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# regexpu-core [](https://travis-ci.org/mathiasbynens/regexpu-core) [](https://codecov.io/gh/mathiasbynens/regexpu-core)
_regexpu_ is a source code transpiler that enables the use of ES2015 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 ES2015 `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
```
Then, `require` it:
```js
const rewritePattern = require('regexpu-core');
```
## API
This module exports a single function named `rewritePattern`.
### `rewritePattern(pattern, flags, options)`
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 ES2015 `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'
```
The optional `options` argument recognizes the following properties:
#### `dotAllFlag` (default: `false`)
Setting this option to `true` enables support for [the `s` (`dotAll`) flag](https://github.com/mathiasbynens/es-regexp-dotall-flag).
```js
rewritePattern('.');
// → '[\\0-\\t\\x0B\\f\\x0E-\\u2027\\u202A-\\uFFFF]'
rewritePattern('.', '', {
'dotAllFlag': true
});
// → '[\\0-\\t\\x0B\\f\\x0E-\\u2027\\u202A-\\uFFFF]'
rewritePattern('.', 's', {
'dotAllFlag': true
});
// → '[\\0-\\uFFFF]'
rewritePattern('.', 'su', {
'dotAllFlag': true
});
// → '(?:[\\0-\\uD7FF\\uE000-\\uFFFF]|[\\uD800-\\uDBFF][\\uDC00-\\uDFFF]|[\\uD800-\\uDBFF](?![\\uDC00-\\uDFFF])|(?:[^\\uD800-\\uDBFF]|^)[\\uDC00-\\uDFFF])'
```
#### `unicodePropertyEscape` (default: `false`)
Setting this option to `true` enables [support for Unicode property escapes](property-escapes.md):
```js
rewritePattern('\\p{Script_Extensions=Anatolian_Hieroglyphs}', 'u', {
'unicodePropertyEscape': true
});
// → '(?:\\uD811[\\uDC00-\\uDE46])'
```
#### `lookbehind` (default: `false`)
Setting this option to `true` enables support for [lookbehind assertions](https://github.com/tc39/proposal-regexp-lookbehind).
```js
rewritePattern('(?<=.)a', '', {
'lookbehind': true
});
// → '(?<=[\\0-\\t\\x0B\\f\\x0E-\\u2027\\u202A-\\uFFFF])a'
```
#### `namedGroup` (default: `false`)
Setting this option to `true` enables support for [named capture groups](https://github.com/tc39/proposal-regexp-named-groups).
```js
rewritePattern('(?<name>.)\k<name>', '', {
'namedGroup': true
});
// → '(.)\1'
```
#### `onNamedGroup`
This option is a function that gets called when a named capture group is found. It receives two parameters:
the name of the group, and its index.
```js
rewritePattern('(?<name>.)\k<name>', '', {
'namedGroup': true,
onNamedGroup(name, index) {
console.log(name, index);
// → 'name', 1
}
});
```
#### `useUnicodeFlag` (default: `false`)
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.
```js
rewritePattern('\\p{Script_Extensions=Anatolian_Hieroglyphs}', 'u', {
'unicodePropertyEscape': true,
'useUnicodeFlag': true
});
// → '[\\u{14400}-\\u{14646}]'
```
## 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.
|
