REGEX is Regular Expressions (Sigil uses the PCRE dialect)
It is an extremely powerful search and replace tool for modifying code / documents. There are some REGEX tutorials here on MR. Look for them

search for a REGEX cheat sheet, it has the terms and some examples
While you are at it, get the CSS (2.x) one also.
Note The CSS we use is a SUBSET of that which is used for the WWW
Then, there are all those wonderful devices that don't fully support those (some due to the way they allow user control to override the publisher.