Lately, out of curiosity, I spent some time to better understand how the C preprocessor works. I admit it, I thought it was a very dumb copy-paste based replace mechanism, only capable of doing the simpler keyword matching and replacement. Boy, was I wrong. Turns out the preprocessor is actually an organically grown pseudo language (as opposed to a properly designed language feature) inside C, which later got standardized through an incredibly complex set of rules and definitions. Rules for recursion, expansion, pattern matching and crazy operators like # and ## are some of the things that I never before knew existed in the preprocessor.