Bash-like brace expansion, implemented in JavaScript. Safer than other brace expansion libs, with complete support for the Bash 4.3 braces specification, without sacrificing speed.

alpha, alphabetical, bash, brace, braces, expand, expansion, filepath, fill, fs, glob, globbing, letter, match, matches, matching, number, numerical, path, range, ranges, sh, brace-expansion, brace-patterns, glob-matcher, glob-pattern, javascript, minimatch, multimatch, node, nodejs, regular-expression
npm install braces@2.3.2


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Bash-like brace expansion, implemented in JavaScript. Safer than other brace expansion libs, with complete support for the Bash 4.3 braces specification, without sacrificing speed.

Please consider following this project's author, Jon Schlinkert, and consider starring the project to show your ❤️ and support.


Install with npm:

$ npm install --save braces

Why use braces?

Brace patterns are great for matching ranges. Users (and implementors) shouldn't have to think about whether or not they will break their application (or yours) from accidentally defining an aggressive brace pattern. Braces is the only library that offers a solution to this problem.


The main export is a function that takes one or more brace patterns and options.

var braces = require('braces');
braces(pattern[, options]);

By default, braces returns an optimized regex-source string. To get an array of brace patterns, use brace.expand().

The following section explains the difference in more detail. (If you're curious about "why" braces does this by default, see brace matching pitfalls.

Optimized vs. expanded braces


By default, patterns are optimized for regex and matching:

//=> ['a/(x|y|z)/b']


To expand patterns the same way as Bash or minimatch, use the .expand method:

//=> ['a/x/b', 'a/y/b', 'a/z/b']

Or use options.expand:

console.log(braces('a/{x,y,z}/b', {expand: true}));
//=> ['a/x/b', 'a/y/b', 'a/z/b']


  • lists: Supports "lists": a/{b,c}/d => ['a/b/d', 'a/c/d']
  • sequences: Supports alphabetical or numerical "sequences" (ranges): {1..3} => ['1', '2', '3']
  • steps: Supports "steps" or increments: {2..10..2} => ['2', '4', '6', '8', '10']
  • escaping
  • options


Uses fill-range for expanding alphabetical or numeric lists:

//=> ['a/(foo|bar|baz)/*.js']

//=> ['a/foo/*.js', 'a/bar/*.js', 'a/baz/*.js']


Uses fill-range for expanding alphabetical or numeric ranges (bash "sequences"):

console.log(braces.expand('{1..3}'));     // ['1', '2', '3']
console.log(braces.expand('a{01..03}b')); // ['a01b', 'a02b', 'a03b']
console.log(braces.expand('a{1..3}b'));   // ['a1b', 'a2b', 'a3b']
console.log(braces.expand('{a..c}'));     // ['a', 'b', 'c']
console.log(braces.expand('foo/{a..c}')); // ['foo/a', 'foo/b', 'foo/c']

// supports padded ranges
console.log(braces('a{01..03}b'));   //=> [ 'a(0[1-3])b' ]
console.log(braces('a{001..300}b')); //=> [ 'a(0{2}[1-9]|0[1-9][0-9]|[12][0-9]{2}|300)b' ]


Steps, or increments, may be used with ranges:

//=> ['2', '4', '6', '8', '10']

//=> ['(2|4|6|8|10)']

When the .optimize method is used, or options.optimize is set to true, sequences are passed to to-regex-range for expansion.


Brace patterns may be nested. The results of each expanded string are not sorted, and left to right order is preserved.

"Expanded" braces

//=> ['ab/e', 'ac/e', 'a/x/e', 'a/y/e']

//=> ['a/x/c', 'a/1/c', 'a/2/c', 'a/3/c', 'a/4/c', 'a/5/c', 'a/y/c']

"Optimized" braces

//=> ['a(b|c|/(x|y))/e']

//=> ['a/(x|([1-5])|y)/c']


Escaping braces

A brace pattern will not be expanded or evaluted if either the opening or closing brace is escaped:

//=> ['a{d,c,b}e']

//=> ['a{d,c,b}e']

Escaping commas

Commas inside braces may also be escaped:

//=> ['a{b,c}d']

//=> ['ad,ce', 'abe']

Single items

Following bash conventions, a brace pattern is also not expanded when it contains a single character:

//=> ['a{b}c']



Type: Number

Default: 65,536

Description: Limit the length of the input string. Useful when the input string is generated or your application allows users to pass a string, et cetera.

console.log(braces('a/{b,c}/d', { maxLength: 3 }));  //=> throws an error


Type: Boolean

Default: undefined

Description: Generate an "expanded" brace pattern (this option is unncessary with the .expand method, which does the same thing).

console.log(braces('a/{b,c}/d', {expand: true}));
//=> [ 'a/b/d', 'a/c/d' ]


Type: Boolean

Default: true

Description: Enabled by default.

//=> [ 'a/(b|c)/d' ]


Type: Boolean

Default: true

Description: Duplicates are removed by default. To keep duplicates, pass {nodupes: false} on the options


Type: Number

Default: 250

Description: When braces.expand() is used, or options.expand is true, brace patterns will automatically be optimized when the difference between the range minimum and range maximum exceeds the rangeLimit. This is to prevent huge ranges from freezing your application.

You can set this to any number, or change options.rangeLimit to Inifinity to disable this altogether.


// pattern exceeds the "rangeLimit", so it's optimized automatically
//=> ['([1-9]|[1-9][0-9]{1,2}|1000)']

// pattern does not exceed "rangeLimit", so it's NOT optimized
//=> ['1', '2', '3', '4', '5', '6', '7', '8', '9', '10', '11', '12', '13', '14', '15', '16', '17', '18', '19', '20', '21', '22', '23', '24', '25', '26', '27', '28', '29', '30', '31', '32', '33', '34', '35', '36', '37', '38', '39', '40', '41', '42', '43', '44', '45', '46', '47', '48', '49', '50', '51', '52', '53', '54', '55', '56', '57', '58', '59', '60', '61', '62', '63', '64', '65', '66', '67', '68', '69', '70', '71', '72', '73', '74', '75', '76', '77', '78', '79', '80', '81', '82', '83', '84', '85', '86', '87', '88', '89', '90', '91', '92', '93', '94', '95', '96', '97', '98', '99', '100']


Type: Function

Default: undefined

Description: Customize range expansion.

var range = braces.expand('x{a..e}y', {
  transform: function(str) {
    return 'foo' + str;

//=> [ 'xfooay', 'xfooby', 'xfoocy', 'xfoody', 'xfooey' ]


Type: Boolean

Default: undefined

Description: In regular expressions, quanitifiers can be used to specify how many times a token can be repeated. For example, a{1,3} will match the letter a one to three times.

Unfortunately, regex quantifiers happen to share the same syntax as Bash lists

The quantifiers option tells braces to detect when regex quantifiers are defined in the given pattern, and not to try to expand them as lists.


var braces = require('braces');
//=> [ 'a/b(1|3)/(x|y|z)' ]
console.log(braces('a/b{1,3}/{x,y,z}', {quantifiers: true}));
//=> [ 'a/b{1,3}/(x|y|z)' ]
console.log(braces('a/b{1,3}/{x,y,z}', {quantifiers: true, expand: true}));
//=> [ 'a/b{1,3}/x', 'a/b{1,3}/y', 'a/b{1,3}/z' ]


Type: Boolean

Default: undefined

Description: Strip backslashes that were used for escaping from the result.

What is "brace expansion"?

Brace expansion is a type of parameter expansion that was made popular by unix shells for generating lists of strings, as well as regex-like matching when used alongside wildcards (globs).

In addition to "expansion", braces are also used for matching. In other words:

More about brace expansion (click to expand)

There are two main types of brace expansion:

  1. lists: which are defined using comma-separated values inside curly braces: {a,b,c}
  2. sequences: which are defined using a starting value and an ending value, separated by two dots: a{1..3}b. Optionally, a third argument may be passed to define a "step" or increment to use: a{1..100..10}b. These are also sometimes referred to as "ranges".

Here are some example brace patterns to illustrate how they work:


{a,b,c}       => a b c
{a,b,c}{1,2}  => a1 a2 b1 b2 c1 c2


{1..9}        => 1 2 3 4 5 6 7 8 9
{4..-4}       => 4 3 2 1 0 -1 -2 -3 -4
{1..20..3}    => 1 4 7 10 13 16 19
{a..j}        => a b c d e f g h i j
{j..a}        => j i h g f e d c b a
{a..z..3}     => a d g j m p s v y


Sets and sequences can be mixed together or used along with any other strings.

{a,b,c}{1..3}   => a1 a2 a3 b1 b2 b3 c1 c2 c3
foo/{a,b,c}/bar => foo/a/bar foo/b/bar foo/c/bar

The fact that braces can be "expanded" from relatively simple patterns makes them ideal for quickly generating test fixtures, file paths, and similar use cases.

Brace matching

In addition to expansion, brace patterns are also useful for performing regular-expression-like matching.

For example, the pattern foo/{1..3}/bar would match any of following strings:


But not:


Braces can also be combined with glob patterns to perform more advanced wildcard matching. For example, the pattern */{1..3}/* would match any of following strings:


Brace matching pitfalls

Although brace patterns offer a user-friendly way of matching ranges or sets of strings, there are also some major disadvantages and potential risks you should be aware of.


"brace bombs"

  • brace expansion can eat up a huge amount of processing resources
  • as brace patterns increase linearly in size, the system resources required to expand the pattern increase exponentially
  • users can accidentally (or intentially) exhaust your system's resources resulting in the equivalent of a DoS attack (bonus: no programming knowledge is required!)

For a more detailed explanation with examples, see the geometric complexity section.

The solution

Jump to the performance section to see how Braces solves this problem in comparison to other libraries.

Geometric complexity

At minimum, brace patterns with sets limited to two elements have quadradic or O(n^2) complexity. But the complexity of the algorithm increases exponentially as the number of sets, and elements per set, increases, which is O(n^c).

For example, the following sets demonstrate quadratic (O(n^2)) complexity:

{1,2}{3,4}      => (2X2)    => 13 14 23 24
{1,2}{3,4}{5,6} => (2X2X2)  => 135 136 145 146 235 236 245 246

But add an element to a set, and we get a n-fold Cartesian product with O(n^c) complexity:

{1,2,3}{4,5,6}{7,8,9} => (3X3X3) => 147 148 149 157 158 159 167 168 169 247 248 
                                    249 257 258 259 267 268 269 347 348 349 357 
                                    358 359 367 368 369

Now, imagine how this complexity grows given that each element is a n-tuple:

{1..100}{1..100}         => (100X100)     => 10,000 elements (38.4 kB)
{1..100}{1..100}{1..100} => (100X100X100) => 1,000,000 elements (5.76 MB)

Although these examples are clearly contrived, they demonstrate how brace patterns can quickly grow out of control.

More information

Interested in learning more about brace expansion?


Braces is not only screaming fast, it's also more accurate the other brace expansion libraries.

Better algorithms

Fortunately there is a solution to the "brace bomb" problem: don't expand brace patterns into an array when they're used for matching.

Instead, convert the pattern into an optimized regular expression. This is easier said than done, and braces is the only library that does this currently.

The proof is in the numbers

Minimatch gets exponentially slower as patterns increase in complexity, braces does not. The following results were generated using braces() and minimatch.braceExpand(), respectively.

Pattern braces minimatch
{1..9007199254740991}[1] 298 B (5ms 459μs) N/A (freezes)
{1..1000000000000000} 41 B (1ms 15μs) N/A (freezes)
{1..100000000000000} 40 B (890μs) N/A (freezes)
{1..10000000000000} 39 B (2ms 49μs) N/A (freezes)
{1..1000000000000} 38 B (608μs) N/A (freezes)
{1..100000000000} 37 B (397μs) N/A (freezes)
{1..10000000000} 35 B (983μs) N/A (freezes)
{1..1000000000} 34 B (798μs) N/A (freezes)
{1..100000000} 33 B (733μs) N/A (freezes)
{1..10000000} 32 B (5ms 632μs) 78.89 MB (16s 388ms 569μs)
{1..1000000} 31 B (1ms 381μs) 6.89 MB (1s 496ms 887μs)
{1..100000} 30 B (950μs) 588.89 kB (146ms 921μs)
{1..10000} 29 B (1ms 114μs) 48.89 kB (14ms 187μs)
{1..1000} 28 B (760μs) 3.89 kB (1ms 453μs)
{1..100} 22 B (345μs) 291 B (196μs)
{1..10} 10 B (533μs) 20 B (37μs)
{1..3} 7 B (190μs) 5 B (27μs)

Faster algorithms

When you need expansion, braces is still much faster.

(the following results were generated using braces.expand() and minimatch.braceExpand(), respectively)

Pattern braces minimatch
{1..10000000} 78.89 MB (2s 698ms 642μs) 78.89 MB (18s 601ms 974μs)
{1..1000000} 6.89 MB (458ms 576μs) 6.89 MB (1s 491ms 621μs)
{1..100000} 588.89 kB (20ms 728μs) 588.89 kB (156ms 919μs)
{1..10000} 48.89 kB (2ms 202μs) 48.89 kB (13ms 641μs)
{1..1000} 3.89 kB (1ms 796μs) 3.89 kB (1ms 958μs)
{1..100} 291 B (424μs) 291 B (211μs)
{1..10} 20 B (487μs) 20 B (72μs)
{1..3} 5 B (166μs) 5 B (27μs)

If you'd like to run these comparisons yourself, see test/support/generate.js.


Running benchmarks

Install dev dependencies:

npm i -d && npm benchmark

Latest results

Benchmarking: (8 of 8)
 · combination-nested
 · combination
 · escaped
 · list-basic
 · list-multiple
 · no-braces
 · sequence-basic
 · sequence-multiple

# benchmark/fixtures/combination-nested.js (52 bytes)
  brace-expansion x 4,756 ops/sec ±1.09% (86 runs sampled)
  braces x 11,202,303 ops/sec ±1.06% (88 runs sampled)
  minimatch x 4,816 ops/sec ±0.99% (87 runs sampled)

  fastest is braces

# benchmark/fixtures/combination.js (51 bytes)
  brace-expansion x 625 ops/sec ±0.87% (87 runs sampled)
  braces x 11,031,884 ops/sec ±0.72% (90 runs sampled)
  minimatch x 637 ops/sec ±0.84% (88 runs sampled)

  fastest is braces

# benchmark/fixtures/escaped.js (44 bytes)
  brace-expansion x 163,325 ops/sec ±1.05% (87 runs sampled)
  braces x 10,655,071 ops/sec ±1.22% (88 runs sampled)
  minimatch x 147,495 ops/sec ±0.96% (88 runs sampled)

  fastest is braces

# benchmark/fixtures/list-basic.js (40 bytes)
  brace-expansion x 99,726 ops/sec ±1.07% (83 runs sampled)
  braces x 10,596,584 ops/sec ±0.98% (88 runs sampled)
  minimatch x 100,069 ops/sec ±1.17% (86 runs sampled)

  fastest is braces

# benchmark/fixtures/list-multiple.js (52 bytes)
  brace-expansion x 34,348 ops/sec ±1.08% (88 runs sampled)
  braces x 9,264,131 ops/sec ±1.12% (88 runs sampled)
  minimatch x 34,893 ops/sec ±0.87% (87 runs sampled)

  fastest is braces

# benchmark/fixtures/no-braces.js (48 bytes)
  brace-expansion x 275,368 ops/sec ±1.18% (89 runs sampled)
  braces x 9,134,677 ops/sec ±0.95% (88 runs sampled)
  minimatch x 3,755,954 ops/sec ±1.13% (89 runs sampled)

  fastest is braces

# benchmark/fixtures/sequence-basic.js (41 bytes)
  brace-expansion x 5,492 ops/sec ±1.35% (87 runs sampled)
  braces x 8,485,034 ops/sec ±1.28% (89 runs sampled)
  minimatch x 5,341 ops/sec ±1.17% (87 runs sampled)

  fastest is braces

# benchmark/fixtures/sequence-multiple.js (51 bytes)
  brace-expansion x 116 ops/sec ±0.77% (77 runs sampled)
  braces x 9,445,118 ops/sec ±1.32% (84 runs sampled)
  minimatch x 109 ops/sec ±1.16% (76 runs sampled)

  fastest is braces



Pull requests and stars are always welcome. For bugs and feature requests, please create an issue.

Running Tests

Running and reviewing unit tests is a great way to get familiarized with a library and its API. You can install dependencies and run tests with the following command:

$ npm install && npm test
Building docs

(This project's is generated by verb, please don't edit the readme directly. Any changes to the readme must be made in the readme template.)

To generate the readme, run the following command:

$ npm install -g verbose/verb#dev verb-generate-readme && verb

Related projects

You might also be interested in these projects:

  • expand-brackets: Expand POSIX bracket expressions (character classes) in glob patterns. | homepage
  • extglob: Extended glob support for JavaScript. Adds (almost) the expressive power of regular expressions to glob… more | homepage
  • fill-range: Fill in a range of numbers or letters, optionally passing an increment or step to… more | homepage
  • micromatch: Glob matching for javascript/node.js. A drop-in replacement and faster alternative to minimatch and multimatch. | homepage
  • nanomatch: Fast, minimal glob matcher for node.js. Similar to micromatch, minimatch and multimatch, but complete Bash… more | homepage


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Jon Schlinkert


Copyright © 2018, Jon Schlinkert. Released under the MIT License.

This file was generated by verb-generate-readme, v0.6.0, on February 17, 2018.

  1. this is the largest safe integer allowed in JavaScript.