Generally, in regex, anything enclosed in parentheses () are stored for replacement. In your replace field, you put \1 for the first set of parentheses, \2 for the second, etc.
Find: The Book, Chapter ([0-9]+):([^\n]+)
Replace: <h2>Chapter \1 -- \2</h2>
would find The Book, Chapter 17: The End of Mayonnaise
and replace it with
<h2>Chapter 17 -- The End of Mayonnaise</h2>
m a r