title: C# regexp matched subexpressions returns empty string
body: <p>I wrote this regexp:</p>
  
  <pre><code>var cellPattern = new Regex(@"(?(?=\d+)\d+|\|)\s(.)\s", RegexOptions.Compiled | RegexOptions.Multiline);
  </code></pre>
  
  <p>And to get cells from this string:</p>
  
  <pre><code>string field = 
  "  A   B   C   D   E \n" +
  "1   | X |   |   |   \n" +
  " ---+---+---+---+---\n" +
  "2   |   |   |   |   \n" +
  " ---+---+---+---+---\n" +
  "3   | O |   |   |   \n" +
  " ---+---+---+---+---\n" +
  "4   |   |   | X |   \n" +
  " ---+---+---+---+---\n" +
  "5   |   |   |   |   \n" +
  "O &gt;&gt;&gt; ";
  </code></pre>
  
  <p>I'm executing <code>cellPattern.Matches(field);</code> It returns MatchCollection with 25 matches, <strong>but why all matches has an empty string as first group?</strong></p>
  
  <p><em>P.S.:
  If I am using a named matched subexpressions, everything works the way I want: all matches has grid cell as "cell" group</em>:</p>
  
  <pre><code>var cellPattern = new Regex(@"(?(?=\d+)\d+|\|)\s(?&lt;cell&gt;.)\s", RegexOptions.Compiled | RegexOptions.Multiline);
  </code></pre>
  
  <p><em>P.P.S: My project framework is <code>.NET Framework 4.5.2</code></em></p>
  
  <p><em>P.P.P.S: On <a href="http://regexstorm.net/tester" rel="nofollow noreferrer">this site</a>, you also can see this behavior</em></p>