At SS64 syntax-regex.html it is stated that \w is equivalent to [a-zA-Z_0-9].
This is not exact.
'müller-riebensehl' -match '^[\w-]+?$' True
Is there any way to reliably only match on ASCII characters, not on umlauts etc. without making it explicit?
I don't think there is, but I have updated the page with some notes about it matching foreign language letters with accents.
I'd like to see a regex switch that could compare a string to the users current language and flag any characters which don't match that language.