# This version replaces multiple hyphens with a single hyphen
echo "$STRING" | iconv -t ascii//TRANSLIT | sed -E 's/[^a-zA-Z0-9]+/-/g' | sed -E 's/^-+|-+$//g' | tr A-Z a-z

# This version maintains the number of hyphens
echo "$STRING" | iconv -t ascii//TRANSLIT | sed -E 's/[^a-zA-Z0-9-]+/-/g' | sed -E 's/^-+|-+$//g' | tr A-Z a-z