These things just seem to find me, this time it was a very large database dump for a media site which was plagued with misnested HTML tags. Seriously. Just shy of 250,000 misnested pairs.

Here’s the pattern I came up with to fix it:

Find:

<(([^ >]+)(?:[^>]*))>(.*)<(([^ >]+)(?:[^>]*))>(.*)</\2>(.*)</\5>

Replace with:
<$1>$3<$4>$6</$5>$7</$2>
or, depending on your regex engine, your replace string might look like this:
<\1>\3<\4>\6</\5>\7</\2>

That handles all of the following cases:

<b><i>text</b></i>
<b>text<i>text</b>text</i>
<b><a href="#" target="_new">link</b>text</a>
<a href="#"><h2>text</a></h2>

Running the final substitution was ridiculously fast, Regular Expressions are magic.