Check if a value is a regular expression

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

JavaScriptNode.js & browsers Version 1.0.0 · MIT · .md

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

Usage

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.

Know the 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 above is the code the usage lines run. It does not validate its arguments; add checks where it receives untrusted input.

Keep exploring

• Check whether all values are equal — Compare every item with the first using Object.is.
• Check whether an object is empty — Check for own enumerable string keys on a JSON object.
• Check whether an integer is even — Test parity for safe integers, including zero and negative values.