Thanks to all who replied in this thread. I used Timur's solution and it works like a charm. Having all caps for TOC entries when there are hundreds of entries is just too hard to read, and using title case instead makes it so much better on my eyes.
There is just one situation left unfixed which I think could be fixed. Whenever there are hyphenated words, the second word is not capped. For example,
CHOCOLATE-GLAZED will become
Chocolate-glazed instead of
Chocolate-Glazed.
So is there any way to construct a separate regex expression that will make every first letter after a hyphen an uppercase letter?
Also, I noticed in the latest version of Sigil, there is no search option for whole words only? I may be mistaken but I thought it had that in the past. I noticed that when I tried to use a "Replace All" to change "nut" to "Nut" that I ended up with a lot of "PeaNut" and ButterNut" results!
I may be misremembering, but I could have sworn the search options are less than before? Or maybe I am missing the option to put the search window in an advanced mode?