An ID3 tag reader implemented in pure Lua. Supports ID3v1 tags and a subset of ID3v2 tags.

id3.lua

--- A simple module to read ID3 tags from MP3 files. 
-- Supports ID3v1 tags and a (meaningful) subset of ID3v2 tags.
-- @class module
-- @name id3
-- @author Michal Kottman
-- @copyright 2011, released under MIT license

local id3 = {}

local function textFrame(name)
	return function (reader, info, frameSize)
		local encoding = reader.readByte()
		info[name] =  reader.readStr(frameSize - 1)
	end
end

-- only decode these ID3v2 frames
local frameDecoders = {
	COMM = function (reader, info, frameSize)
		local encoding = reader.readByte()
		local language = reader.readStr(4)
		info.comment = reader.readStr(frameSize - 5)
	end,
	TALB = textFrame 'album',
	TBPM = textFrame 'bpm',
	TENC = textFrame 'encoder',
	TLEN = textFrame 'length',
	TIT2 = textFrame 'title',
	TPE1 = textFrame 'artist',
	TRCK = textFrame 'track',
	TYER = textFrame 'year',
}

local function unpad(str)
	return (str:gsub('[%s%z]+$', ''))
end

function isbitset(x, p)
	local b = 2 ^ (p - 1)
	return x % (b + b) >= b       
end

--- Read ID3 tags from MP3 file. First tries ID3v2 tags, then ID3v1 and returns those
-- which are found first. Returns the following tags (if they are contained in the file):
-- <ul><li>title</li><li>artist</li><li>album</li><li>year</li><li>comment</li></ul>
-- @name readtags
-- @param file Either string (filename) or a file object opened by io.open()
-- @return Table containing the metadata from ID3 tag, or nil.
function id3.readtags(file)
	if type(file) == 'string' then
		file = assert(io.open(file, 'rb'))
	elseif type(file) ~= 'userdata' then
		error('Expecting file or filename as #1, not '..type(file), 2)
	end

	local position = file:seek()
	
	local function decodeID3v2(reader)
		local info = {}
		local rb = reader.readByte
		local version = reader.readInt(2)
		local flags = rb()
		local size = reader.readInt(4, 128)
		
		if isbitset(flags, 7) then
			local mult = version >= 0x0400 and 128 or 256
			local extendedSize = reader.readInt(4, mult)
			local extendedFlags = reader.readInt(2)
			local padding = reader.readInt(4)
			reader.skip(extendedSize - 10)
		end
		while reader.position() < size + 3 do
			local frameID = reader.readStr(4)
			local frameSize = reader.readInt(4)
			local frameFlags = reader.readInt(2)
			if frameDecoders[frameID] then
				frameDecoders[frameID](reader, info, frameSize)
			else
				reader.skip(frameSize)
			end
		end
		file:seek('set', position)
		return info
	end
	
	local function decodeID3v1(reader)
		local info = {}
		info.title = reader.readStr(30)
		info.artist = reader.readStr(30)
		info.album = reader.readStr(30)
		info.year = reader.readStr(4)
		info.comment = reader.readStr(28)
		local zero = reader.readByte()
		local track = reader.readByte()
		local genre = reader.readByte()
		if zero == 0 then
			info.track = track
			info.genre = genre
		else
			info.comment = unpad(info.comment .. string.char(zero, track, genre))
		end
		
		file:seek('end', -128 - 227)
		local hdr = reader.readStr(4)
		if hdr == "TAG+" then
			info.title = unpad(info.title .. reader.readStr(60))
			info.artist = unpad(info.artist .. reader.readStr(60))
			info.album = unpad(info.album .. reader.readStr(60))
			-- some other tags omitted
		end
		
		file:seek('set', position)
		return info
	end
	
	local function readByte()
		local byte = assert(file:read(1), 'Could not read byte.')
		return string.byte(byte)
	end
	local reader = {
		readStr = function(len)
			local str = assert(file:read(len), 'Could not read '..len..'-byte string.')
			return unpad(str)
		end,
		readByte = readByte,
		readInt = function(size, mult)
			mult = mult or 256
			local n = readByte()
			for i=2, size do
				n = n*mult + readByte()
			end
			return n
		end,
		position = function() return file:seek() end,
		skip = function(offset) file:seek('cur', offset) end
	}
	
	-- try ID3v2
	file:seek('set', 0)
	local header = file:read(3)
	if header == "ID3" then
		return decodeID3v2(reader)
	end
	
	-- try ID3v1
	file:seek('end', -128)
	header = file:read(3)
	if header == "TAG" then
		return decodeID3v1(reader)
	end
end

if not ... then
	-- debug if not called by 'require' as module
	require 'pl.pretty'.dump(id3.readtags('song.mp3'))
else
	return id3
end

song.mp3

It seems that id3 tags form bandcamp are a little different

if not ... then
	-- debug if not called by 'require' as module
	metadata = id3.readtags('No Trigger - Adult Braces - 01 Sleeping Bag.mp3')
	--metadata = id3.readtags('song.mp3')
	-- print(#metadata.title)
	-- print(string.sub(metadata.title,3,-1))
	
	for k,v in pairs(metadata) do
		print(k)
		print(v)
	end
else
	return id3
end


$ lua id3.lua 
title
��Sleeping Bag
comment
���Visit http://birdattackrecords.bandcamp.com
year
��2017
album
��Adult Braces
track
��1
artist
��No Trigger

There is some waste at the beginning

if frameDecoders[frameID] always skips