I don't think there is one in Sigil. The best I've come up with is <h2>\L\1\E</h2> which puts them all in lowercase, then you can do multiple cycles of <h2>(.*?) ([a-z])(.*?)</h2> to <h2>\1 \u\2\3</h2>.
It's not perfect, it will miss things like ", III, the, and unless you specifically code for them. I've found it is overall faster if you do the first step for all lowercase, then just manually highlight and use the Ab button.
The better answer is to use Calibre's Book Editor. They have Regex functions that will do that built in.
The BEST answer is to get one of our enterprising coders to get regex functions for Sigil...
