One important thing here has to be remembered: Whenever you use parentheses in Pattern or in one of the CondPattern, back-references are internally created which can be used with the strings $N
and %N (see Section 25.3). These are available for creating the strings Substitution and TestString. Figure 25-3 shows to which locations the backreferences are transfered for expansion.
We know this was a crash course on mod_rewrite's internal processing. But you will benefit from this knowledge when reading the following. mod_rewrite's behaviour can be controlled with numerous
directives, they are all described in the Apache Documentation of mod_rewrite. Further, real world examples of
mod_rewrite use are presented in the Apache URL Rewriting Guide.
But besides the directives available for mod_rewrite, you will also need a grasp of regular expressions (Section 25.3) and the .htaccess file (Section 25.4) to understand the solution for a search engine friendly PHP-Nuke site that we will present later on, in Section 25.5.
Not so fast! "Regular expression?" Right. If you've used "wildcards" with MS-DOS or Windows, somefile.* or some?.exe for example, you already understand the concept. But regex (regular
expressions) are much more powerful. And complex. Read on! 
