Replace two space indentation with tabs

Question

I'd like to replace all two-space indents with tabs

I can't do :%s/ /^I/g bc this replaces within lines too.

I can't do :%s/^ /^I/g bc this replaces only the first two spaces of each line, not each level of indentation.

If there were a way to count how many matches were found, something like this could work :%s/^\( \)*/\count^I/g, but I don't know whether any of that exists and I've tried looking it up but couldn't find it.

Is there a way I can replace X spaces at the start of a line with X/2 tabs?

Answer 1

Using :h sub-replace-special:

%s@^ \+@\=repeat("\t", strlen(submatch(0)) / &ts)