I don't mind paying for quality content (and I mind paying in cash a lot less than paying with personal data), but if someone wants a free alternative to this,

I don't mind paying for quality content (and I mind paying in cash a lot less than paying with personal data), but if someone wants a free alternative to this, I recommend https://craftinginterpreters.com/ https://craftinginterpreters.com/ . Bit lighter on the theory side, but exactly what you need if you want to write a parser in practice.