I'm not sure exactly where I learn't about it but it is used by PCRE which is the regex engine now used by Sigil.
Have a look at this link for background info that might help explain a bit about PCRE.
http://en.wikipedia.org/wiki/Perl_Co...ar_Expressions
It took me about 10 weeks to start to get my head around regex but once I started to get it I found that I could cut up to 80% off my conversion times (I take web based yarns and convert them to epub for my later re-reading).
Look for regex tutorials on the web and keep butting your head against it and eventually it will get through.