I'm very new to both helix and kakoune, but reading kakoune's Migrating from Vim doc which is linked in the helix docs I found the following snippet to substitute in a selection: s(word1):(word2):(word3):(word4)<ret>c<c-r>4<c-r>2<c-r>3<c-r>1<esc>

But when I tried it in helix, it didn't work. I think helix might be lacking support for kakoune's Integer Registers which reference match groups in the last used regex expression.