Hi this was surprisingly easy as you can see from the screen shots attached.
I have tested this successfully., a surprisingly powerful feature, thanks :)
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You should be able to remove newlines from all clips by adding a
replacement rule to shelve.sqlite (untested):
insert into replacements ("klass", "rx", "subst") values ("clip", "\n", " ");
Shelve doesn't currently implement a gui for that though. You'd have
to use some sqlite editor (eg the SQLite Manager Firefox addon).