RPL is a variant of modern Regular Expressions (regex) that is designed to scale to big data, many developers, and large collections of patterns. If you use regex, you already know a lot of RPL. Additional features over regex found in RPL:

Looks like a programming language, and plays well with development tools
Comes with a library of dozens of useful patterns (timestamps, network addresses, and more)
Has development tools: tracing, REPL, color-coded match output
Produces JSON output (and other formats)