Duh, I should have looked more carefully in preferences.
I actually did look in preferences first (because I know the Calibre developers don't overlook much!) but I went straight to "Tweaks" to look for an adjustment.
So thanks to chaley, ducks and mobovenka for the quick reply.
I've been using Calibre for over a year and have often been vexed by the overly broad results with default searches. I can't believe I didn't inquire about this before.
So this is a change many others might want to do as it makes searching the library a lot easier.