$re = '/(?<=<[^>]*)(?<=\s)(on\w+|formaction|action)=["\']?(.*?)["\']?(?=[\n>])/m';
$str = '<img src="/name">
<a href="javascript:void">some url</a>
<a href="https://convertkit.com">URL</a>
<img src=x onerror="alert(document.domain)">
<img src=x onerror=" alert(document.domain) ">
<img src=x
onerror="alert(document.domain)"
>
<img src=x onerror=alert(document.domain)>
<button form="form1" formaction="javascript:alert(1)">Submit</button>
<form action=""></form>
<button onclick="hacked()">Hack</button>
<a href="https://convertkit.com/about?utm_content=bazard">click men</a>
<div>onmouseover=alert(1)</div>
<div> onmouseover=alert(1) </div>';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
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 PHP, please visit: http://php.net/manual/en/ref.pcre.php