
For example, {"A", "-", "Z"} would lead to "[A-Z]" in the regex

It's been a while so you've probably learned this, but you can change the tests.

It doesn't fail the test case so it's technically fine 🤷

Real ninja :D

Could you explain?
This is too complex and so much better than mine

The spaces are quite easy to get around.

HINT: You should probably make your function space independent, as in working the same regardless of spacing

<_>::into

Great use of the Turbofish !

When reporting an issue, please, state the language.

This happends at least in Rust.

Yes it's pretty clear.

Somehow I thought one test case was wrong and asked the last one, but it seems I'm wrong and the test is right.

Sorry about that

In the description:
"Your task is to return the first longest string consisting of k consecutive strings taken in the array."
seems to be clear:-) At least it seems have been for lots of people.
Moreover the example given removes ambiguity if there was one:

...
Two strings are the longest: "folingtrashy" and "abcdefuvwxyz".
The first that came is "folingtrashy" so 
...

It really seems not everyone means the same thing by first