I know you've found it won't work anyway, in the packages you're using, but...
"\n\r" should do what I want
Are you sure? There's a difference between Unix and MS systems, in that the 0x0D 0x0A (IIRC[1]) of DOS-and-onwards systems is CarriageReturn+LineFeed, as opposed to just a single LineFeed character being used in Unix. But DOS-implemented Perl, for example, will treat "\n" as the two characters, and Unix-implemented Perl will treat it as the one. I'd expect DOS (and Windows) editors implementing the multiline-capable regexping that you're after to do the same.
(Although I remember microEmacs needing to specific about the two control characters, when I used that in the Early '90s. That wasn't the same regexp notation, however, so probably doesn't count. However, if you can get a suitable variant of Emacs and get around to learning all the controlling escape-sequences you need, that might do what you want. I'll admit that I'm not sure I can remember them all, so you might be on your own if there isn't someone out there currently an Emacs buff. Or, alternatively, there might be a vi one wanting to drag you into their own little Cabal.
)
Also, I believe it should be in the order "\r\n". Again, ICBW, but it looks more correct at first glance. Lazy me having been lazily using "\n" alone on all the different platforms I've used recently, knowing that (for my purposes, at least) it does what I want it to do!
[1] Long time since I hex edited them, so I'm very much open to being corrected.