Simple REGEX replacement fails

OS X 10.11.6 latest Scrivener

I have document titles which contain “Monday” and “Sunday”. I want to strip out the “day”, leaving “Mon” and “Sun”. I know that I can do this with two simple Project Replace operations, but I instead tried:
Search: (Sun|Mon)day
Replace with:

\1

“\1” is a backreference to the value inside the parentheses (Manual, page 294). This operation gives the expected result in both BBEdit and Nisus Writer Pro. However in Scrivener the replacement text is “1”.

That is an error in the manual, you should find Perl-style ($1, $2…) works for replacement values. Sorry about that, I’ve fixed it for the next revision.