Match linebreaks - \n or \r\n?

Question

Gonna answer in 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.

In PCRE \R matches \n, \r and \r\n.

You have different line endings in the example texts in Debuggex. What is especially interesting is that Debuggex seems to have identified which line ending style you used first, and it converts all additional line endings entered to that style.

I used Notepad++ to paste sample text in Unix and Windows format into Debuggex, and whichever I pasted first is what that session of Debuggex stuck with.

So, you should wash your text through your text editor before pasting it into Debuggex. Ensure that you're pasting the style you want. Debuggex defaults to Unix style (\n).

Also, NEL (\u0085) is something different entirely: https://en.wikipedia.org/wiki/Newline#Unicode

(\r?\n) will cover Unix and Windows. You'll need something more complex, like (\r\n|\r|\n), if you want to match old Mac too.

This only applies to question 1.

I have an app that runs on Windows and uses a multi-line MFC editor box.
The editor box expects CRLF linebreaks, but I need to parse the text enterred
with some really big/nasty regexs'.

I didn't want to be stressing about this while writing the regex, so
I ended up normalizing back and forth between the parser and editor so that
the regexs' just use \n. I also trap paste operations and convert them for the boxes.

This does not take much time.
This is what I use.

 boost::regex  CRLFCRtoLF (
     " \\r\\n | \\r(?!\\n) "
     , MODx);

 boost::regex  CRLFCRtoCRLF (
     " \\r\\n?+ | \\n "
     , MODx);


 // Convert (All style) linebreaks to linefeeds 
 // ---------------------------------------
 void ReplaceCRLFCRtoLF( string& strSrc, string& strDest )
 {
    strDest  = boost::regex_replace ( strSrc, CRLFCRtoLF, "\\n" );
 }

 // Convert linefeeds to linebreaks (Windows) 
 // ---------------------------------------
 void ReplaceCRLFCRtoCRLF( string& strSrc, string& strDest )
 {
    strDest  = boost::regex_replace ( strSrc, CRLFCRtoCRLF, "\\r\\n" );
 }

In Python:

# as Peter van der Wal's answer
re.split(r'\r\n|\r|\n', text, flags=re.M) 

or more rigorous:

# https://docs.python.org/3/library/stdtypes.html#str.splitlines
str.splitlines()