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.
The easiest way to try out Retina is to use it right in your browser at Try It Online!
There is an up-to-date .NET binary of Retina in the root directory of the repository. Alternatively, you can build it yourself from the C# sources. The code requires .NET 4.5. I do not regularly test Retina with Mono, but previous versions have worked without problems.
Source files can simply be passed as command-line arguments. For details of ways to invoke Retina see the docs.
How does it work?
Full documentation of the language can be found in the Wiki. For a (hopefully) more accessible introduction to the language's basic features see the annotated example programs. It might also be worth having a look at the changelog.