title: search and replace with sed not working
body: <p>I would like to use sed for search and replace, but it doesn't seem to be working.
  I guess I'm making a stupid mistake I can't spot.</p>
  
  <p>here is my file.txt:</p>
  
  <pre><code>PFPR03  PA1448770
  PFPR03  PA1448780
  PFPR03  PA1448790
  PFPR03  PA1448800
  PFPR03  PA1448810
  PFPR03  PA1448830
  PFPR03  PA1448840
  PFPR03  PA1448850
  PFPR03  PA1448860
  PFPR03  PA1448870
  </code></pre>
  
  <p>I want to change PA1448770 to PA14_48770, ie. adding an underscore after the PA14.</p>
  
  <p>Here are a few examples of what I've tried, none of them are affecting the file at all:</p>
  
  <pre><code>sed 's/^PA14/PA14_/g' file.txt
  sed 's/^PA14([0-9]*)/PA14_\1/' file.txt 
  </code></pre>
  
  <p>Any help is appreciated,</p>