JavaScript RegExp Word Boundary Metacharacter (\b)

What Is \b?

\b is a word-boundary anchor. It matches a position (zero width)—not a character—where a word character meets a non-word character, or the start/end of the string.

\b     → word boundary (zero-width)
\B     → NOT a word boundary
\w     → word char: [A-Za-z0-9_]

/\bcat\b/ matches "a cat" — not "catalog"
/cat/     matches "catalog" (partial)

\b does not consume spaces or letters

How to Use \b

Wrap the target token with boundaries when you need a whole-word match.

1. Write the word or token you want (for example cat).
2. Add \b on both sides: /\bcat\b/.
3. Test with regex.test(str) or extract with str.match(...).
4. Remember: \b is a position — it never eats a space or letter.

Pseudocode

word char = letter, digit, or underscore

\b succeeds at a position when:
  (left is word AND right is non-word)
  OR (left is non-word AND right is word)
  OR (start of string AND first char is word)
  OR (end of string AND last char is word)

\b never advances the cursor (zero width)

/\bcat\b/ means:
  boundary → "cat" → boundary

Cheat Sheet

Keep these facts handy while writing whole-word patterns.

Live Preview

Pick a pattern and see whether it matches with RegExp.prototype.test.

pattern = /\bcat\b/
text = "a cat"
pattern.test(text)
→ true

/* boundaries around the whole word "cat" */

JavaScript Examples

Short, copy-ready snippets. Open DevTools Console, or use Try It Yourself for a live editor.

const pattern = /\bcat\b/;

console.log(pattern.test("a cat"));    // true
console.log(pattern.test("the cat!")); // true
console.log(pattern.test("catalog"));  // false
console.log(pattern.test("scat"));     // false

true
true
false
false

const text = "catalog";

console.log(/cat/.test(text));      // true  (substring)
console.log(/\bcat\b/.test(text));  // false (not a whole word)

console.log(/\bcat\b/.test("cat-dog")); // true (hyphen is non-word)

true
false
true

const text = "the cat sat";
const words = text.match(/\b\w+\b/g);

console.log(words);
// [ "the", "cat", "sat" ]

console.log(/\bjs\b/i.test("Learn JS now")); // true

[ 'the', 'cat', 'sat' ]
true

Performance Notes

Anchors like \b are inexpensive. Cost usually comes from the rest of the pattern (greedy quantifiers, backtracking), not from the boundary itself.

Key Takeaways