I think it's important to write your own parser, at some point. Though, for the purpose of learning you need someone to guide you.

If you're learning this stuff you need a simple challenge to work through the end.

With regards to lex and yacc, I absolut detest these tools. They are horrible but they do work. And if you just want a functioning parser they'll do. My main criticism of these tools are the horrible errors you might end up with and the lack of sensible extension points.

If it is your first foray into parsers, I do think a simple grammar and handwritten lexer/parser is a good first step. Iterate some on that, then use tools to help with the verbose stuff.

Though to be honest, I enjoy writing parsers by hand. So, I'm a bit biased.