How I can use %b match pattern with modifiers?

Hi guys.
Do you know how string.match() captures work? I hope yes.
Do you know how %bxy works? If nope, then:

Let’s look at this pattern: "%b(\]" looks like gibberrish (also I put \ to escape ] symbol)
But let apply it to this string: "Hi([I'm some test) string]! Bye"
Pattern above will output the following: "([I'm some test) string]"

Same works for any character. And this also accords for folded brackets, like:
"I'm ( folded (bracket ))" with "%b()" will output "( folded (bracket ))"

But I have not found any way to use it with modifiers, like:
+, -, *, ?

Can anyone help me with it, cuz this breaks my narraitor logic a lot? =C