[KevinH]

What you are looking for is called Regular Expression Find and Replace (aka regex).
This mode enable placeholders for specific character classes. Digits are represented by \d,
and a following + sign means 1 or more.

So set Sigil's Find and Replace Mode to Regex.

Try using something like the following in the Find Field:

DSF Chapter \d+

Before making any big Find Replace changes, I would make a Checkpoint just in case a big find and replace does not work the way you expect.

Sigil also has a DryRun Find and Replace command that will show you all the Find and Replaces done in context but not actually perform them, so you can test your regex expressions first.