Regex Flags in JavaScript: g, i, m, s, u, y Explained

Regex flags change how a pattern matches — the same pattern with different flags can pass or fail on identical input. Use the Regex Tester to toggle flags and see matches update in real time.

What regex flags do

Flags are modifiers appended after the closing / in JavaScript literal syntax, or passed as the second argument to new RegExp(pattern, flags).

/pattern/flags
//     ^^^^^ gimsuvy in JavaScript

Without the right flags, valid patterns look broken — especially for multiline logs, case-insensitive validation, and Unicode property escapes.

The g flag (global)

Global — find all matches, not just the first.

const text = 'id-1 id-2 id-3';
text.match(/id-\d/);      // ['id-1'] — first only
text.match(/id-\d/g);     // ['id-1', 'id-2', 'id-3']

Pitfall: String.prototype.match with g returns an array of matches without capture groups. Use matchAll when you need groups from every match.

[...text.matchAll(/id-(\d)/g)].map((m) => m[1]); // ['1', '2', '3']

The i flag (ignore case)

Case insensitive matching.

/ERROR/i.test('error line 42'); // true

Use for user-facing validation (emails, slugs) where case should not matter. Do not rely on i alone for security filters — normalize input separately when needed.

The m flag (multiline)

Multiline — ^ and $ match line boundaries, not only string start/end.

const log = 'OK first\nERROR second\nOK third';
log.match(/^ERROR/m); // matches 'ERROR' at line start

Essential for parsing stack traces, config files, and linter output line by line.

The s flag (dotall)

Dotall — . matches newline characters (\n, \r).

const block = 'start\nmiddle\nend';
/start.*end/s.test(block); // true — dot crosses newlines

Without s, . stops at the first newline. Combine with m carefully when anchoring lines vs blocks.

The u flag (unicode)

Unicode — enables \u{...} code points and Unicode property escapes \p{...}.

/\p{Script=Greek}/u.test('δ'); // true

Required for emoji, combining marks, and non-Latin scripts. Without u, surrogate pairs and properties behave incorrectly.

The y flag (sticky)

Sticky — match only at lastIndex (advanced tokenizer use).

const re = /\w+/y;
re.lastIndex = 0;
re.exec('abc def'); // ['abc'] — stops at space, lastIndex = 3

Rare in application code; useful for manual parsers. Misuse causes silent non-matches when lastIndex is wrong.

Flag combinations that matter in practice

// Validate full string (not substring) — use anchors
/^[a-z0-9-]+$/.test(userSlug);

Common flag mistakes

const re = /foo/g;
re.test('foo'); // true — lastIndex advanced
re.test('foo'); // false — starts mid-string
re.lastIndex = 0; // reset when reusing global regex

How to use ByteToolBox Regex Tester

1. Open Regex Tester
2. Enter pattern and test string
3. Toggle g, i, m (and others your engine supports)
4. Inspect match list, groups, and highlights
5. Copy the working pattern into your codebase

Test edge cases: empty string, Unicode emoji, Windows \r\n newlines.

See also: Mastering Regular Expressions for pattern syntax depth.

Related tools

• Regex Tester — live flag toggling and match inspection
• JSON Formatter — validate JSON extracted by regex before parsing

Try Regex Tester

Debug your next validation regex with flags enabled in the Regex Tester.