Regular Expressions: Basic Character Sets

• With "or" expressions, we can recognize a whole set of characters.

• >

  /^c(a|o|u)t$/.test('cat');

  Result:

  true

• This gets tiresome if we need many options in the "or". Fortunately, we can use a character set to simplify it. The set [aou] is equivalent to (a|o|u).

• >

  /^c[aou]t$/.test('cat');

  Result:

  true

• >

  /^c[aou]t$/.test('cot');

  Result:

  true

• >

  /^c[aou]t$/.test('cet');

  Result:

  false

• What if we want to allow any string of lower case letters? We'd have to write /(a|b|c|d|e| and so on. Instead, we can write another character set.

• >

  /[abcdefghijklmnopqrstuvwxyz]/.test('a');

  Result:

  true

• >

  /[abcdefghijklmnopqrstuvwxyz]/.test('g');

  Result:

  true

• That was shorter, but still wordy. We can specify an entire range of characters by using -.

• >

  /[a-z]/.test('g');

  Result:

  true

• >

  /[1-3]/.test('1');

  Result:

  true

• >

  /[1-3]/.test('a');

  Result:

  false

• >

  /[1-3]/.test('2');

  Result:

  true

• As usual, we escape special characters when we want them to be literal. This range contains only one character, an escaped ] written as \].

• >

  /[\]]/.test(']');

  Result:

  true

• Character sets can be negated to mean "everything not in the set".

• We negate with ^, a character that we already saw. Normally it means "beginning of line". But inside [square brackets], it means "negate the character set". (There are only so many symbols on a keyboard, so some get reused.)

• >

  /[^a]/.test('a');

  Result:

  false

• >

  /[^a]/.test('5');

  Result:

  true

• Negation applies to the entire character set. The regex /[^ab]/ means "any character other than a or b".

• >

  /[^ab]/.test('a');

  Result:

  false

• >

  /[^ab]/.test('c');

  Result:

  true

• >

  /[^ab]/.test('_');

  Result:

  true

• Negation also applies to ranges.

• >

  /[a-z]/.test('h');

  Result:

  true

• >

  /[^a-z]/.test('h');

  Result:

  false

• >

  /[^a-z]/.test('5');

  Result:

  true

• Character sets match exactly one character in the string. (This is like character classes, which also match only one character.) To match more than one character, we can use + or *.

• >

  /^[a-z]$/.test('cat');

  Result:

  false

• >

  /^[a-z]+$/.test('cat');

  Result:

  true