Regular Expressions: Literals

• Regular expressions (regexes) are patterns that describe strings. We might write a regex for filenames ending in ".jpg". Or we might write one that recognizes phone numbers. We'll use JavaScript's regexes, but it's OK if you don't know JavaScript.

• We can ask whether a regex matches a given string: /a/.test('cat'). The regex /a/ means "any string containing an a". We asked about 'cat', which contains an "a", so we'll get true back.

• >

  /a/.test('a');

  Result:

  true

• >

  /a/.test('b');

  Result:

  false

• >

  /a/.test('xax');

  Result:

  true

• Regexes are case-sensitive, so "a" and "A" are different characters.

• >

  /A/.test('a');

  Result:

  false

• >

  /A/.test('A');

  Result:

  true

• Multi-character regexes test that the characters appear together. They have to be adjacent, with no other characters between them.

• >

  /cat/.test('cart');

  Result:

  false

• >

  /cat/.test('cat');

  Result:

  true

• >

  /cat/.test('the cat there');

  Result:

  true

• Spaces are treated as normal characters. If they're in the regex, they'll have to be in the matched string too. Likewise for _, -, and some other punctuation. (But some punctuation has special meaning, which we'll see soon.)

• >

  /a cat/.test('that is a cat');

  Result:

  true

• >

  /_/.test('happy-cat');

  Result:

  false

• >

  /_/.test('happy_cat');

  Result:

  true