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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)
_regexpu_ is a source code transpiler that enables the use of ES6 Unicode regular expressions in JavaScript-of-today (ES5).
_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.
## Installation
To use _regexpu-core_ programmatically, install it as a dependency via [npm](https://www.npmjs.com/):
```bash
npm install regexpu-core --save-dev
```
Then, `require` it:
```js
const rewritePattern = require('regexpu-core');
```
## API
This module exports a single function named `rewritePattern`.
### `rewritePattern(pattern, flags)`
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.
```js
rewritePattern('foo.bar', 'u');
// → 'foo(?:[\\0-\\t\\x0B\\f\\x0E-\\u2027\\u202A-\\uD7FF\\uDC00-\\uFFFF]|[\\uD800-\\uDBFF][\\uDC00-\\uDFFF]|[\\uD800-\\uDBFF])bar'
rewritePattern('[\\u{1D306}-\\u{1D308}a-z]', 'u');
// → '(?:[a-z]|\\uD834[\\uDF06-\\uDF08])'
rewritePattern('[\\u{1D306}-\\u{1D308}a-z]', 'ui');
// → '(?:[a-z\\u017F\\u212A]|\\uD834[\\uDF06-\\uDF08])'
```
_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:
```js
// In ES5, the dot operator only matches BMP symbols:
rewritePattern('foo.bar');
// → 'foo(?:[\\0-\\t\\x0B\\f\\x0E-\\u2027\\u202A-\\uFFFF])bar'
// But with the ES6 `u` flag, it matches astral symbols too:
rewritePattern('foo.bar', 'u');
// → 'foo(?:[\\0-\\t\\x0B\\f\\x0E-\\u2027\\u202A-\\uD7FF\\uDC00-\\uFFFF]|[\\uD800-\\uDBFF][\\uDC00-\\uDFFF]|[\\uD800-\\uDBFF])bar'
```
`rewritePattern` uses [regjsgen](https://github.com/d10/regjsgen), [regjsparser](https://github.com/jviereck/regjsparser), and [regenerate](https://github.com/mathiasbynens/regenerate) as internal dependencies.
## Author
| [](https://twitter.com/mathias "Follow @mathias on Twitter") |
|---|
| [Mathias Bynens](https://mathiasbynens.be/) |
## License
_regexpu-core_ is available under the [MIT](https://mths.be/mit) license.
|
