I know that grep will display the matched line, but how about if I want to get the matched characted, for example:

file.txt contains:
Hello 123 World
This is a56 letter34
56are you su345re ?
1000 yes I do

I want to list all instances of digits that appear e.g:
123
56
34
56
345
1000

do I have to use sed ?

Thanks !