title: add character to the middle of a file name
body: <p>I need to add a character to the middle of a file name, I need to do this for about 8000 files all in the same directory.  I'm looking for a command-line option:</p>
  
  <p>Example (all files that need renaming are five digits, they are in a directory that contains other six-digit filenames that do not need to be renamed):</p>
  
  <pre><code>01011
  02022
  12193
  </code></pre>
  
  <p>To:</p>
  
  <pre><code>010101
  020202
  121903
  </code></pre>
  
  <p>I've tried a couple of things: rename, mv, etc.  Similar to this (<a href="https://stackoverflow.com/questions/5242877/bash-adding-0s-in-the-middle-of-a-file-name">Bash - Adding 0&#39;s in the middle of a file name</a>) but not exactly</p>