title: How to join() words from a string?
body: <p>I want to  transform the string <code>'one two three'</code> into <code>one_two_three</code>.</p>
  
  <p>I've tried <code>"_".join('one two three')</code>, but that gives me <code>o_n_e_ _t_w_o_ _t_h_r_e_e_</code>...</p>
  
  <p>how do I insert the <code>"_"</code> only at spaces between words in a string?</p>