require "uchar"

local s  = uchar.of("αβγδεζηθ")
local n = 3
local m = 3
local kc = "δ"   -- known character
local ks = "δε"  -- known string
-- For reference.
print("Index of characters:  12345678")
print($"Complete string:      {s}")
-- Starting from n characters in and of m length
print($"Start {n}, length {m}:    {s:sub(n, n + m - 1)}")
-- Starting from n characters in, up to the end of the string
print($"Start {n}, to end:      {s:sub(n)}")
-- Whole string minus last character.
print($"All but last:         {s:sub(1, -2)}")
-- Starting from a known character within the string and of m length.
local dx = s:find(kc)
if dx then
    print($"Start '{kc}', length {m}:  {s:sub(dx, dx + m - 1)}")
end
-- Starting from a known substring within the string and of m length.
local sx = s:find(ks)
if sx then
    print($"Start '{ks}', length {m}: {s:sub(sx, sx + m - 1)}")
end