Character Classes

You will be able to use JavaScript regular expression character classes to match one character from a set, a range, or a shorthand group such as digits or word characters.

What it is

A character class is written inside square brackets, such as [aeiou]. It matches exactly one character from the allowed set. A range like [a-z] means every lowercase letter from a to z. A leading caret, [^aeiou], negates the class and matches one character that is not in the set. JavaScript also provides shorthand classes: \d for digits, \w for word characters, and \s for whitespace. Related terms include quantifiers, flags, and alternation.

Why it matters

• Validate user input, such as checking whether a field contains only letters.
• Extract specific characters, such as vowels, digits, or punctuation.
• Replace unwanted characters, such as removing non-alphanumeric text.
• Build compact patterns instead of listing many alternatives.
• Support case-insensitive matching when combined with the i flag.

Syntax or steps

The smallest useful pattern is:

/[aeiou]/g

Read it as: match one character from the set a, e, i, o, or u, and do it globally.

• [abc] matches one of a, b, or c.
• [a-z] matches one lowercase letter.
• [A-Za-z0-9] matches one alphanumeric character.
• [^0-9] matches one non-digit character.
• \d, \w, and \s are shorthand classes.

Inside a character class, most symbols are literal, but ^, -, ], and \ have special meaning. Put - first or last, or escape it, if you want a literal hyphen.

Example

const text = "Hello, World! 2024";

const vowels = /[aeiou]/g;
const digits = /\d/g;
const nonAlphanumeric = /[^A-Za-z0-9]/g;

console.log(text.match(vowels));
console.log(text.match(digits));
console.log(text.match(nonAlphanumeric));

Expected output:

[ 'e', 'o', 'o' ]
[ '2', '0', '2', '4' ]
[ ',', ' ', '!', ' ' ]

Part by part:

• /[aeiou]/g finds each lowercase vowel. The g flag returns all matches, not just the first.
• /\d/g finds each digit.
• /[^A-Za-z0-9]/g finds each character that is not a letter or digit.
• text.match(...) returns an array of matched characters, or null if there are no matches.

Common mistakes

• Forgetting the g flag when you need every match. Without g, match returns only the first match.
• Using [a-z] when uppercase letters should also match. Fix it by using [A-Za-z] or adding the i flag.
• Placing - in the middle of a class accidentally, such as [a-z-0-9]. If you want a literal hyphen, put it at the end: [a-z0-9-].
• Thinking a class matches multiple characters. [aeiou] matches one vowel; use a quantifier like [aeiou]+ for one or more.

When to use it

Use a character class when you need to match one character from a set or range. Use alternation when you need to match whole words or multi-character options.

Practice

Guided exercise: write a pattern that matches every lowercase consonant in "JavaScript".

const word = "JavaScript";
const consonants = /[bcdfghjklmnpqrstvwxyz]/g;
console.log(word.match(consonants));

Expected output:

[ 'v', 'c', 'r', 'p', 't' ]

Challenge: extract all word-like tokens from "user_42, email@site.com". Hint: use \w+. Expected output: [ 'user_42', 'email', 'site', 'com' ].

Quick check

What does /[^0-9]/g match?

It matches every character that is not a digit.

Summary

Character classes let you describe a single allowed character compactly using sets, ranges, negation, or shorthand groups. They are useful for validation, extraction, and replacement, especially when combined with flags and quantifiers.