Question about using a pattern in ReplacePart

I noticed that "ReplacePart" can use pattern. For example:

ReplacePart[{{0, 0, 0}, {0, 0, 0}, {0, 0, 0}}, {i_, i_} -> x]
{{x, 0, 0}, {0, x, 0}, {0, 0, x}}

So I naively think I can do a little change like

ReplacePart[{{0, 0, 0}, {0, 0, 0}, {0, 0, 0}}, {i_+1, i_} -> x]

But it gives

{x, x, x}

Why? I anticipated that the result whould be

{{0, 0, 0}, {x,0,0}, {0, x, 0}}

Can anyone help?

Comments

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