The Complete Guide to Regular Expressions -...

Regular expressions are a tiny language for describing text patterns. They power search-and-replace, validation, and parsing in nearly every language and editor — and they're famously cryptic. This guide builds them up piece by piece so the symbols stop looking like line noise.

Prefer a focused read? Jump to Character Classes, Quantifiers & Anchors for the building blocks, or Groups, Backreferences & Lookarounds for the advanced features.

What a regex is

A regular expression is a pattern matched against text. The simplest patterns are literals — cat matches the letters c-a-t anywhere in the input. The power comes from metacharacters, symbols that mean "a kind of character" or "a repetition" rather than themselves.

In most languages a pattern is written between slashes with optional flags: /pattern/flags. In JavaScript, /cat/i matches "cat" case-insensitively.

Metacharacters and escaping

A handful of characters are special: . ^ $ * + ? ( ) [ ] { } | \. To match one literally, escape it with a backslash — \. matches a real dot, \$ a real dollar sign. The most useful metacharacter is ., which matches any single character (except a newline, by default).

Character classes

A character class in square brackets matches one character from a set:

[aeiou] — any one vowel
[a-z] — any lowercase letter (a range)
[^0-9] — any character that is not a digit (a negated class)

Common sets have shorthands: \d (digit), \w (word character), \s (whitespace), and their negations \D, \W, \S. Full details in Character Classes, Quantifiers & Anchors.

Quantifiers

Quantifiers say how many times the preceding item may repeat:

* — zero or more · + — one or more · ? — zero or one
{3} — exactly 3 · {2,4} — 2 to 4 · {2,} — 2 or more

By default quantifiers are greedy (they match as much as possible); adding ? makes them lazy (as little as possible). So \d+ grabs a whole number, while \d+? grabs one digit at a time.

Anchors and boundaries

Anchors match a position, not a character: ^ is the start of the string (or line), $ is the end, and \b is a word boundary. ^\d+$ means "the entire string is digits"; \bcat\b matches "cat" as a whole word but not inside "category".

Groups, alternation, and flags

Parentheses group part of a pattern and capture what it matched: (\d{4}) captures a four-digit year. A pipe means alternation (or): cat|dog matches either. For capturing, backreferences, and lookarounds, see Groups, Backreferences & Lookarounds.

Flags change how the whole pattern behaves — g (find all matches), i (ignore case), m (^/$ match per line), s (let . match newlines).

Build a pattern incrementally and watch it match with the Regex Tester, and keep the Regex Cheat Sheet handy — all in your browser.

Try it

Type a pattern and paste real text to see matches highlight live as you go — nothing is uploaded.

The Complete Guide to Regular Expressions