Regex in JavaScript

Q: What is the difference between test() and match()?

test() returns a boolean (true/false) and is faster when you only need to know if a pattern matches. match() returns an array with the matched text, capture groups, and index — use it when you need the actual match data.

Q: How do I escape special regex characters in JavaScript?

Use a backslash before special characters: \. \* \+ \?  \[ \] \{ \} \^ \$ \| \\. For dynamic patterns, escape user input with: str.replace(/[.*+?^${}()|[\]\\]/g, '\\$&').

Q: What flags are available in JavaScript regex?

g (global), i (case-insensitive), m (multiline), s (dotAll — makes . match newlines), u (Unicode), y (sticky — matches at lastIndex), d (hasIndices — ES2022, provides start/end indices for groups), and v (unicodeSets — ES2024, extends u with set notation).

Q: Why does my regex match differently in JavaScript than in Python?

JavaScript and Python use different regex engines. Key differences: JS requires double escaping in RegExp strings, JS lookbehinds must be fixed-length in older engines (variable-length supported in V8 since 2020), and Python has features like atomic groups and possessive quantifiers (3.11+) that JS lacks.

JavaScript uses the built-in RegExp object for regular expressions. Patterns can be created with literal syntax (/pattern/flags) or the RegExp constructor. The engine supports lookaheads, lookbehinds, named capture groups, and Unicode property escapes.

Code Examples

Test if a string matches a pattern

const regex = /^[a-z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2,}$/i;
const isValid = regex.test("user@example.com");
console.log(isValid); // true

The test() method returns true or false. It's the fastest way to check whether a pattern matches anywhere in a string.

Extract all matches with matchAll()

const text = "Prices: $12.99, $5.50, $120.00";
const regex = /\$(\d+\.\d{2})/g;

for (const match of text.matchAll(regex)) {
  console.log(match[0], "->", match[1]);
}
// $12.99 -> 12.99
// $5.50 -> 5.50
// $120.00 -> 120.00

matchAll() returns an iterator of all matches including capture groups. Requires the g flag. Available in ES2020+.

Named capture groups

const dateRegex = /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/;
const match = "2026-03-08".match(dateRegex);

console.log(match.groups.year);  // "2026"
console.log(match.groups.month); // "03"
console.log(match.groups.day);   // "08"

Named groups use (?<name>...) syntax. Access them via match.groups.name instead of numeric indices — much cleaner for complex patterns.

Replace with a callback function

const text = "hello world";
const result = text.replace(/\b\w/g, (char) => char.toUpperCase());
console.log(result); // "Hello World"

String.replace() accepts a function as the second argument. The function receives each match and can return a dynamic replacement.

Split a string with regex

const csv = "one, two ,three,  four";
const parts = csv.split(/\s*,\s*/);
console.log(parts); // ["one", "two", "three", "four"]

String.split() accepts a regex, which is useful for splitting on variable-width delimiters like commas with optional whitespace.

Lookahead and lookbehind

// Positive lookahead: match digits followed by "px"
"12px 3em 100px".match(/\d+(?=px)/g); // ["12", "100"]

// Positive lookbehind: match digits preceded by "$"
"$50 and $120".match(/(?<=\$)\d+/g); // ["50", "120"]

// Negative lookahead: match digits NOT followed by "px"
"12px 3em 100px".match(/\d+(?!px)/g); // ["1", "3", "10"]

Lookaheads (?=...) and lookbehinds (?<=...) match a position without consuming characters. Negative variants use (?!...) and (?<!...).

Note

JavaScript regex literals use forward slashes: /pattern/flags. The RegExp constructor takes a string, so backslashes must be doubled: new RegExp('\\d+'). The u flag enables full Unicode support including \p{...} property escapes. The v flag (ES2024) adds set notation and improved Unicode handling.

Regex in Other Languages

Regex in Python →Regex in Java →Regex in Go →Regex in PHP →

Frequently Asked Questions

What is the difference between test() and match()?

test() returns a boolean (true/false) and is faster when you only need to know if a pattern matches. match() returns an array with the matched text, capture groups, and index — use it when you need the actual match data.

How do I escape special regex characters in JavaScript?

Use a backslash before special characters: \. \* \+ \?  \[ \] \{ \} \^ \$ \| \\. For dynamic patterns, escape user input with: str.replace(/[.*+?^${}()|[\]\\]/g, '\\$&').

What flags are available in JavaScript regex?

g (global), i (case-insensitive), m (multiline), s (dotAll — makes . match newlines), u (Unicode), y (sticky — matches at lastIndex), d (hasIndices — ES2022, provides start/end indices for groups), and v (unicodeSets — ES2024, extends u with set notation).

Why does my regex match differently in JavaScript than in Python?