Last names are also not limited to ASCII. There all sorts of special caracters depending on the language like üáàãéèç-' (the single quote is specially important because its normally blocked due to SQL injection, but its a common character in italian/french/spanish names).
Preventing people from writing their own names because you decide to use a regexp from stackoverflow is also bad.
Preventing people from writing their own names because you decide to use a regexp from stackoverflow is also bad.