Match linebreaks – \n or \r\n?

I will answer in the opposite direction.

2. For a full explanation about \r and \n I have to refer to this question, which is far more complete than I will post here: Difference between \n and \r?

Long story short, Linux uses \n for a new-line, Windows \r\n and old Macs \r. So there are multiple ways to write a newline. Your second tool (RegExr) does for example match on the single \r.

1. [\r\n]+ as Ilya suggested will work, but will also match multiple consecutive new-lines. (\r\n|\r|\n) is more correct.