title: Regex - How to replace period in file name with an underscore
body: <p>I'm looking to build a regular expression that will allow me to replace the periods (.) in a filename with an underscore (_).</p>
  
  <p><strong>Example:</strong> replace filename.doc with filename_doc</p>
  
  <p>I've had a play around in Expresso, and have worked out that it will be something like .\w{3,4) - to pick up 3 or 4 character extentions eg doc, docx, xls, xlsx etc.</p>
  
  <p>But I cannot figure out how to replace only the period and retain the extention.</p>