title: Splitting letters from numbers within a string
body: <p>I'm processing strings like this: <code>"125A12C15"</code>
  I need to split them at boundaries between letters and numbers, e.g. this one should become <code>["125","A","12","C","15"]</code>.</p>
  
  <p>Is there a more elegant way to do this in Python than going through it position by position and checking whether it's a letter or a number, and then concatenating accordingly? E.g. a built-in function or module for this kind of thing?</p>
  
  <p>Thanks for any pointers!</p>