$re = '/<(h[1-6])>(.+)(\s+)([^\s]+)<\/\1>/im';
$str = '<h2>This is a test heading</h2>
<p>Here is some text</p>
<div>
<h3>Here is a another heading</h3>
<p>Some more paragraph text which shouldn\'t match</p>
</div>
<h2>This is a test heading</h2>
<p>Here is some text</p>
<div>
<h3>Here is a another heading</h3>
<p>Some more paragraph text which shouldn\'t match</p>
</div>';
$subst = "<$1>$2 $4<\/$1>";
$result = preg_replace($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