title: While replacing using regex, How to keep a part of matched string?
body: <p>I have</p>
  
  <pre><code>12.hello.mp3
  21.true.mp3
  35.good.mp3
  .
  .
  .
  </code></pre>
  
  <p>so on as file names in listed in a text file.</p>
  
  <p>I need to replace only those dots(.) infront of numbers with a space.(e.g. 12.hello.mp3 => 12 hello.mp3). 
  If I have regex as "[0-9].", it replaces number also.
  Please help me.</p>