title: Help with parsing lines of a file and replacing a substring
body: <p>I need to replace certain characters in each line of a file.  The file is not delimited, but each line has a fixed format.  For example, I need to replace 5 questionmarks into 'x's.  The 5 questionmarks that need to be replaced in each line are found in position 10.  So for example:</p>
  
  <p>Input file:</p>
  
  <pre><code>abdfg trr?????456
  g?????dhs?????diu
  eerrttyycdhjjhddd
  </code></pre>
  
  <p>The output file should be:</p>
  
  <pre><code>abdfg trrxxxxx456
  g?????dhsxxxxxdiu
  eerrttyycdhjjhddd
  </code></pre>
  
  <p>The output file will be saved as a different file to a specific location</p>
  
  <p>What's the best way to do this in VB.NET (I'm a little new to VB, so any code sample would help)? </p>