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0.5.3 • Public • Published


randexp will generate a random string that matches a given RegExp Javascript object.

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const RandExp = require('randexp');
// supports grouping and piping
new RandExp(/hello+ (world|to you)/).gen();
// => hellooooooooooooooooooo world
// sets and ranges and references
new RandExp(/<([a-z]\w{0,20})>foo<\1>/).gen();
// => <m5xhdg>foo<m5xhdg>
// wildcard
new RandExp(/random stuff: .+/).gen();
// => random stuff: l3m;Hf9XYbI [YPaxV>U*4-_F!WXQh9>;rH3i l!8.zoh?[utt1OWFQrE ^~8zEQm]~tK
// ignore case
new RandExp(/xxx xtreme dragon warrior xxx/i).gen();
// => xxx xtReME dRAGON warRiOR xXX
// dynamic regexp shortcut
new RandExp('(sun|mon|tue|wednes|thurs|fri|satur)day', 'i');
// is the same as
new RandExp(new RegExp('(sun|mon|tue|wednes|thurs|fri|satur)day', 'i'));

If you're only going to use gen() once with a regexp and want slightly shorter syntax for it

const randexp = require('randexp').randexp;
randexp(/[1-6]/); // 4
randexp('great|good( job)?|excellent'); // great

If you miss the old syntax

/yes|no|maybe|i don't know/.gen(); // maybe


Regular expressions are used in every language, every programmer is familiar with them. Regex can be used to easily express complex strings. What better way to generate a random string than with a language you can use to express the string you want?

Thanks to String-Random for giving me the idea to make this in the first place and randexp for the sweet .gen() syntax.

Default Range

The default generated character range includes printable ASCII. In order to add or remove characters, a defaultRange attribute is exposed. you can subtract(from, to) and add(from, to)

const randexp = new RandExp(/random stuff: .+/);
randexp.defaultRange.subtract(32, 126);
randexp.defaultRange.add(0, 65535);
// => random stuff: 湐箻ໜ䫴␩⶛㳸長���邓蕲뤀쑡篷皇硬剈궦佔칗븛뀃匫鴔事좍ﯣ⭼ꝏ䭍詳蒂䥂뽭

You can also change the default range by changing RandExp.prototype.defaultRange.

Custom PRNG

The default randomness is provided by Math.random(). If you need to use a seedable or cryptographic PRNG, you can override RandExp.prototype.randInt or randexp.randInt (where randexp is an instance of RandExp). randInt(from, to) accepts an inclusive range and returns a randomly selected number within that range.

Infinite Repetitionals

Repetitional tokens such as *, +, and {3,} have an infinite max range. In this case, randexp looks at its min and adds 100 to it to get a useable max value. If you want to use another int other than 100 you can change the max property in RandExp.prototype or the RandExp instance.

const randexp = new RandExp(/no{1,}/);
randexp.max = 1000000;

With RandExp.sugar()

const regexp = /(hi)*/;
regexp.max = 1000000;

Bad Regular Expressions

There are some regular expressions which can never match any string.

  • Ones with badly placed positionals such as /a^/ and /$c/m. Randexp will ignore positional tokens.

  • Back references to non-existing groups like /(a)\1\2/. Randexp will ignore those references, returning an empty string for them. If the group exists only after the reference is used such as in /\1 (hey)/, it will too be ignored.

  • Custom negated character sets with two sets inside that cancel each other out. Example: /[^\w\W]/. If you give this to randexp, it will return an empty string for this set since it can't match anything.

Projects based on randexp.js

JSON-Schema Faker

Use generators to populate JSON Schema samples. See: jsf on github and jsf demo page.



npm install randexp


Download the minified version from the latest release.


Tests are written with mocha

npm test

Integration with TypeScript

RandExp includes TypeScript definitions.

import * as RandExp from "randexp";
const randexp = new RandExp(/[a-z]{6}/);

Use dtslint to check the definition file.

npm install -g dtslint
npm run dtslint

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