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Generate strings that match a Regular Expression pattern. Efficiently generate all possible matches, or only the quantity you need.

Have you ever:

  • scrutinized a Regular Expression while trying to visualize what it does and doesn't match?
  • crafted a list of strings to test whether a Regular Expression is working as intended?
  • needed to generate filler data like phone numbers, addresses, zip codes, and email addresses?

regex-to-strings helps with problems like these!




The regExPattern parameter supports three formats:

  1. A RegExp object, like /[a-z]/i
  2. A string that looks like a RegExp object, like "/[a-z]/i"
  3. A string containing just a Regular Expression pattern, like "[a-z]"

The returned object contains two properties:

  • count: The total number of strings that match regExPattern
  • getIterator(): A generator that yields strings matched by regExPattern
import { expand } from 'regex-to-strings';

const phoneNumberPattern = /((\(555\) ?)|(555-))?\d{3}-\d{4}/;
const phoneNumberExpander = expand(phoneNumberPattern);

console.log(phoneNumberExpander.count); // 40000000

for (const phoneNumber of phoneNumberExpander.getIterator()) {
	// (555)547-4836
	// 476-2063
	// 467-2475
	// (555) 194-2532
	// (555)403-4986
	// 555-838-9771
	// etc.


A shortcut to the count property of expand(regExPattern).

import { count } from 'regex-to-strings';

const numStrings = count(/[a-z]{5}/i);
console.log(numStrings); // 380204032

expandN(regExPattern, n)

A shortcut to take n strings from expand(regExPattern).getIterator().

import { expandN } from 'regex-to-strings';

const strings = expandN(/\d{3,5}/, 5);
console.log(strings); // ['84504', '94481', '3971', '69398', '7792']

If the Regular Expression matches fewer than n strings, the returned array will contain fewer than n elements.

import { expandN } from 'regex-to-strings';

const strings = expandN(/[abc]/, 100);
console.log(strings); // ['b', 'a', 'c']


A shortcut to get all strings from expand(regExPattern).getIterator().

import { expandAll } from 'regex-to-strings';

const strings = expandAll(/\d/);
console.log(strings); // ['6', '5', '0', '2', '7', '9', '4', '3', '1', '8']


Supported Regular Expression syntax

regex-to-strings uses regexp-tree to parse your Regular Expression, and so the Regular Expression syntax you can use is largely determined by that library. If your pattern is not recognized by regex-to-strings, try parsing it with regexp-tree to see if the syntax is supported.

regex-to-strings also includes extensive positive and negative tests that track which Regular Expression features are supported.

Regular Expressions with unbounded repetition

Regular Expressions support many techniques for matching an unlimited number of characters. For example, the following patterns will match as many a's as possible:

  • /a*/
  • /a+/
  • /a{7,}/

When regex-to-strings encounters a repetition with no upper bound, it artificially sets an upper bound of 100. This is done for many reasons, chiefly so that a simple pattern like expandAll(/a+/) will not cause an infinite loop.

This also affects the count calculation; expand(/a+/).count returns 100 rather than Infinity.

Randomizing the results

regex-to-strings goes to great lengths to randomize the generated strings. Otherwise the results would be predictable, uninteresting, and probably unhelpful.

// How results might appear without randomization
const strings = expandN(/\d+/, 10);
console.log(strings); // ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9']

Random selections occur throughout the string generation process to give you a thorough sampling of matching strings.


regex-to-strings relies heavily on the regexp-tree Regular Expression parser by Dmitry Soshnikov. Thanks!