<?php

echo preg_replace('#(?<!\*)\*([a-zA-Z\s\.]+)\*(?!\*)#i', '<em>$1</em>', '**bold text with *italic* **');

echo '<br>';
echo '<br>';

echo preg_replace('#(?<!\*)\*([a-zA-Z\s\.]+)\*?(?!\*)#i', '<em>$1</em>', '*italic text **with bold** *');