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

RegExp.prototype.test(str) returns true/false. String.prototype.match(re) returns the match array or null. With the g flag, match() returns all values but without group details — use matchAll() to access groups for each match.

Why does my regex with the g flag not work in a loop?

With the g flag, RegExp.exec() advances lastIndex after each call. If you reuse the same RegExp object across multiple calls, reset lastIndex to 0 before each new search, or use String.matchAll() which handles this automatically.

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Q: What is the difference between greedy and lazy quantifiers?

Greedy quantifiers (* + {n,m}) match as many characters as possible. Lazy quantifiers (*? +? {n,m}?) match as few as possible. Example: matches the entire ' text ' (greedy), while matches only ' ' (lazy).

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Testing a JavaScript regex online

The JavaScript RegExp engine supports all modern features. Common methods for using regex in JS code:

const re = /\d{4}-\d{2}-\d{2}/g;

const text = 'Ship 2024-03-15, return 2024-04-01';

// All matches with matchAll (ES2020)

for (const m of text.matchAll(re)) {

  console.log(m[0], 'at index', m.index);

}

// Quick test

/^\d{4}-\d{2}-\d{2}$/.test('2024-01-01'); // → true

// Replace

text.replace(/\d{4}-\d{2}-\d{2}/g, '[DATE]');

// → "Ship [DATE], return [DATE]"

Frequently asked questions

What is the difference between greedy and lazy quantifiers?

Greedy quantifiers (*, +, {n,m}) match as many characters as possible. Lazy quantifiers (*?, +?, {n,m}?) match as few as possible. Example: <.*> matches the entire <b>text</b> (greedy), while <.*?> matches only <b> (lazy).

What is the s (dotAll) flag?

By default, . does not match newline characters (\n, \r). The s flag (ES2018) makes . match any character including newlines, which is useful when matching patterns that span multiple lines.

When should I use the u flag?

Use u for any pattern involving non-ASCII characters, emoji, or Unicode property escapes (\p{Letter}). It also enforces strict escaping, catching typos like invalid escape sequences that the engine would otherwise silently ignore.

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