Here is a really interesting article about how to write one: (Even better, he gives the full source to a JavaScript formatter and colorizer.)
Implementing a syntax-higlighting JavaScript editor in JavaScript
or
A brutal odyssey to the dark side of the DOM tree
How does one do decent syntax
highlighting? A very simple scanning
can tell the difference between
strings, comments, keywords, and other
code. But this time I wanted to
actually be able to recognize regular
expressions, so that I didn't have any
blatant incorrect behaviour anymore.
Importantly, it handles regex correctly. Also of interest is that he used a continuation passing style lexer/parser instead of the more typical lex (or regex) based lexers that you'll see in the wild.
As a bonus he discusses a lot of real-world issues you'll run into when working with JavaScript in the browser.
