Try the following regular expression
[^a-zA-Z0-9]+
The example follows
echo "abc---012" | sed -e 's/[^0-9a-zA-Z][^0-9a-zA-Z]*/ /g'
#output
abc 012
FYI: .+ is equivalent to ..*
[^a-zA-Z0-9]
example follows
echo "abc---012" | sed -e 's/[^0-9a-zA-Z]/ /g'
#output
abc 012