<div class="someClass"><span class="someOtherClass">Text</span></div>

<table>
  <tbody>
    <tr>
      <td>label</td>
      <td>data</td>
    </tr>
    <tr>
      <td>label</td>
      <td>data</td>
    </tr>
    <tr>
      <td>label</td>
      <td>data</td>
    </tr>
  </tbody>
</table>

<div class="someClass"><span class="someOtherClass">Text</span></div>
      <table>
  <tbody>
    <tr>
      <td>label</td>
      <td>data</td>
    </tr>
    <tr>
      <td>label</td>
      <td>data</td>
    </tr>
    <tr>
      <td>label</td>
      <td>data</td>
    </tr>
  </tbody>
</table>
<div class="someClass"><span class="someOtherClass">Text</span></div>

var regex = Regex.Matches("<div class='someClass'>(.*?)<div class='someClass'>", html);

var text = doc.DocumentNode
            .SelectNodes("//span[@class='someOtherClass']")
            .Select(x => x.InnerText)
            .ToList();

var doc = new HtmlAgilityPack.HtmlDocument();
doc.LoadHtml(htmlstring);

var tables = doc.DocumentNode
             .SelectNodes("//span[@class='someOtherClass']/following::table").ToList();
foreach (var table in tables)
{
    var list = table.Descendants("tr")
                    .Select(tr => tr.Descendants("td")
                    .Select(td => td.InnerText).ToList())
                    .ToList();
}

<div s+ class s* = s* ['"] someClass ['"] [^>]* >
 ( [Ss]*? )                  # (1)
 <div s+ class s* = s* ['"] someClass ['"] [^>]* >