<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/133360>133360</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [libc++] `<regex>`: Undefined backreferences should match empty string
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            libc++
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          Alcaro
      </td>
    </tr>
</table>

<pre>
    ```
> /(a)?\1/.exec("")
Array [ "", undefined ]
> /(a)?\1/.exec("a")
Array [ "", undefined ]
> /(a)?\1/.exec("aa")
Array [ "aa", "a" ]
```
~Firefox

But in libc++, the equivalent operations return `null`, `null` and `[ "aa", "a" ]`. The spec is unambiguous; https://262.ecma-international.org/5.1/#sec-15.10.2.9 5.3 says unmatched backreferences should match the empty string.

Full example: https://godbolt.org/z/ndrqej718 (libstdc++ has the same bug)

Found in [this battery of tests](https://godbolt.org/z/6PnbKzTrn) - most or all of the libc++ failures are this, #133314, or both.
</pre>
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