Where can I learn the basics of writing a lexer?

Question

I want to learn how to write a lexer. My university course had an assignment where we had to write a parser (and a lexer to go along with it) but this was given to us with no instruction or feedback (beyond the mark) so I didn't really learn much from it.

After searching for this topic, I can only find fairly advanced write ups which focus on areas which I feel are a few steps ahead of where I am at. I want a discussion on the basics of writing a lexer for a very simple language which I can use as a basis for investigating tokenising more complex languages.

At this stage I'm not really interested in best practices or optimisation techniques but instead prefer a focus on the essentials. What are some good resources to get me started?

Answer 1

Basically there are two main approaches to writing a lexer:

1. Creating a hand-written one in which case I recommend this small tutorial.
2. Using some lexer generator tools such as lex. In this case, I recommend reading the tutorials to the particular tool of choice.

Also I would like to recommend the Kaleidoscope tutorial from the LLVM documentation. It runs through the implementation of a simple language and in particular demonstrates how to write a small lexer. There is a C++ and an Objective Caml version of the tutorial.

The classical textbook on the subject is Compilers: Principles, Techniques, and Tools also known as the Dragon Book. However this probably falls under the category of "fairly advanced write ups".