Character classes and ranges in regex

Character Classes Match One of Many

Square brackets define a character class — a set of characters where exactly one must match at that position. [aeiou] matches any single lowercase vowel. [0-9] matches any digit via a range shorthand.

/[aeiou]/.test('sky') // false — no vowels
/[aeiou]/.test('the') // true  — 'e' matches

/[a-z]/.test('A')     // false — lowercase only
/[a-zA-Z]/.test('A')  // true  — upper and lower combined

Negated Classes

A caret ^ at the start of a class negates it: match any character NOT in the set.

/[^0-9]/.test('abc') // true  — no digits
/[^0-9]/.test('a1b') // true  — has non-digit chars
/[^a-z]+/.test('123') // true

Shorthand Classes

Three shorthands cover the most common sets:

• \d — [0-9]
• \w — [a-zA-Z0-9_]
• \s — whitespace (space, tab, newline)

Their uppercase inverses negate them: \D matches any non-digit, \W any non-word character, \S any non-whitespace.

/\d{3}/.test('abc')  // false
/\d{3}/.test('123')  // true
/\w+/.test('hello_world') // true

Character classes compose well with quantifiers. [a-z]+ matches one or more lowercase letters; [0-9]{4} matches exactly four digits. You can also mix ranges and individual characters in one class: [a-z0-9_] matches any lowercase letter, digit, or underscore (which is equivalent to \w minus uppercase). When building patterns for identifiers, slugs, or usernames, combining ranges in a single class is cleaner and faster than alternation with multiple sub-patterns.