Match linebreaks – \n or \r\n?

While writing this answer, I had to match exclusively on linebreaks instead of using the s-flag (dotall – dot matches linebreaks).

The sites usually used to test regular expressions behave differently when trying to match on \n or \r\n.

I noticed

• Regex101 matches linebreaks only on \n
  (example – delete \r and it matches)

• RegExr matches linebreaks neither on \n nor on \r\n
  and I can’t find something to make it match a linebreak, except for the m-flag and \s
  (example)

• Debuggex behaves even more different:
  in this example it matches only on \r\n, while
  here it only matches on \n, with the same flags and engine specified

I’m fully aware of the m-flag (multiline – makes ^ match the start and $ the end of a line), but sometimes this is not an option. Same with \s, as it matches tabs and spaces, too.

My thought to use the unicode newline character (\u0085) wasn’t successful, so:

1. Is there a failsafe way to integrate the match on a linebreak (preferably regardless of the language used) into a regular expression?
2. Why do the above mentioned sites behave differently (especially Debuggex, matching once only on \n and once only on \r\n)?