<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/135048>135048</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [libc++] `<regex>`: `\00` should be rejected
        </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>
    ```c++
#include <regex>
#include <stdio.h>

int main() {
    try {
        std::regex r{"\\00"};
        puts("valid");
    } catch (const std::exception& e) {
        printf("not valid: %s\n", e.what());
    }
    try {
 std::regex r{"\\01"};
        puts("valid");
    } catch (const std::exception& e) {
        printf("not valid: %s\n", e.what());
 }
}
```
Expected: Reject them. 00 and 01 do not match DecimalIntegerLiteral in the JS spec, and lookahead can't be a digit either.
Actual: Both are valid. (Can't find what they're actually parsed as, though.)

https://godbolt.org/z/heM1o1aGe (MS-STL is busy fixing it https://github.com/microsoft/STL/pull/5380)
</pre>
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