$re = '~(?:src|action|href)=[\'"]\K/(?!/)[^\'"]*~';
$str = '<img src="/relative/url/img.jpg" />
<form action="/">
<form action="/contact-us/">
<a href=\'/relative/url/\'>Note the Single Quote</a>
<img src="//site.com/protocol-relative-img.jpg" />
';
$subst = "https://example.com$0";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$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 PHP, please visit: http://php.net/manual/en/ref.pcre.php