Regular Expressions

Regular Expressions

What it means

A regular expression (regex) is a pattern used to match, extract, or replace text. JavaScript represents them as RegExp objects, either via a literal (/pattern/flags) or the RegExp constructor (useful when the pattern is built dynamically from a string). Common flags: g (global — find all matches, not just the first), i (case-insensitive).

Examples

const pattern = /\d+/;                     // matches one or more digits
console.log(pattern.test("abc123"));        // true

const match = "abc123def456".match(/\d+/g); // global flag -- find all matches
console.log(match);                         // ["123", "456"]

const email = /^[\w.-]+@[\w.-]+\.\w+$/;
console.log(email.test("alice@example.com"));   // true

const cleaned = "Hello   World".replace(/\s+/g, " ");   // collapse whitespace
console.log(cleaned);                       // "Hello World"

// Capture groups
const dateMatch = "2026-06-15".match(/(\d{4})-(\d{2})-(\d{2})/);
console.log(dateMatch[1], dateMatch[2], dateMatch[3]);   // 2026 06 15

Common mistake

Forgetting the g flag when you need all matches, not just the first one — .match() without g returns only the first match (plus capture-group details); with g it returns every match but drops capture-group info.

const text = "cat, bat, hat";

console.log(text.match(/\w+at/));      // only the first match: ["cat", ...]
console.log(text.match(/\w+at/g));     // all matches: ["cat", "bat", "hat"]

Quick practice

1. What does the g flag do on a regex?

   Answer

   Makes the match "global" — instead of stopping at the first match, methods like .match() and .replace() find/act on every match in the string.

2. What’s the difference between .test() and .match()?

   Answer

   .test() (called on the regex) returns a boolean — whether the pattern matches; .match() (called on the string) returns the actual matched text (or null if there's no match).

3. How do you extract just the year from "2026-06-15" using a regex capture group?

   Answer

   "2026-06-15".match(/(\d{4})-\d{2}-\d{2}/)[1] — the parenthesized group captures "2026", accessible at index 1 of the match result.