cat filename | sed 's|[^0-9]||g'

cat filename | sed 's|[^0-9]||g' | sort | uniq

while read line; do
    echo ${line//[^0-9]/};
done < filename

cat filename | sed 's|[^0-9]*||'

grep -Po "[0-9]*(..*)?" filename

cat filename | tr -d [:alpha:]

grep -o '[0-9]*' filenames

sed 's/[^0-9]*//' filename

echo filename123onetwothree.999 | sed 's/.*[^0-9]([0-9]*)$/1/'
999

sed 's/.*[^[:digit:]]([[:digit:]]*)$/1/' file