Course 04 — Regex, visually

Regularexpressions you can see.

Each lesson is a puzzle: strings that must match and strings that must not. Type a pattern and every test lights up as it changes, while a diagram draws the pattern as a path the engine follows.

Start lesson one Open the sandbox

16 lessons · 5 chapters · free

Regex — a pattern taking shape
  1. 01 · Read

    Each lesson is one idea, explained in a few short paragraphs — no walls of theory.

  2. 02 · Do

    Type into a live environment and watch it react. Tab completes, ↑ recalls, undo takes back any step.

  3. 03 · Match

    Reach the goal any way you like — you're judged on the result, not the keystrokes. Par is the shortest route.

01

Characters

Literals, the dot, character classes and their shorthands.

  1. 01 Find a word The simplest pattern is just the text you're looking for.
  2. 02 One of several characters [ae] — a character class.
  3. 03 Ranges and negation [0-9a-f], and [^…] for "anything but".
  4. 04 Shorthand classes \d, \w and \s — the classes you'll type most.
  5. 05 The dot, and escaping it . matches anything — so a real dot needs \.
02

Repetition

Quantifiers — how many, and how greedily.

  1. 06 Optional characters ? means zero or one.
  2. 07 One or more, zero or more + and *.
  3. 08 Exact counts {3}, {2,4} and {2,}.
03

Anchors & boundaries

Matching at the start, the end, or the edge of a word.

  1. 09 Start and end ^ and $ — the whole string, nothing more.
  2. 10 Whole words only \b — the edge of a word.
04

Groups

Alternation, capture groups and backreferences.

  1. 11 Either this or that a|b, and grouping with ( ).
  2. 12 Capture the parts Groups don't just group — they remember.
  3. 13 Repeat what you matched \1 — the same text again.
05

Advanced

Lookarounds, lazy quantifiers and real-world patterns.

  1. 14 Greedy and lazy Why .* eats too much, and how ? fixes it.
  2. 15 Lookahead Check for something without consuming it.
  3. 16 A real-world pattern Putting it together: a practical email check.

Patterns run on your browser's own JavaScript regex engine. The syntax taught here is shared by Python, Ruby, PCRE and most editors; the lessons flag the few places they disagree.