title: regex expression to replace first occurrence of “#” with “ _”
body: <p>How can I  build a regex expression to replace first  occurrence of <code>#</code> with <code>_</code>.</p>
  
  <p>I  need to  change <code>f2b1_last_name_buyer</code> to  <code>f2b1#last_name_buyer</code></p>
  
  <pre><code>resultString = Regex.Replace(subjectString, "^([^_]+)_", "#", RegexOptions.Singleline);
  </code></pre>
  
  <p>but it scubs all of the characters to the first  _</p>