Lua中获取字符长度

function UITool.getCharLength(str)
    str = str or ""
    local strLength = 0
    local len = string.len(str)
    while str do
        local fontUTF = string.byte(str,1)

        if fontUTF == nil then
            break
        end
        --lua中字符占1byte,中文占3byte
        if fontUTF > 127 then 
            local tmp = string.sub(str,1,3)
            strLength = strLength+1
            str = string.sub(str,4,len)
        else
            local tmp = string.sub(str,1,1)
            strLength = strLength+1
            str = string.sub(str,2,len)
        end
    end
    return strLength
end