Regular Expressions, Explained

You've seen them: a wall of slashes, backslashes, and dollar signs like ^\d{4}-\d{2}-\d{2}$ - and your stomach sank a little. Regular expressions have a reputation for being write-only magic that only wizards understand. That reputation is undeserved. A regex is one small idea wearing a scary costume, and once you see the idea, the costume stops working on you.

This guide makes regex readable instead of terrifying. We'll start with what a regex actually is (a pattern that describes a shape of text), learn the handful of pieces you'll reach for almost every time, and then meet the real-world traps - greedy matching, escaping, and regex that turns into gibberish - so they don't bite you.

How to read this

Just need to recognize the pieces? Skim Phase 2: The Core Toolkit - it's a tour of every symbol you'll actually use, each with a tiny example.
Want it to finally make sense? Read in order. Each phase builds on the last, and the whole thing rests on the one idea in Phase 1.

The phases

What a Regex Actually Is - the mental model: you're describing the shape of text, not writing code. With a tiny first example you can see match and not-match against.
The Core Toolkit - the pieces you'll use 90% of the time: literals, character classes, quantifiers, anchors, and groups - built up to matching something real, and an honest word on why "the perfect email regex" is a trap.
Using Regex for Real (and the Gotchas) - where you meet regex (editors, grep, code), and the classic traps: greedy vs lazy matching, escaping special characters, and regex becoming unreadable - with the cure for each.

Deeper material - lookahead/lookbehind, backreferences, the differences between regex flavors (PCRE vs JavaScript vs POSIX), and catastrophic backtracking - is deliberately left for a follow-up guide. You can do an enormous amount of real work with only what's here.

Related reading: Programming From Zero for the basics underneath this, and The Terminal and Shell for using regex with tools like grep.