The preceding section described the alias module and its allies. Everything these directives can do, and more, can be done instead by mod_rewrite.c, an extremely compendious module that is almost a complete software product in its own right.[†] The documentation is thorough, and the reader is referred to http://www. engelschall.com/pw/apache/rewriteguide/ for any serious work. This section is intended for orientation only.
[†] But for simple tasks Alias and friends are much easier to use.
Rewrite takes a rewriting pattern and applies it to the URL. If it matches, a rewriting substitution is applied to the URL. The patterns are regular expressions familiar to us all in their simplest form; for example, mod.*\.c , which matches any module filename. The complete science of regular expressions is somewhat extensive, and the reader is referred to ... /src/regex/regex.7, a manpage that can be read with nroff -man regex.7 (on FreeBSD, at least). Regular expressions are also described in the POSIX specification and in Jeffrey Friedl's Mastering Regular Expressions (O'Reilly & Associates). The essence of regular expressions is that a number of special characters can be used to match parts of incoming URLs.
The substitutions can include mapping functions that take bits of the incoming URL and look them up in databases or even apply programs to them. The rules can be applied repetitively and recursively to the evolving URL. It is possible (as the documentation says) to ...