Regex (short for regular expression) is a pattern language for matching text. It lets you describe what a string should look like — “starts with a letter, followed by 3-5 digits, ends with .com” — and then search, validate, or replace text that matches that pattern.

Every programming language supports regex. You’ll use it for input validation, text parsing, search-and-replace, log analysis, and data extraction.

What does regex look like?

A regex pattern is a string of characters where some characters have special meaning:

/^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/

That’s an email validation pattern. It looks intimidating, but it breaks down into simple pieces:

• ^ — start of string
• [a-zA-Z0-9._%+-]+ — one or more letters, digits, or special chars (the local part)
• @ — literal @ sign
• [a-zA-Z0-9.-]+ — the domain name
• \. — literal dot
• [a-zA-Z]{2,} — two or more letters (the TLD like .com, .org)
• $ — end of string

Common patterns you’ll actually use

\d          → any digit (0-9)
\w          → any word character (letter, digit, underscore)
\s          → any whitespace (space, tab, newline)
.           → any character except newline
+           → one or more of the previous
*           → zero or more of the previous
?           → zero or one of the previous
{3}         → exactly 3 of the previous
{2,5}       → between 2 and 5 of the previous
[abc]       → a, b, or c
[^abc]      → anything except a, b, or c
(group)     → capture group
|           → or

Real-world examples

// Validate a phone number (US format)
/^\d{3}-\d{3}-\d{4}$/

// Extract all URLs from text
/https?:\/\/[^\s]+/g

// Replace multiple spaces with one
text.replace(/\s+/g, ' ')

// Check if a string is a valid hex color
/^#([0-9a-fA-F]{3}|[0-9a-fA-F]{6})$/

// Extract numbers from a string
"Price: $42.99".match(/\d+\.?\d*/g)  // ["42.99"]

Using regex in JavaScript

// Test if a pattern matches
const isEmail = /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test('user@example.com'); // true

// Find matches
const matches = 'abc 123 def 456'.match(/\d+/g); // ['123', '456']

// Replace
const clean = 'Hello   World'.replace(/\s+/g, ' '); // 'Hello World'

// Named capture groups
const match = '2026-03-15'.match(/(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/);
console.log(match.groups.year); // '2026'

When NOT to use regex

Regex is powerful but not always the right tool:

• Parsing HTML/XML — use a proper parser. Regex can’t handle nested tags reliably.
• Complex validation — email validation with regex is famously incomplete. Use a library.
• Readability matters — if your regex is longer than a line, consider breaking the logic into code instead.
• Performance-critical paths — complex regex with backtracking can be slow. The regex CPU spike postmortem is a real example of this going wrong.

Learn more

• Regex cheat sheet — all patterns on one page
• Regex tester — test patterns in your browser
• Regex explainer — paste a regex and get a plain-English breakdown
• Regex debugger — step through matches visually