def f(s)
  b = ->op, a, b {op.to_proc.(a.to_i, b.to_i).to_s}
  s = s.dup
  loop do
    next if s.sub!(/\(([^\(\)]*)\)/) {f $1}
    next if s.sub!(/(-?\d+)\*(-?\d+)/) {b.(:*, $1, $2)}
    next if s.sub!(/(-?\d+)\/(-?\d+)/) {b.(:/, $1, $2)}
    next if s.sub!(/(-?\d+)\+(-?\d+)/) {b.(:+, $1, $2)}
    next if s.sub!(/(-?\d+)-(-?\d+)/) {b.(:-, $1, $2)}
    break s
  end
end
p ['1+2+3+4', '4+5*6-2*3', '5+12*(2+9/3)/((2+3)*5-7*3)'].map {|s| [s, f(s)]}
 

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