Try:
Find: <title>.*?Chapter\s*(\d*?)\s*.*?</title>
Replace: <title>Chapter \1</title>
<title> & </title> are just the bounding tags
.*? is any number (or none) of any characters
\s* is any number (or none) spaces
(\d+?) is 1 or more numerical digits and it remembers what it finds
The Replace simply inserts the remembered digits into the phrase using the \1.
There are some
good regex tutorials that will really help when editing code. It doesn't take long to get the basics....it does take forever to become a master... I'm definitely not there yet!
Make sure you focus on the PCRE flavor of Regex...that is what Sigil uses.
Cheers,