No random tests

Added this (plus some other perfomance tests) to all languages: many (most?) of the existing solutions were wiped out...

Negative numbers are not tested.

Just added a test case similar to that mentioned by you.

Just added test cases similar to those mentioned by you (Ruby version pending fixes).
Edit: Now ok.

Just added.

Just added a test case similar to that mentioned by you.

I'd disagree. It took me a bit to understand it, but when it clicked, it made me smile. I think that's what clever solutions are about.

Of course, it would not be good production code at all.

Hello,
This is due to the way that the Codewars testing framework processes data from stdout. Printf prints data without a new line - something that Codewars doesn't like. You can use printf, but you will need to add a new line (\n) sequence at the end of your string. See man printf(1) for more information.
Thanks!

It seems that solutions are accepted that find the following files: foopng and foo.pngbar when the first argument is png. That is wrong and should be checked.

This solution returns wrong results! If you have a file foopng and your first argument is png this file foopng is found, but it should not! The dot must be escaped: grep "\.$1"

And if you have a file bar.pngGGG the file is found, but it should not. So check for end of line: grep "\.$1$"

NOT best practices!

Neither solutions with echo, or echo -e nor printf work. I also tried pipeline it to sed 's%$%\r\n%', to no avail.

Why does printf not work. I had to use echo. Why?

I get a Segmentation fault when I use this solution. The size of result is never defined. How could that have worked?