Sigh. I completely reinstalled lm19.3, and have added no new applications except kate.
The new version of kate (Kate version 17.12.13, libraries KDE Frameworks 5.44.0, Qt 5.9.5 (built against 5.9.5), The xcb windowing system) now shows the regex button.
note that the version I got using synaptic was version 17.12.3! WHY the different versions??
Still doesn't allow inputting Unicode via Ctrl-Shift-u, though.
So now I'll start installing my standard set of applications, and see if somethning affects kate.
BTW I seem to recall once before I was doing some regex stuff with kate, sed, and komodo, and something happened, the screen locked up or something, or I had to cycle power because i got everything stopped working (this happens to me just about once a day), and it was after that when I discovered kate no longer did regex.
sabbe sattā bhavantu sukhitattā. LN 19.3-64 Cinn 4.4.8, Mobo: ASUSTeK STRIX B250G GAMING v, Dual core Pntm G4560, Intel Gfx. Laptop:ASUS, Core i3, 4G RAM Intel Gfx