======
[regsub -all {[s]} "s" "a" new_str]
======
substitutes to 1, whereas
======
[regsub -all {[ß]} "ß" "a" new_str]
======
substitutes to 2.

I guess it's some encoding stuff, as äöü trigger this, too. But I don't know how tcl handles special encodings.

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[MG] For me, both [regsub] calls return 1 as expected, with Tcl 8.5.7.