The token table contains interface functions to TEX's handling of tokens. These functions are most
useful when combined with the token_filter callback, but they could be used standalone as well.
A token is represented in Lua as a small table. For the moment, this table consists of three numeric
entries:
index meaning description
1 command code this is a value between 0 and 130 (approximately)
2 command modifier this is a value between 0 and 221
3 control sequence id for commands that are not the result of control sequences, like letters and

\documentclass{article}
\usepackage{luacode}
\begin{document}
\luaexec{%
local upper = unicode.utf8.upper
local char = unicode.utf8.char
local unpack = unpack or table.unpack
bracket_level = 0
start = true
stop_bracket = false
mytokens = function()
local t = token.get_next()
local command, modifier, cs = unpack(t)
if cs == 0 and command == 11 then
if start or bracket_level > 0 then
print(upper(char(t[2])), t[1], t[3], bracket_level, start)
end
elseif command == 1 then
if not stop_bracket then
bracket_level = bracket_level + 1