Iit could be more robust, but at a certain point working with regex's is infuriating... I can think of more valid gdscript syntax this wouldn't currently handle, but shouldn't be the mod system's job to handle everything, only the sane things, like all multilines must be in form:
var blah = {
}
or the same with []'s, and not allow inserting anything extra after the starting or ending []{} characters. I'm sure at some point someone will write this in a mod though and bitch it doesn't work...
var fizz = { buzz = [
"moo"
]}
What I've proposed though should work with modifying any of the existing code's current bits as they always follow that good standard as far as I've seen.
Though realized one thing I did not update. I made AddTo work, but did not test RemoveFrom. Best guess, it works but maybe not as expected because the first line will be considered to be the { or [.