\w matches stuff not considered "letters" by Unicode. If you want to use Unicode's definitions, it's quite easy to do since it provides properties relevant to your question. 

L   Letters
Nd  Number, Decimal Digit

[download]

So you'd use 

s/[^\p{L}\p{Nd}_]//g

[download]

Update: "^" was missing!

In reply to Re: Strip utf-8 dangerous url chars by ikegami
in thread Strip utf-8 dangerous url chars by AlfaProject

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