> > Hi - I am thinking of a function where you can highlight region, only
> > lets say of one color (ie category) and then be able to replace
> > strings or regexp within those region only.
> 
> AFAIR, isearch+ can do this out of the box.

Yes, in particular in combination with `zones.el'.

And it's mainly the `isearch-prop.el' part of
Isearch+ that works with zones.

Isearch+ description:

https://www.emacswiki.org/emacs/IsearchPlus

`isearch+.el':

https://www.emacswiki.org/emacs/download/isearch%2b.el

`isearch-prop.el':

https://www.emacswiki.org/emacs/download/isearch-prop.el
 
> Isearch itself needs some help: You would first have to mark the
> region(s) to treat yourself - using hi-lock for example, or something
> even more basic (text properties or overlays).  And then use an
> according `isearch-filter-predicate' to limit isearch operations to
> these regions.

Yes.