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Regex — The Definitive Reference

From literals to lookaheads, from catastrophic backtracking to ReDoS hardening.
Everything you need to think in regex.

A regular expression (regex) is a sequence of characters that defines a search pattern. They are a formal language rooted in Kleene's theory of regular sets (1956), later formalized by Ken Thompson into the first practical implementation for Unix's ed editor in 1968.

Despite the name, most modern "regex" engines far exceed true regular languages — they support backreferences, lookarounds, and recursion, making them computationally more powerful (and more dangerous) than pure finite automata.

What Regex Can and Cannot Do

✓ Great for ✗ Cannot parse ⚠ Be careful with
Pattern matching, validation, extraction Recursive structures (HTML/XML) Untrusted input
Search-and-replace, tokenizing Properly nested parens (without extensions) Catastrophic backtracking
Log parsing, lightweight transforms Context-sensitive grammar Over-engineering simple string ops

Anatomy of a Pattern

/^(\d{4})-(0[1-9]|1[0-2])-(\d{2})$/gm
 │  │       │               │         │└─ flags
 │  │       │               │         └── delimiter (JS)
 │  │       │               └──────────── group 3: day
 │  │       └──────────────────────────── group 2: month (alternation)
 │  └──────────────────────────────────── group 1: year
 └─────────────────────────────────────── start anchor

Guide Structure

This reference is organized into seven areas:

  1. Foundations — syntax, character classes, anchors, quantifiers, alternation
  2. Grouping — captures, named groups, backreferences, lookarounds
  3. Flags & Unicode — modifiers, Unicode property escapes, encoding traps
  4. Advanced — greediness internals, atomic groups, conditionals, recursion
  5. Engines — NFA vs DFA, performance, catastrophic backtracking, ReDoS
  6. Languages — JS, Python, Go, Java, PCRE/PHP specifics
  7. Practical — production patterns, best practices, gotchas, cheat sheet

Start Here

New to regex? Read Syntax Fundamentals → Characters & Classes → Quantifiers in order.
Experienced? Jump straight to Catastrophic Backtracking or the Cheat Sheet.