Regular Expressions in JavaScript

Also Known As

TL;DR

Explanation

JS regex: /pattern/flags or new RegExp(pattern, flags). Key methods: test() (returns boolean), match() (returns array or null), matchAll() (returns iterator of all matches — requires g flag), replace()/replaceAll() (string replacement), split() (split by pattern). Flags: g (global — all matches), i (case-insensitive), m (multiline — ^ and $ match line boundaries), s (dotAll — . matches newlines), u (Unicode — enables \u{1F600} and correct Unicode handling), d (indices — adds match index info). Named capture groups: (?<name>pattern). Lookahead: (?=) positive, (?!) negative.

Common Misconception

Why It Matters

Common Mistakes

• Using RegExp.test() with g flag in a loop — lastIndex advances causing false negatives.
• Not using the u flag for Unicode strings — \w does not match accented characters without u.
• match() without g flag returns only first match — use matchAll() for all matches.
• Not anchoring patterns — /cat/ matches concatenate, not just the word cat.

Code Examples

// Stateful g flag bug — alternating results:
const re = /hello/g; // g flag makes lastIndex stateful
console.log(re.test('hello world')); // true  (lastIndex=5)
console.log(re.test('hello world')); // false (lastIndex=0 after miss)
console.log(re.test('hello world')); // true  (lastIndex=5 again)

// match() only returns first:
'aaa'.match(/a/);  // ['a'] — not all three!

// Avoid g flag with test() — or reset lastIndex:
const hasHello = /hello/.test('hello world'); // No g — safe

// matchAll for all matches:
const matches = [...'aaa'.matchAll(/a/g)]; // [{...}, {...}, {...}]

// Named capture groups:
const dateRe = /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/u;
const { groups: { year, month, day } } = '2026-03-16'.match(dateRe);

References

• ↗ https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_expressions

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