title: Finding the indexes of multiple/overlapping matching substrings
body: <p>I have a string, <code>s="CCCGTGCC"</code> and a subtstring <code>ss="CC"</code>. I want to get all the indexes in <code>s</code> that start the string <code>ss</code>. In my example I would want to get back the array <code>c(1,2,6)</code>.</p>
  
  <p>Is there any string function that achieves this? Notice that my string is in the form <code>"CCCGTGCC"</code>, and not <code>c("C","C","C","G","T","G","C","C")</code>.</p>
  
  <p><code>grep</code> only returns whether there is a match anywhere in the string, and not the indexes of the matches within the string, unless I'm missing something. </p>