If you just want to change a prefix from one string to another you can use a simple wildcard like abc* -> 123*
Rename PATTERN="abc*" TO="123*"
Nothing complex is needed just for that. (Regular expressions are not needed either.)
If you still don't have what you need from Blueroly's post or mine, please give some example filenames, before and after renaming, so we know the full operation you're aiming for.