@deanishe TextExpander has this feature, but I see your points.
@VeroThank you. This is exactly what I looking for. It's like a "key trigger" for expansion. From the docs, took note of prefixes, but never considered using both together.
Why is one snippet OK (in black) - but the other is not (in red) -- see link to screenshot? Do the 2nd, 3rd, etc. characters have to be unique also for a Snippet to expand? Both are set to Auto Expansion.
// Working snippet in black text
:ux
// Not valid snippet in red text
:uxd
Screenshot