<block>
    <foo val="bar"/>
    <foo val="bar"/>
</block>
<block>
    <foo val="bar"/>
    <foo val="bar"/>
</block>

<block>
    <foo val="bar1"/>
    <foo val="bar"/>
</block>
<block>
    <foo val="bar1"/>
    <foo val="bar"/>
</block>

:let a = ['', '1']
:%s/barzs/=reverse(a)[0]/g

:let a = ['a', 'b', 'c']
:%s/barzs/=add(a, remove(a, 0))[-1]/g

:h :s
:h range
:h /zs
:h :s=
:h reverse(
:h :s_flags
:h Lists
:h add(
:h remove

:%s/bar/bar1/gc

qv            start recording in register v
/"bar"/e<cr>  search for "bar" and position the cursor at the end of the match
i1<esc>       insert 1 before the cursor and go back to normal mode
n             jump to next match
q             stop recording

:%s/bar(.*)n(.*)bar/bar11r2bar

:let dosubs=1
:%s/bar/=[dosubs?'bar1':submatch(0),extend(g:,{'dosubs':!dosubs})][0]/g

:%SubstituteSelected/<bar>/&1/ yn

":[range]SubstituteSelected/{pattern}/{string}/[flags] {answers}
"           Replace matches of {pattern} in the current line /
"           [range] with {string}, determining whether a particular
"           match should be replaced on the sequence of "y" or "n"
"           in {answers}. I.e. with "ynn", the first match is
"           replaced, the second and third are not, the fourth is
"           again replaced, ...
"           Handles & and , 1 .. 9 in {string}.
function! CountedReplace()
    let l:index = s:SubstituteSelected.count % len(s:SubstituteSelected.answers)
    let s:SubstituteSelected.count += 1

    if s:SubstituteSelected.answers[l:index] ==# 'y'
        if s:SubstituteSelected.replacement =~# '^\='
            " Handle sub-replace-special.
            return eval(s:SubstituteSelected.replacement[2:])
        else
            " Handle & and , 1 .. 9 (but not u, U, n, etc.)
            let l:replacement = s:SubstituteSelected.replacement
            for l:submatch in range(0, 9)
                let l:replacement = substitute(l:replacement,
                   '%(%(^|[^\])%(\\)*\)@<!' .
                       (l:submatch == 0 ?
                           '%(&|\'.l:submatch.')' :
                           '\' . l:submatch
                       ),
                   submatch(l:submatch), 'g'
                )
            endfor
            return l:replacement
        endif
    elseif s:SubstituteSelected.answers[l:index] ==# 'n'
        return submatch(0)
    else
        throw 'ASSERT: Invalid answer: ' . string(s:SubstituteSelected.answers[l:index])
    endif
endfunction
function! s:SubstituteSelected( range, arguments )
    let l:matches = matchlist(a:arguments, '^(i@!S)(.*)%(%(^|[^\])%(\\)*\)@<!1(.*)%(%(^|[^\])%(\\)*\)@<!1(S*)s+([yn]+)$')
    if empty(l:matches)
        echoerr 'Invalid arguments'
        return
    endif
    let s:SubstituteSelected = {'count': 0}
    let [l:separator, l:pattern, s:SubstituteSelected.replacement, l:flags, s:SubstituteSelected.answers] = l:matches[1:5]

    execute printf('%ssubstitute %s%s%s=CountedReplace()%s%s',
       a:range, l:separator, l:pattern, l:separator, l:separator, l:flags
    )
endfunction
command! -bar -range -nargs=1 SubstituteSelected call <SID>SubstituteSelected('<line1>,<line2>', <q-args>)

v%(block>_{-})zsbar
%s,,&1,g



   v ............ very magic (avoid lots of scapes)
   %  ............ ignore whats flows
   (  ............ starts (ignored) group
   _ ............ multiline search
   .{-} .......... non-greedy
   zs ........... start pattern for substituition
   bar  .......... pattern we want to change

   % ............. whole file
   s ............. substituition
   ,, ............ use last search (could be //)
   & ............. use searched pattern
   1 ............. add 1 after it