regex word boundary \b — match whole words only

Q: What counts as a word boundary

\b matches at a position between a word character ( \w : [a-zA-Z0-9_] ) and a non-word character. ``` // Boundaries in "hello world": // |hello| |world| // ^ between start-of-string and 'h' → \b // ^ between 'o' and ' ' → \b // ^ between ' ' and 'w' → \b // ^ between 'd' and end-of-string → \b ```

Quick Answer

// Match "cat" as a whole word — not "cats", "concatenate"
/\bcat\b/

// JavaScript
'The cat concatenate scat'.match(/\bcat\b/g)
// ['cat']

# Python
import re
re.findall(r'\bcat\b', 'The cat concatenate scat')
# ['cat']

Usage

Without \b, a pattern like /cat/ matches inside "concatenate" and "scat".

Other causes & fixes

What counts as a word boundary

\b matches at a position between a word character (\w: [a-zA-Z0-9_]) and a non-word character.

// Boundaries in "hello world":
// |hello| |world|
// ^ between start-of-string and 'h' → \b
// ^ between 'o' and ' '            → \b
// ^ between ' ' and 'w'            → \b
// ^ between 'd' and end-of-string  → \b

\B — match NON-word boundary

// Match "cat" only inside a longer word
/\Bcat\B/.test('concatenate')   // true
/\Bcat\B/.test('cat')           // false

Replace whole words only

// JavaScript
'foo foobar foo'.replace(/\bfoo\b/g, 'baz')
// 'baz foobar baz'  — foobar is untouched

# Python
import re
re.sub(r'\bfoo\b', 'baz', 'foo foobar foo')
# 'baz foobar baz'