capturing sub-expressions?

[Linda Walsh]

I was wondering -- in the bash substitute commands ${..%%|##|//} etc,
is there a way to "capture" a subexpression, so that I can
use the subexpression in the replacement string so I can
end up 'only' with the the subexpression?

If the full expression was in a shellvar "Options", I thought about
using something like:
    ${Option//!(expr)}
which I hoped would have "!(expr) match anything but my desired
expression, and the double "/" would say allow it to match multiple
times, but it appears the "!()" construct is limited to pathname
expansion?  Presuming that is true (limited to pathname expansion),
is there anyway to do it on Shell vars?

I keep confusing the path-matching regular expressions with
string-matching expressions and keep getting disappointed when I'm
re-reminded of the limitation...:-(

Maybe an "RFE"?...(*sigh*)
Linda