| Issue |
131516
|
| Summary |
[libc++] `<regex>`: Character class `[\W\D]` fails to match alphabetic characters
|
| Labels |
libc++
|
| Assignees |
|
| Reporter |
muellerj2
|
The (ECMAScript) regular _expression_ `[\W\D]` describes a character class that matches the union of (a) all non-alphanumeric characters and (b) all non-digits. So effectively, the character class should be equivalent to `[\D]` and thus match all non-digits. However, libc++'s regex implementation only matches non-alphanumeric characters.
Test case:
```
#include <iostream>
#include <regex>
using namespace std;
int main()
{
regex re(R"([\W\D])");
cout << "matches alphabetic: " << regex_match("a", re) << '\n'
<< "matches digit: " << regex_match("0", re) << '\n'
<< "matches non-alphanumeric: " << regex_match(".", re);
return 0;
}
```
https://godbolt.org/z/YdvY4Pb6a
This prints:
```
matches alphabetic: 0
matches digit: 0
matches non-alphanumeric: 1
```
But it should print (as MSVC STL and libstdc++ do here):
```
matches alphabetic: 1
matches digit: 0
matches non-alphanumeric: 1
```
The problem lies here:
https://github.com/llvm/llvm-project/blob/215c0d2b651dc757378209a3edaff1a130338dd8/libcxx/include/regex#L2139-L2141
The negated character classes are bitwise or'ed, but De Morgan's law says that `(not w) or (not d) = not (w and d)`, so the bit masks should really be bitwise and'ed.
But bitwise and'ing is problematic as well, because the standard only provides a guarantee that bitwise or'ing works, but doesn't state that bitwise and'ing corresponds to the intersection of the character classes (see [\[re.grammar/9\]](https://eel.is/c++draft/re.grammar#9)). Maybe and'ing will still work for libc++'s `std::regex_traits<char>` and `std::regex_traits<wchar_t>` traits classes (although I haven't checked that), but it might not do the right thing for some user-provided traits classes.
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