string="CBX-1234--CBX-5678"
shortenedString=$(echo "$string" | sed "s/^([A-Za-z]+-[0-9]+)/1/")

sed -E 's/^([A-Za-z]+-[0-9]+)/1/'

sed -E 's/^([A-Za-z]+-[0-9]+).*$/1/'

shortenedString="${string%%--*}"

$ echo "abhellocd" | sed -r "s/.*(hello).*/1/g"
hello

-r, --regexp-extended
          use extended regular expressions in the script.