# Filename Extension
> [Regular expression][regexp] to capture a filename extension.
## Usage
```javascript
var reExtname = require( '@stdlib/regexp/extname' );
```
#### reExtname( \[platform] )
Returns a [regular expression][regexp] to capture a filename extension.
```javascript
var RE = reExtname();
// returns
RE = reExtname( 'posix' );
// returns
var ext = RE.exec( '/foo/bar/index.js' )[ 1 ];
// returns '.js'
RE = reExtname( 'win32' );
// returns
ext = RE.exec( 'C:\\foo\\bar\\index.js' )[ 1 ];
// returns '.js'
```
#### reExtname.REGEXP
[Regular expression][regexp] to capture a filename extension.
```javascript
var bool = ( reExtname.REGEXP.toString() === reExtname().toString() );
// returns true
```
#### reExtname.REGEXP_POSIX
[Regular expression][@stdlib/regexp/extname-posix] to capture a [POSIX][posix] filename extension.
```javascript
var ext = reExtname.REGEXP_POSIX.exec( '/foo/bar/index.js' )[ 1 ];
// returns '.js'
```
#### reExtname.REGEXP_WIN32
[Regular expression][@stdlib/regexp/extname-windows] to capture a Windows filename extension.
```javascript
var ext = reExtname.REGEXP_WIN32.exec( 'C:\\foo\\bar\\index.js' )[ 1 ];
// returns '.js'
```
## Notes
- The as `REGEXP` exported [regular expression][regexp] is [platform][@stdlib/assert/is-windows]-dependent. If the current process is running on Windows, `REGEXP === REGEXP_WIN32`; otherwise, `REGEXP === REGEXP_POSIX`.
## Examples
```javascript
var reExtname = require( '@stdlib/regexp/extname' );
var RE_EXTNAME = reExtname();
var ext;
// Assuming a POSIX platform...
ext = RE_EXTNAME.exec( '/foo/bar/index.js' )[ 1 ];
// returns '.js'
ext = reExtname.REGEXP_WIN32.exec( '/foo/bar/home.html' )[ 1 ];
// returns '.html'
ext = reExtname.REGEXP_WIN32.exec( 'C:\\foo\\bar\\home.html' )[ 1 ];
// returns '.html'
```
[regexp]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions
[posix]: https://en.wikipedia.org/wiki/POSIX
[@stdlib/assert/is-windows]: https://www.npmjs.com/package/@stdlib/assert-is-windows
[@stdlib/regexp/extname-posix]: https://www.npmjs.com/package/@stdlib/regexp/tree/main/extname-posix
[@stdlib/regexp/extname-windows]: https://www.npmjs.com/package/@stdlib/regexp/tree/main/extname-windows