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# regexpu-core [](https://travis-ci.org/mathiasbynens/regexpu-core) [](https://coveralls.io/r/mathiasbynens/regexpu-core) [](https://gemnasium.com/mathiasbynens/regexpu-core)
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_regexpu_ is a source code transpiler that enables the use of ES6 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 ES6 `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-dev
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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)`
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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 ES6 `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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`rewritePattern` uses [regjsgen](https://github.com/d10/regjsgen), [regjsparser](https://github.com/jviereck/regjsparser), and [regenerate](https://github.com/mathiasbynens/regenerate) as internal dependencies.
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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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