This one will be short, I could be part of the previous one, but it feels like it’s not substitution, but it’s still one. There is an n flag which will reports the number of matches, but does not actually substitute. It can be useful if we want to count them.

For example :% s/it//gn reports back:

10 matches on 6 lines

Without g, it’s only 6 because without g it will stop at the first match per line:

6 matches on 6 lines