Regex replace text outside brackets only

regex-outside-brackets.php

<?php

$text = 'I love eating CO2 for breakfast. <a href="http://www.co2health.com" class="co2" id="co2">CO2 health effects</a>. Here is a pic of CO2 <img src="http://site.com/co2.jpg" alt="CO2">';
$text = preg_replace('~<[^>]*>(*SKIP)(*F)|CO2~', 'CO<sub>2</sub>', $text);
echo $text;

/*
Output:
I love eating CO<sub>2</sub> for breakfast. <a href="http://www.co2health.com" class="co2" id="co2">CO<sub>2</sub> health effects</a>. Here is a pic of CO<sub>2</sub> <img src="http://site.com/co2.jpg" alt="CO2">
*/