stable-regex

Fast, deterministic regex checks for hot paths.

Why this package exists

When a regex has g or y, .test() mutates lastIndex.

const pattern = /hello/g

pattern.test('hello') // true
pattern.test('hello') // false
pattern.test('hello') // true

In hot code paths, developers usually pick one of three options:

• pattern.test(input) (fast but wrong for reused stateful regexes)
• new RegExp(...).test(input) each call (correct but allocates and is slower)
• manual pattern.lastIndex = 0 before test (correct but repetitive boilerplate)

The stable-regex library resets the index just when needed, keeping the performance stable and predictable.primitive.

Use stable-regex when you:

• Reuse precompiled regexes in request handlers/parsers/normalizers.
• Need deterministic boolean checks with g/y patterns.
• Want high throughput without repeating reset boilerplate.

This library is 2.6x faster than recreating a regex each call, which is the solution most used.

Install

npm install stable-regex

Usage

const stableRegex = require('stable-regex')

const pattern = /hello/g

stableRegex(pattern, 'hello') // true
stableRegex(pattern, 'hello') // true
stableRegex(pattern, 'hello') // true

API

stableRegex(pattern, input)

pattern

Required Type: RegExp or RegExp-like object exposing .test()

If lastIndex is numeric, it is reset before testing.

input

Required Type: string

Returns boolean.

Throws TypeError when pattern does not expose .test().

Benchmark

Naive alternatives

Node v25.6.1 on Apple Silicon, 12,000,000 iterations:

Case	Ops/sec	ns/op	Correct for reused /.../g
pattern.test(input)	78,175,641	12.79	❌
new RegExp(...).test(input) each call	18,652,815	53.61	✅
manual lastIndex = 0; test()	48,754,401	20.51	✅
stableRegex(pattern, input)	48,406,721	20.66	✅

Real-world numbers

From the @unavatar/core hot path (src/avatar/auto.js) comparing:

• stableRegex(precompiledPattern, input)
• dataUriRegex().test(input) (fresh regex each call)

Setup: Node v25.6.1, Apple Silicon, 10,000,000 iterations, 7 rounds, median.

Case	stable-regex (ms)	fresh regex each call (ms)	Speedup
Data URI match (data:image/...)	288.50	976.70	3.39x
Non-match (https://example.com/...)	67.07	730.39	10.89x

That translates to:

• 70.46% less time on matching inputs.
• 90.82% less time on non-matching inputs.

License

stable-regex © Kiko Beats, released under the MIT License.

Maintained by Kiko Beats with help from contributors.

kikobeats.com · GitHub Kiko Beats · Twitter @kikobeats