oops, mixed up brackets and parantheses (it's not my native language...)
Glad you found out by yourself

I learned regex by this excellent tutorial:
http://www.regular-expressions.info/tutorial.html
I am still not sure which regex version Sigil uses (especially since there was a change from 0.4.2 to 0.4905), but until you reach the differences its a long way, don't worry about this now. If unsure stick with the Perl 5 version, it's the most popular one.