これだけ
s = "０９８７６５４３２１"
s.tr!("０-９", "0-9")
p s # => 0987654321
NKFとか使わなくても、数字くらいなら、trで変換するのがお手軽。
（カタカナとかになると、全半角で文字数も変わってくるのでtrでは難しい）
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