Here are a few regex examples that might help:
Mobileread
There are also some links on the Sigil-Regex
help page.
There are also several sources online that may help, such as
Regex Buddy.
You just need to make sure you are using the same "flavor" of regex as that used by Sigil... some of the expressions that other types of regex use aren't supported by Sigil.
I hope that helps!
Cheers,