Regular Expressions: Word Boundaries

• When using a regular expression to search for a word, we usually don't want to match those letters inside another word.

• >

  /cat/.test("I couldn't locate Mr. Meow.");

  Result:

  true

• The word 'locate' contains 'cat', so this sentence matches. But in this case, we only intended to match the actual word 'cat'. Regexes provide a way to do this using the word boundary \b:

• >

  /\bcat\b/.test('It was difficult to locate, but');

  Result:

  false

• >

  /\bcat\b/.test('the cat returned.');

  Result:

  true

• \b only matches where a word character is next to a non-word character. (Remember that word-characters are letters, numbers and underscores.)

• >

  /\bcat\b/.test("var cat_name = 'Mr. Meow';");

  Result:

  false

• >

  /\bcat\b/.test("Where's the cat's toy?");

  Result:

  true

• >

  /\bcat\b/.test('cat-like reflexes');

  Result:

  true

• In the last example above, the "c" in "cat" was also the first character in the string. Still, that string matched /\bcat\b/. It matches because the beginning and end of the string are considered non-word characters.

• Like most character classes, \b can be negated by capitalizing it. \B only matches between two word characters. It's pronounced "non-word-boundary".

• >

  /\Bcat\B/.test('The cat over there');

  Result:

  false

• >

  /\Bcat\B/.test('concatenate');

  Result:

  true

• \B can be used to find out if a word has 'cat' in particular places, which could help win Scrabble games.

• If you'd like to find words that contain "cat", but don't end with "cat":

• >

  /cat\B/.test('publication');

  Result:

  true

• >

  /cat\B/.test('wildcat');

  Result:

  false

• >

  /cat\B/.test('catenary');

  Result:

  true

• Or words that contain "cat" only in the middle:

• >

  /\Bcat\B/.test('cathode');

  Result:

  false

• >

  /\Bcat\B/.test('muscat');

  Result:

  false

• >

  /\Bcat\B/.test('hecatomb');

  Result:

  true