const regex = new RegExp('{%\\ (if)[\\s\\S]*?{%\\ end\\1\\ %}|
{%.+?%}|
{{.+?}}', 'g')
const str = `<div class="post-preview">
<div class="post-title">
<div class="post-name">
<a href="{{ post.url }}">{{ post.title }}</a>
</div>
<div class="post-date">
{% include time.html %}
</div>
</div>
<div class="post-snippet">
{% if post.content contains '<!--break-->' %}
{{ post.content | split:'<!--break-->' | first }}
<div class="post-readmore">
<a href="{{ post.url }}">read more-></a>
</div>
{% endif %}
</div>
{% include post-meta.html %}
</div>`;
const subst = ``;
// The substituted value will be contained in the result variable
const result = str.replace(regex, subst);
console.log('Substitution result: ', result);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for JavaScript, please visit: https://developer.mozilla.org/en/docs/Web/JavaScript/Guide/Regular_Expressions