M4

M4 is a general-purpose macro processing language. It is probably most often encountered in the context of configuring sendmail or autoconf, both of which use it to generate their inscrutable native configuration files.

Software

 * GNU M4

Sample files

 * https://telparia.com/fileFormatSamples/text/m4/

Resources

 * Wikipedia article
 * Notes on the M4 Macro Language