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