Regex Lookahead and Lookbehind: Patterns Every Developer Should Know

Lookaheads and lookbehinds are the regex features that separate "I know regex" from "I know regex." They let you match patterns based on what comes before or after — without including that context in the match itself. Once you understand them, problems that seemed impossible with basic regex become straightforward. Let's break them down. What Are Lookaheads and Lookbehinds? Think of regular expressions as a cursor moving through text. Normally, every part of your pattern consumes characters — the cursor advances as it matches. Lookaheads and lookbehinds are different: they peek at surrounding text without consuming it. The cursor looks, but doesn't move. An analogy: Imagine you're searching a bookshelf for books with red covers. A normal regex is like pulling each book off the shelf to check it. A lookahead is like glancing at the next book without pulling it out. A lookbehind is like glancing at the previous book. You're gathering information, but you're not removing anything from the