title: Generate different permutations/combinations in a string
body: <p>I am given a string (eg "12345678").
  I want to generate different combinations using <code>+</code>,<code>-</code>,<code>*</code>,<code>/</code>.
  Like :</p>
  
  <pre><code>'1+2+3+4+5+6+7+8'
  '1+2*3-4+5+6-7+8'
  '1-2+3+4*5+6-7*8'
  '1-2-3-4+5*6+7+8'
  '1+2+3+4+5+6*7*8'
  '1-2+3-4+5-6+7-8'
  </code></pre>
  
  <p>Any idea how do i generate all different combinations like above?</p>