<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/55357>55357</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            libc++ regex invalid back reference
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          jeremy-rifkin
      </td>
    </tr>
</table>

<pre>
    `(x*)\s+(?!\1).+` should be a valid regex for std::regex but this causes a runtime error in libc++: https://godbolt.org/z/vPqWaEhGc

Not sure if this is an already known issue.
</pre>
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