--根据首字节获取UTF8需要的字节数
local function GetUTF8CharLength(ch)
	local utf8_look_for_table = {
		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
		2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
		2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
		3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
		4, 4, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 6, 6, 1, 1,
	}
	return utf8_look_for_table[ch]
end

--根据UTF8流获取字符串长度
--GetUTF8Length("一二三四五六七") 返回7
local function GetUTF8Length(str)
	local len = 0
	local ptr = 1
	repeat
		local char = string.byte(str, ptr)
		local char_len = GetUTF8CharLength(char)
		len = len + 1
		ptr = ptr + char_len
	until(ptr>#str)
	return len
end

--截取UTF8字符串
--SubUTF8String("一二三四五六七",1,3) 返回一二三
local function SubUTF8String(str, begin, length)
	begin = begin or 1
	length = length or -1 --length为-1时代表不限制长度
	local ret = ""
	local len = 0
	local ptr = 1
	repeat
		local char = string.byte(str, ptr)
		local char_len = GetUTF8CharLength(char)
		len = len + 1

		if len>=begin and (length==-1 or len<begin+length) then
			for i=0,char_len-1 do
				ret = ret .. string.char( string.byte(str, ptr + i) )
			end
		end

		ptr = ptr + char_len
	until(ptr>#str)
	return ret
end


local function test(str, len, add)
	local ret = ""
	for i=1,math.ceil(GetUTF8Length(str)/len) do
		ret = ret .. SubUTF8String(str, (i-1)*len + 1, len) .. add
	end
	return ret
end

local str = "一二三四五六七一二三四五六七一二三四五六七一二三四五六七123"
print( test(str, 7, "~~\n") )
--[[
输出结果：
一二三四五六七~~
一二三四五六七~~
一二三四五六七~~
一二三四五六七~~
123~~
]]

一二三四五六七~~
一二三四五六七~~
一二三四五六七~~
一二三四五六七~~
123~~