| Issue |
133360
|
| Summary |
[libc++] `<regex>`: Undefined backreferences should match empty string
|
| Labels |
libc++
|
| Assignees |
|
| Reporter |
Alcaro
|
```
> /(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.
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