title: Matching the nth occurrence of a single character in regex
body: <p>I am having trouble finding any regex which actually matches what I need, despite how easy this request seemed when I started out. Hoping to get help here.</p>
  
  <p>An early mistake in our code is sending dates in the format <code>YYYY-MM-DD HH:mm:ss:SSS</code>. This is not valid ISO, as the milliseconds are proceeded by a colon, not a period.</p>
  
  <p>I wanted a quick regex to use in conjunction with JS .replace() to match the third colon <code>:</code> and replace it with a period <code>.</code> As I said, this sounded pretty basic -- but I have found nothing that works, and no other places where the suggested fix for a similar problem has worked in my case.</p>
  
  <p>Any help is appreciated.</p>