Regular Expressions: Boundaries

• Often, we want to match text at the beginning and end of strings. We'll use boundaries for that. The first is ^, which means beginning of string.

• >

  /^cat/.test('cat');

  Result:

  true

• >

  /^cat/.test('cats are cute');

  Result:

  true

• >

  /^cat/.test('I like cats');

  Result:

  false

• >

  /^cat/.test('cats like dogs');

  Result:

  true

• >

  /^cat/.test('dogs like cats');

  Result:

  false

• The second boundary is $, which means end of string.

• >

  /cat$/.test('a dog');

  Result:

  false

• >

  /cat$/.test('a cat');

  Result:

  true

• >

  /cat$/.test('cats');

  Result:

  false

• Usually, we want to match an entire string. Most regexes should include these boundaries, ^ and $.

• >

  /^a$/.test('a');

  Result:

  true

• >

  /^a$/.test('the letter a');

  Result:

  false

• >

  /^a$/.test('a cat');

  Result:

  false

• To match only the empty string, we can smash the ^ and $ together.

• >

  /^$/.test('a');

  Result:

  false

• >

  /^$/.test(' ');

  Result:

  false

• >

  /^$/.test('');

  Result:

  true

• If we use the wrong boundaries, there's no error. The regex always returns false.

• >

  /$cat^/.test('cat');

  Result:

  false

• If we use boundaries in the wrong place, the regex will always return false.

• >

  /cat$s/.test('cats');

  Result:

  false

• >

  /cat^s/.test('cat');

  Result:

  false

• One final note about the ^ and $ boundaries. Above, we said that they match the beginning and end of strings. In most cases, that's true, but only because most regex engines default to "single-line mode": a regex only considers the first line in the string.

• Regex engines also have "multi-line mode", where a single regex can match any part of a multi-line string. In that mode, ^ means "beginning of line" and $ means "end of line." They can match characters that are at the beginning or end of a line, even if those characters aren't at the beginning or end of the entire string.

• Multi-line regexes are their own topic, so we won't cover them in detail here. But in practice, you may see ^ referred to as both "beginning of line" and "beginning of string", and likewise for $.