Eskil

    grid columnconfigure $w.fb {0 2} -uniform a
    grid columnconfigure $w.fb 1 -weight 1

    # Top layout
    pack $w.b -side "top" -anchor "w" -padx 3 -pady 3
    pack $w.fb $w.res -side bottom -fill x -padx 3 -pady 3

    # Fill in existing or an empty line
    if {[llength $::Pref(regsub)] == 0} {
        AddPrefRegsub $top $w
    } else {
        set t 1
        foreach {RE Sub} $::Pref(regsub) {
            set ::diff($top,prefregexp$t) $RE
            set ::diff($top,prefregsub$t) $Sub
            AddPrefRegsub $top $w
            incr t
        }
    }

    trace add variable ::diff($top,prefregexa) write \
            [list EditPrefRegsubUpdate $top]
    trace add variable ::diff($top,prefregexa2) write \
            [list EditPrefRegsubUpdate $top]
    EditPrefRegsubUpdate $top
}