Lookaround

An assertion that tests the text beside the current position and consumes none of it. Four spellings — (?=…), (?!…), (?<=…), (?<!…) — and one genuinely hard question underneath them: how wide may a lookbehind be?

Lesson	The question
Lookahead and lookbehind	What zero-width buys you, the two traps that catch everyone (\d+(?! EUR) matching 25), and the width rule in seven engines.

If you came looking for forward references: they are in the backreferences chapter. Lookaround is about position; a forward reference is about captured text. The first lesson there sets the four names side by side.