Syntax Updated November 2026

Word boundary \b

\b matches the position between a word character (\w: letter, digit or underscore) and a non-word character or the edge of the string, without consuming anything. \bcat\b matches cat in the cat sat but not in concatenate. \B is the opposite: a position that is not a word boundary.

Syntax

\bword\b      \B

Examples

"the cat concatenated cats".match(/\bcat\b/g)

Result: ["cat"]

"the cat concatenated cats".replace(/\bcat\b/g, "dog")

Result: "the dog concatenated cats"

"price $45 and 450".match(/\b\d+\b/g)

Result: ["45", "450"]

"cat concatenate".match(/\Bcat\B/g)

Result: ["cat"]

Only the cat inside concatenate: letters on both sides, so no boundary.

Common mistakes

Writing "\bcat\b" in a plain JavaScript string for new RegExp: \b is a backspace there, so write "\\bcat\\b".
Expecting \b before a symbol: in "pay $5", \b\$ fails because both the space and $ are non-word characters and there is no boundary between them.
Assuming \b understands Unicode: in JavaScript it is ASCII-based even with the u flag, so é counts as a non-word character.
Forgetting that a hyphen is a non-word character: \bwell\b matches inside well-known.

Practice Word boundary \b

These drills check your answer on live data, free, no account needed.

Word boundaries: replace a whole wordReplace every whole word cat with dog. Words that only contain cat, like catalog or bobcat, stay unchanged.
Anchors: lines that start with ERRORA match anywhere counts, so anchor the pattern yourself. Match lines that begin with the word ERROR in capitals.
Alternation: cat, dog or birdThe whole line must match. Match exactly one of the words cat, dog or bird.

Questions

What is \b in regex?

A zero-width assertion for a word boundary: the spot where a word character meets a non-word character or the string edge. It lets you match whole words without consuming the surrounding spaces.

How do I match whole words only?

Wrap the word in boundaries: /\bword\b/. For a list of words use /\b(?:red|green|blue)\b/g.

Why does \b not work with accented or non-Latin letters?

JavaScript treats only [A-Za-z0-9_] as word characters for \b. Use lookarounds with Unicode classes instead: (?<!\p{L})word(?!\p{L}) with the u flag.

Anchors ^ and $What ^ and $ match, how they change with the m flag, whole-string versus per-line matching, and the mistakes with alternation and trailing newlines.
Character classesWhat a character class matches, how ranges and negation work, what \d \w \s and their uppercase opposites mean, and what the dot does and does not match.
Escaping special charactersThe regex metacharacters that must be escaped, how to match a literal dot, dollar, plus or slash, what changes inside brackets, and how to escape user input.
Lookbehind (?<=...) and (?<!...)How lookbehind checks what precedes the match, grabbing the number after a dollar sign or excluding negatives, browser and Python support, and width limits.