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Strings are immutable. Single or double quotes work; both behave identically. Multiline strings use [[ ... ]].

Concatenation uses .. (NOT +):

local first = "Grace"
local last  = "Hopper"
print(first .. " " .. last)   -- Grace Hopper

Length uses the # operator:

print(#"hello")   -- 5

String library essentials:

string.upper("hi")          -- HI
string.lower("HI")          -- hi
string.sub("hello", 2, 4)   -- ell  (1-indexed, inclusive)
string.rep("ab", 3)         -- ababab
string.find("hello", "ll")  -- 3, 4

Method-style works too: ("hi"):upper().