Welcome to the Retina wiki!
Retina is a regex-based recreational programming language. Every program works by reading a (finite) string from standard input, transforming it via a series of regex operations (e.g. counting matches, filtering lines, and most of all substituting). Retina was built on top of .NET's regex engine, but provides its own, more powerful substitution syntax.
Retina was mainly developed for code golf which may explain its very terse configuration syntax and some weird design decisions.
See The Language for complete documentation of all language features. I will try to write a more accessible tutorial for anyone new to the language soon.