How many times does the Vim editor match a string appear

Vim combines regular expressions to achieve complex functions, such as
Common find substitution%s/matching strings/replacement strings in vim/g
If you want to match only the number of strings:
%s/string/&/gn
N, which represents only the number of matches to be reported, without actual substitution.
What if you want only the number of occurrences of this string in the current line?
: s/string/&/gn
If the number of occurrences of a string is found between M and n rows
: m,ns/string/&/gn

If there are no more than 2 BBB per line, this can be done simply:
:%s/(bbb.*) bbb/1/
This command is better understood, under ordinary VI should also be able to use. But because of the greed of the *, when BBB exceeds 2, it will remove the last BBB instead of the second. To avoid this problem, the above command can be rewritten as:
:%s/(bbb.{ -}) bbb/1/
(bbb.*) @<= is a so-called 0-width "assertion" (to be honest, I have always been very sick of translating some of the terms in the regular term), which is popularly said to match not the string but the "position". What kind of position? This is where the front matches the bbb.*, which is where the BBB was previously seen. Note that "those" are used here because there are a lot of positions in the back of the first BBB that match the conditions. Then the second BBB appears in the same position, and naturally the second BBB matches the following expression:
(bbb.*) @<=bbb
Of course the third fourth (if any) also matches, but the first match is the second BBB.
It seems that the quantifiers appearing in the assertion should be careful when discussing its greed.

Vim is really powerful, the regular expression of the grammar expansion is also outside the mainstream of their own system, it is inevitable to us and add some learning costs