Regular Expression Character Classes

In JavaScript, regular expressions (RegEx) allow you to match patterns in strings. Character classes are a fundamental concept that define a set of characters you want to match. They are enclosed in square brackets [] or use predefined shorthand notations for common patterns.

1. Basic Character Classes

Character classes let you match any one character from a specified set or range.

Syntax	Description	Example
[abc]	Matches any one character in the set (a, b, or c)	"cat".match(/[abc]/g) → ["c","a"]
[a-z]	Matches any lowercase letter	"hello".match(/[a-z]/g) → ["h","e","l","l","o"]
[A-Z]	Matches any uppercase letter	"HELLO".match(/[A-Z]/g) → ["H","E","L","L","O"]
[0-9]	Matches any digit	"123".match(/[0-9]/g) → ["1","2","3"]
[a-zA-Z0-9]	Matches any alphanumeric character	"Ab1".match(/[a-zA-Z0-9]/g) → ["A","b","1"]

2. Negated Character Class

Use ^ inside brackets to negate the set. It matches any character not listed.

const regex = /[^0-9]/; // Matches any non-digit
console.log(regex.test("a")); // true
console.log(regex.test("5")); // false

3. Predefined Shorthand Character Classes

JavaScript provides shortcuts for commonly used character sets:

Syntax	Matches	Example
\\d	Any digit [0-9]	/\\d/.test('5') → true
\\D	Any non-digit [^0-9]	/\\D/.test('a') → true
\\w	Any word character [a-zA-Z0-9_]	/\\w/.test('G') → true
\\W	Any non-word character	/\\W/.test('@') → true
\\s	Any whitespace (space, \\t, \\n)	/\\s/.test(' ') → true
\\S	Any non-whitespace character	/\\S/.test('a') → true

4. Dot . Character

• Matches any character except newline (\\n)
• Use the s flag to include newlines

const regex = /./;
console.log(regex.test("a")); // true
console.log(regex.test("\\n")); // false

5. Unicode Property Escapes (ES2018+)

For advanced Unicode matching, use \\p{...} with the u flag.

const emojiRegex = /\\p{Emoji}/u;
console.log(emojiRegex.test("?")); // true

6. Combining Character Classes

You can combine ranges, negations, and shorthand classes in a single set:

const regex = /[A-Za-z\\d_]/; // Matches letters, digits, or underscore
console.log(regex.test('Z')); // true
console.log(regex.test('9')); // true
console.log(regex.test('-')); // false

7. Character Class Quantifiers

Character classes can be combined with quantifiers to match multiple occurrences:

Quantifier	Meaning	Example
+	One or more	/[a-z]+/.test('abc') → true
*	Zero or more	/[a-z]*/.test('') → true
?	Zero or one	/[a-z]?/.test('a') → true
{n,m}	Between n and m times	/[a-z]{2,4}/.test('abc') → true

8. Escaping Special Characters

Special characters like ., *, [, ] need to be escaped with \\ if you want to match them literally.

console.log("[test]".match(/[\[\]]/g)); // ["[", "]"]