# Check if a value is a regular expression

Read the internal tag, which also works for values from another realm.

Canonical: https://1loc.dev/validator/check-if-a-value-is-a-regular-expression/
Version: 1.0.0
Runtime: JavaScript · Node.js 22 / modern browsers
Verification: Usage lines executed in Node.js 22 and headless Chromium; no external services
Updated: 2026-10-28

## Implementation

```javascript
function isRegExp(value) { return Object.prototype.toString.call(value) === '[object RegExp]'; }
```

## Usage

```javascript
isRegExp(/^[a-z]+$/); // → true
isRegExp(new RegExp('^[a-z]+$')); // → true
isRegExp('^[a-z]+$'); // → false
```

Lines marked → were executed in Node.js 22 and in headless Chromium, and each result was compared with the value shown. Lines marked e.g. are illustrative.

## Limits

- Works across realms (iframes, vm contexts), where instanceof RegExp fails. Symbol.toStringTag can spoof the tag.
- Never build a RegExp from untrusted text without escaping it: some patterns take long enough to stall a server (ReDoS).

The function does not validate its arguments. Add checks where it receives untrusted input.

## Source and license

Original: https://github.com/phuocng/1loc/blob/14b481cf60dff1620ab1bb99a43b1f68068cdd9b/contents/check-if-a-value-is-a-regular-expression.mdx
Author: Nguyen Huu Phuoc and original contributors
Contributors: See original repository history
Source commit: 14b481cf60dff1620ab1bb99a43b1f68068cdd9b
Copyright (c) 2020 phuoc-ng
License: MIT — https://1loc.dev/licenses/
Maintainer: Oveyon
Modifications: Documented as a reference with executed usage lines. Original MDX retained as inert source evidence.