如何使用preg_replace在第3和第4段之间插入文本字符串?

I'm trying to figure out how to create a common newspaper device called a 'pullquote' in a Wordpress post. (But this isn't strictly a Wordpress question; it's more a generic Regex question.) I have a tag to surround the text in the post. I want to copy the text between the tags (which I know how to do) and the insert it between the 3rd and 4th instances of the p tags in the post.I'm trying to figure out how to create a common