From the README:
The re package is a small, portable, lightweight, and quick, regular expression library for Common Lisp. It is a non-recursive, backtracing VM. The syntax is similar to Lua-style pattern patching (found here), but has added support for additional regex features (see below). It’s certainly not the fastest, but is very easy to understand and extend.
It makes heavy use of the monadic parse combinator library for parsing the regular expressions. If you’d like to understand the parsing and compiling of regular expressions, I recommend reading up on that library as well.