$re = '/(<(span).+?><(strong|em)>(.+?)<\/\3><\/\2>)/m';
$str = '<p>Alligators much like birds, have been shown to exhibit unidirectional movement of air through their lungs. Most other amniotes are believed to exhibit bidirectional, or tidal breathing. For <span style="color: #3a9ee3;"><strong>alligator</strong></span> tidal breathing <span style="color: #3a9ee3;"><strong>alligator</strong></span> animal, such as a mammal, air flows into and out of the <span style="color: #169179;"><em>alligators</em></span> lungs through alligator branching bronchi which terminate in small <em>alligator</em> dead-end chambers called alveoli. As the alveoli represent <span style="color: #3a9ee3;"><strong>Lion</strong></span> dead-ends to flow, the inspired air must move back out the same way it came in. In contrast, air in <span style="color: #3a9ee3;"><strong>alligator</strong></span> lungs makes a circuit, moving in only one direction through the parabronchi. The <span style="color: #3a9ee3;"><strong>alligators</strong></span> lungs are no joke! The air first enters the outer branch, moves through the parabronchi, and exits <span style="color: #3a9ee3;"><strong>alligators</strong></span> the lung through the inner branch. Oxygen alligator exchange takes place <span style="color: #3a9ee3;"><strong>Alligators</strong></span> in extensive vasculature around the alligator.</p>';
preg_match_all($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