Put ... at the end of a string if it gets too large

I’m really bad with manipulating strings and I was wondering if I could get some help with something. How would I see if a string is larger than 11 characters and if it is, go to 8 characters and replace the last three characters with “…”?

[lua]
local str = “This is a very large string!”

if str:len() > 11 then
str = str:sub(1, 8) … “…”
end

print(str) – This is …
[/lua]

Thank you!