Replace using VIM, reuse part of the search pattern

Reference: Https://unix.stackexchange.com/questions/35206/replace-using-vim-reuse-part-of-the-search-pattern

\0is the whole match. To use the IT need to set it as this and use \1

.s/(\([0-9]*\))/{\1}/

Like trying to test_12 the file test_2323.

A string with a number after the Test_, followed by an X

Into test_12x test_2323x.

%s/\ (test_\d\+\)/\0x/g

Notice that the brackets and the + sign are escaped.

Replace using VIM, reuse part of the search pattern