utf8 substring by index five

utf8substring.lua

local utf8 = require("utf8")
local utf8pos, utf8len = utf8.offset, utf8.len
local sub = string.sub
local max, min = math.max, math.min

local posrelat = function(pos, len)
    if pos >= 0 then return pos end
    if -pos > len then return 0 end
    return pos + len + 1
end

utf8.sub = function(str, i, j)
    local len = utf8len(str)
    i, j = max(posrelat(i, len), 1), j and min(posrelat(j, len), len) or len
    if i <= j  then
        return sub(str, utf8pos(str, i), utf8pos(str, j + 1) - 1)
    end
    return ""
end