function reExtnamePosix() {
	return /^(?:\/?|)(?:[\s\S]*?)(?:(?:\.{1,2}|[^\/]+?|)(\.[^.\/]*|))(?:[\/]*)$/; // eslint-disable-line no-useless-escape
}
Returns a regular expression to capture a POSIX filename extension.

@returns {RegExp} regular expression

@example
var RE_EXTNAME_POSIX = reExtnamePosix();
var ext = RE_EXTNAME_POSIX.exec( 'index.js' )[ 1 ];
// returns '.js'