And remove brackets []?
Well... somehow you use the backslash sign in your work, don't you? Most probably, it is a matter of autocorrection. Just don't let it autocorrect, and don't  omit brackets.

but still, you may have identified the problem at least. Looking for a way to override this setting doesn't look simple. funny that I can type \\\ here but in Trados, set to English, it comes up as \\\\\\\\\  

when I put this .[\n]+

it looks like this:

when I put this .\n+

it looks like this

And if I restart with translate single document with the above rule in place in the top TM, I still get this:

There seems to be something weird going on with the segmentation in Studio recently. I see changes in segmentation being mentioned in release notes (e.g. Studio 2015 SR3), but it's still not working correctly, IMO.

Additionally, if you look through various SDL knowledge base articles, the mentioned regexes vary, probably depending on the actual writer of the article... :-\

I have consistent problems with segmentation... I ended up with defining this obscure Linebreak rule
.?[\r\n][\r\n]
because anything else simply doesn't separate segments as expected.
One would expect that e.g. [\r\n]+ would give the same result with CRLF linebreaks, but it doesn't...

And even with this rule I get weird stuff like this:

SOURCE:

RESULT:

This the beginning of the file.
Then the segmentation "catches up" and works "sort-of okay" throughout the file... and at the end of the file I get again this:

SOURCE:

RESULT:

Now, this has obviously nothing to do with the regex, there is something going on in the segmntation engine, or something...

Just wanted to add an example of that "sort-of okay" work throughout the file - from time to time the segmentation simply "chokes" at certain places, lik ee.g. this:

SOURCE and TARGET for alignment:

RESULT: