Don't Look Back, part 3: Back references in regex and grammars

This post is part of a series, though it can be read independently of the others: Part 1: Regex and Unicode Part 2: The Four Eras of Regex Part 3: Back references in regex and grammars (this post) Part 4: Why is everything on fire? (forthcoming) Today, I want to talk about the back-reference feature of regex, and how we can extend it to grammars, in particular to Parsing Expression Grammars. For a few years now, we have had an experimental implementation of back references in the Rosie project. With some experience using the feature, we are ready to remove its experimental status. In the next release, Rosie v1.5, it will be a supported feature, so it’s time to describe it in detail.