Thanks. Our elements with one indent are identical. But since it looks like not every preference of the huge preference dialog is contained in prefs.oxc, this approach may still be fruitful. Let's do the lines with two indents also. Again, replace every occurrence with nothing:
^(?!\t\t<\w+\b|\w+\b).*\r?\n|^\t\t<(?=\w+\b)|[ >].*
My element names with two indents are (sorted and deduped with VS Code command palette, which you don't have to do): actions, auto, auto_backup, bg_commands, code, command, dual, email, ext, fdb, fields, foldertabs, gridlines, hide, hideext, icon_command, icon_layout, initial_dual, layout, left, lister_title, log, max_threads, modes, mouse_hook_delay, name, named_color_bools, named_colors, named_dwords, pass, port, powermode, renamemacro, right, server, single, spacing_scheme, startup, std.filedisplay, std.foldertabs, std.foldertree, std.metapane, std.statusbar, std.toolbar, std.viewerpane, system_colors, system_colors_enabled, tab_new, text, tortoise_set, user, viewmodes, win_e_command, win_e_layout.
And while I'm at it, here's the regex for the elements with three indents:
^(?!\t\t\t<\w+\b|\w+\b).*\r?\n|^\t\t\t<(?=\w+\b)|[ >].*
My elements with three indents are: details, event, field, file, folder, frozen, lmb_alt, lmb_ctrl, lmb_normal, mmb_alt, mmb_ctrl, mmb_normal, mode, pathstring, pidl, power.