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# regexpu-core [](https://travis-ci.org/mathiasbynens/regexpu-core) [](https://codecov.io/gh/mathiasbynens/regexpu-core)
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_regexpu_ is a source code transpiler that enables the use of ES2015 Unicode regular expressions in JavaScript-of-today (ES5).
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_regexpu-core_ contains _regexpu_’s core functionality, i.e. `rewritePattern(pattern, flag)`, which enables rewriting regular expressions that make use of [the ES2015 `u` flag](https://mathiasbynens.be/notes/es6-unicode-regex) into equivalent ES5-compatible regular expression patterns.
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## Installation
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To use _regexpu-core_ programmatically, install it as a dependency via [npm](https://www.npmjs.com/):
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```bash
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npm install regexpu-core --save
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```
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Then, `require` it:
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```js
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const rewritePattern = require('regexpu-core');
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```
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## API
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This module exports a single function named `rewritePattern`.
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### `rewritePattern(pattern, flags, options)`
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This function takes a string that represents a regular expression pattern as well as a string representing its flags, and returns an ES5-compatible version of the pattern.
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```js
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rewritePattern('foo.bar', 'u');
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// → 'foo(?:[\\0-\\t\\x0B\\f\\x0E-\\u2027\\u202A-\\uD7FF\\uDC00-\\uFFFF]|[\\uD800-\\uDBFF][\\uDC00-\\uDFFF]|[\\uD800-\\uDBFF])bar'
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rewritePattern('[\\u{1D306}-\\u{1D308}a-z]', 'u');
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// → '(?:[a-z]|\\uD834[\\uDF06-\\uDF08])'
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rewritePattern('[\\u{1D306}-\\u{1D308}a-z]', 'ui');
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// → '(?:[a-z\\u017F\\u212A]|\\uD834[\\uDF06-\\uDF08])'
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```
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_regexpu-core_ can rewrite non-ES6 regular expressions too, which is useful to demonstrate how their behavior changes once the `u` and `i` flags are added:
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```js
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// In ES5, the dot operator only matches BMP symbols:
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rewritePattern('foo.bar');
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// → 'foo(?:[\\0-\\t\\x0B\\f\\x0E-\\u2027\\u202A-\\uFFFF])bar'
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// But with the ES2015 `u` flag, it matches astral symbols too:
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rewritePattern('foo.bar', 'u');
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// → 'foo(?:[\\0-\\t\\x0B\\f\\x0E-\\u2027\\u202A-\\uD7FF\\uDC00-\\uFFFF]|[\\uD800-\\uDBFF][\\uDC00-\\uDFFF]|[\\uD800-\\uDBFF])bar'
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```
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The optional `options` argument recognizes the following properties:
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#### `dotAllFlag` (default: `false`)
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Setting this option to `true` enables support for [the `s` (`dotAll`) flag](https://github.com/mathiasbynens/es-regexp-dotall-flag).
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```js
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rewritePattern('.');
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// → '[\\0-\\t\\x0B\\f\\x0E-\\u2027\\u202A-\\uFFFF]'
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rewritePattern('.', '', {
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'dotAllFlag': true
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});
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// → '[\\0-\\t\\x0B\\f\\x0E-\\u2027\\u202A-\\uFFFF]'
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rewritePattern('.', 's', {
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'dotAllFlag': true
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});
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// → '[\\0-\\uFFFF]'
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rewritePattern('.', 'su', {
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'dotAllFlag': true
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});
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// → '(?:[\\0-\\uD7FF\\uE000-\\uFFFF]|[\\uD800-\\uDBFF][\\uDC00-\\uDFFF]|[\\uD800-\\uDBFF](?![\\uDC00-\\uDFFF])|(?:[^\\uD800-\\uDBFF]|^)[\\uDC00-\\uDFFF])'
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```
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#### `unicodePropertyEscape` (default: `false`)
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Setting this option to `true` enables [support for Unicode property escapes](property-escapes.md):
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```js
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rewritePattern('\\p{Script_Extensions=Anatolian_Hieroglyphs}', 'u', {
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'unicodePropertyEscape': true
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});
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// → '(?:\\uD811[\\uDC00-\\uDE46])'
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```
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#### `lookbehind` (default: `false`)
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Setting this option to `true` enables support for [lookbehind assertions](https://github.com/tc39/proposal-regexp-lookbehind).
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```js
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rewritePattern('(?<=.)a', '', {
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'lookbehind': true
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});
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// → '(?<=[\\0-\\t\\x0B\\f\\x0E-\\u2027\\u202A-\\uFFFF])a'
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```
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#### `namedGroup` (default: `false`)
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Setting this option to `true` enables support for [named capture groups](https://github.com/tc39/proposal-regexp-named-groups).
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```js
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rewritePattern('(?<name>.)\k<name>', '', {
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'namedGroups': true
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});
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// → '(.)\1'
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```
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#### `onNamedGroup`
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This option is a function that gets called when a named capture group is found. It receives two parameters:
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the name of the group, and its index.
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```js
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rewritePattern('(?<name>.)\k<name>', '', {
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'namedGroups': true,
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onNamedGroup(name, index) {
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console.log(name, index);
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// → 'name', 1
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}
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});
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```
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#### `useUnicodeFlag` (default: `false`)
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Setting this option to `true` enables the use of Unicode code point escapes of the form `\u{…}`. Note that in regular expressions, such escape sequences only work correctly when the ES2015 `u` flag is set. Enabling this setting often results in more compact output, although there are cases (such as `\p{Lu}`) where it actually _increases_ the output size.
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```js
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rewritePattern('\\p{Script_Extensions=Anatolian_Hieroglyphs}', 'u', {
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'unicodePropertyEscape': true,
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'useUnicodeFlag': true
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});
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// → '[\\u{14400}-\\u{14646}]'
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```
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## Author
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| [](https://twitter.com/mathias "Follow @mathias on Twitter") |
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| [Mathias Bynens](https://mathiasbynens.be/) |
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## License
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_regexpu-core_ is available under the [MIT](https://mths.be/mit) license.
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