How do you fish out certain strings in a string? LUA

So I want to fish out a string within a string that looks kind of like this:

local x = "define x = 2; output('Hello World')"
x:gsub("output%(PATTERN", function(c)
print(c)
end)

I basically want to fish out: "output('Hello World')" But I do not understand how patterns work. So if someone could please tell me that would be great