When substituting in a visual block selection, the automatically
inserted range `'<,'>` is not enough as the pattern will also match
outside of the selection on the same lines. To only match inside the
selection, insert the `\%V` atom.
Theoretically the pattern will also need the atom before the last
character to make sure that everything is inside the selection, but this
can be inserted manually when needed (see `:h /\%V`).