title: apply substitute to a string variable in a map
body: <p>I have a <code>noremap</code> that sends the current full file path to an interpreter.  </p>
  
  <p>The problem is that on windows the <code>\</code> need to be <code>/</code> -- so when i pass <code>expand("%:p")</code> an error is thrown. </p>
  
  <p>I have figured how to do this with a string in the editing frame in vim using <code>:s@\\@/@g</code>, however, i cannot figure out how to apply the substitute command to the string before sending it from vim to the interpreter. </p>
  
  <p>Is this possible? </p>