How do you get the YAML frontmatter of a file?

preg_match_yaml_frontmatter.php

<?php
$string = '---
layout: default
title: Some title - yeah, right
---

<h1>Tester</h1>
<p>Some content</p>';

preg_match("'/---.*?---/s'", $content, $yaml_frontmatter);

print_r($yaml_frontmatter);