--返回當前字元實際佔用的字元數
local function SubStringGetByteCount(str, index)
    local curByte = string.byte(str, index)
    local byteCount = 1;
    if curByte == nil then
        byteCount = 0
    elseif curByte > 0 and curByte <= 127 then
        byteCount = 1
    elseif curByte >= 192 and curByte <= 223
 
 then
        byteCount = 2
    elseif curByte >= 224 and curByte <= 239 then
        byteCount = 3
    elseif curByte >= 240 and curByte <= 247 then
        byteCount = 4
    end
    return byteCount;
end 


--獲取中英混合UTF8字串的真實字元數量
local function SubStringGetTotalIndex(str)
    local curIndex = 0
 
;
    local i = 1;
    local lastCount = 1;
    repeat
        lastCount = SubStringGetByteCount(str, i)
        i = i + lastCount;
        curIndex = curIndex + 1;
    until(lastCount == 0);
    return curIndex - 1;
end

--獲取字串的真實索引值
local function SubStringGetTrueIndex(str, index)
    local curIndex = 0
 
;
    local i = 1;
    local lastCount = 1;
    repeat
        lastCount = SubStringGetByteCount(str, i)
        i = i + lastCount;
        curIndex = curIndex + 1;
    until(curIndex >= index);
    return i - lastCount;
end



--擷取中英混合的UTF8字串，endIndex可預設
function StringUtil.SubStringUTF8(str, startIndex, endIndex)
    if startIndex < 0 then
        startIndex = SubStringGetTotalIndex(str) + startIndex + 1;
    end

    if endIndex ~= nil and endIndex < 0 then
        endIndex = SubStringGetTotalIndex(str) + endIndex + 1;
    end

    if endIndex == nil then
        return string.sub(str, SubStringGetTrueIndex(str, startIndex));
    else
        return string.sub(str, SubStringGetTrueIndex(str, startIndex), SubStringGetTrueIndex(str, endIndex + 1) - 1);
    end
end

呼叫：
local str = “abc中文”
print(StringUtil.SubStringUTF8(str,4,5))
print(StringUtil.SubStringUTF8(str,1,4))
輸出結果：
中文
abc中
[Finished in 0.1s]