The only post I can find for this I think relates to the script console which I think is different to what I want to do.
I have a utility called Keytext which I have used extensively over time.
But I am aware that a lot of what it does for me can be implemented in Opus.
One function it does is replace one set of text for another on the fly.
For instance it will replace
eu
with
EURUSD
Can this be done in Opus?
I setup a key command with the hotkey
A,U
And in Standard Function (Opus or External) mode I entered the entered the text
EURUSD
I realised when I tried this I have to enter an Opus function here.
Any ideas?
Or do I need to stick with Keytext 
PS Should I have posted this in Off-Topic?
I did think about that...
Replace it where? In the clipboard?
Opus scripts can read and write text in the clipboard, so you could do that with Opus.
Anywhere within DOpus where I am entering text.
Mostly it would be when I am editing a filename e.g. adding a suffix
I am thinking this is not possible...?
You could have a rename preset which applied that kind of rule. Or you could use autohotkey or something to copy whatever text is selected to the clipboard, modify it, then paste it back. There isn't a completely general way in Windows to do what you want to do though.