参考项目：https://github.com/Pogs/lua-utf8-simple

UTF-8字符串工具类

Lua代码：utf8_simple.lua

-- ABNF from RFC 3629
--
-- UTF8-octets = *( UTF8-char )
-- UTF8-char = UTF8-1 / UTF8-2 / UTF8-3 / UTF8-4
-- UTF8-1 = %x00-7F
-- UTF8-2 = %xC2-DF UTF8-tail
-- UTF8-3 = %xE0 %xA0-BF UTF8-tail / %xE1-EC 2( UTF8-tail ) /
-- %xED %x80-9F UTF8-tail / %xEE-EF 2( UTF8-tail )
-- UTF8-4 = %xF0 %x90-BF 2( UTF8-tail ) / %xF1-F3 3( UTF8-tail ) /
-- %xF4 %x80-8F 2( UTF8-tail )
-- UTF8-tail = %x80-BF

-- 0xxxxxxx                            | 007F   (127)
-- 110xxxxx	10xxxxxx                   | 07FF   (2047)
-- 1110xxxx	10xxxxxx 10xxxxxx          | FFFF   (65535)
-- 11110xxx	10xxxxxx 10xxxxxx 10xxxxxx | 10FFFF (1114111)

local pattern = '[%z\1-\127\194-\244][\128-\191]*'

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

		return pos
	end

local utf8 = {}

-- THE MEAT

-- maps f over s's utf8 characters f can accept args: (visual_index, utf8_character, byte_index)
utf8.map =
	function (s, f, no_subs)
		local i = 0

		if no_subs then
			for b, e in s:gmatch('()' .. pattern .. '()') do
				i = i + 1
				local c = e - b
				f(i, c, b)
			end
		else
			for b, c in s:gmatch('()(' .. pattern .. ')') do
				i = i + 1
				f(i, c, b)
			end
		end
	end

-- THE REST

-- generator for the above -- to iterate over all utf8 chars
utf8.chars =
	function (s, no_subs)
		return coroutine.wrap(function () return utf8.map(s, coroutine.yield, no_subs) end)
	end

-- returns the number of characters in a UTF-8 string
utf8.len =
	function (s)
		-- count the number of non-continuing bytes
		return select(2, s:gsub('[^\128-\193]', ''))
	end

-- replace all utf8 chars with mapping
utf8.replace =
	function (s, map)
		return s:gsub(pattern, map)
	end

-- reverse a utf8 string
utf8.reverse =
	function (s)
		-- reverse the individual greater-than-single-byte characters
		s = s:gsub(pattern, function (c) return #c > 1 and c:reverse() end)

		return s:reverse()
	end

-- strip non-ascii characters from a utf8 string
utf8.strip =
	function (s)
		return s:gsub(pattern, function (c) return #c > 1 and '' end)
	end

-- like string.sub() but i, j are utf8 strings
-- a utf8-safe string.sub()
utf8.sub =
	function (s, i, j)
		local l = utf8.len(s)

		i =       posrelat(i, l)
		j = j and posrelat(j, l) or l

		if i < 1 then i = 1 end
		if j > l then j = l end

		if i > j then return '' end

		local diff = j - i
		local iter = utf8.chars(s, true)

		-- advance up to i
		for _ = 1, i - 1 do iter() end

		local c, b = select(2, iter())

		-- i and j are the same, single-charaacter sub
		if diff == 0 then
			return string.sub(s, b, b + c - 1)
		end

		i = b

		-- advance up to j
		for _ = 1, diff - 1 do iter() end

		c, b = select(2, iter())

		return string.sub(s, i, b + c - 1)
	end

return utf8

示例代码

local utf8 = require('utf8_simple')
local str = "你好！"
-- 获取字符串长度
print(str .. " len = " .. utf8.len(str))
-- 截取字符串（Lua中字符串索引从1开始）
print("sub str = " .. utf8.sub(str,1,2))