https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119505
Bug ID: 119505
Summary: <regex>: Undefined backreferences should match empty
string
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: blubban at gmail dot com
Target Milestone: ---
#include <regex>
#include <stdio.h>
int main()
{
std::string s{"a"};
std::regex r{"(b)?\\1"};
std::smatch m;
bool result = std::regex_search(s, m, r);
printf("search=%d\n", result);
}
Expected: Return true, because the capture group fails to match and is skipped,
and the backreference then matches the empty string, per
https://262.ecma-international.org/5.1/#sec-15.10.2.9 5.3.
Actual: Returns false.
https://godbolt.org/z/5v8Yjqnd6
libc++ has the same bug. https://github.com/llvm/llvm-project/issues/133360
