https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102667
Bug ID: 102667
Summary: Inconsistent result of std::regex_match
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: fchelnokov at gmail dot com
Target Milestone: ---
This program:
```
#include <iostream>
#include <string>
#include <regex>
#include <assert.h>
int main()
{
std::string input("4321");
std::regex rg("^([0-9])");
std::smatch sm;
bool found = std::regex_match(input, sm, rg);
assert(!sm.size() == sm.empty());
std::cout << "ready: " << sm.ready() << ", found: " <<
found << ", size: " << sm.size() << std::endl;
for (auto it = sm.begin(); it != sm.end(); ++it)
{
std::cout << "iterate '" << *it << "'\n";
}
}
```
prints rather unexpected:
```
ready: 1, found: 0, size: 0
iterate ''
iterate ''
iterate ''
```
So std::smatch contains 3 entries while its size is zero.
Expected result:
```
ready: 1, found: 0, size: 0
```
Demo: https://gcc.godbolt.org/z/Wfh1vaPqq
Related discussion: https://stackoverflow.com/q/66611132/7325599
