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Merged
StephanTLavavej merged 2 commits into from
Apr 17, 2026
Merged

<regex>: Avoid generating empty groups when parsing empty alternatives #6249

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c0c2df2
`<regex>`: Avoid generating empty groups when parsing empty alternatives
muellerj2 Apr 11, 2026
65f52e1
fix comment
muellerj2 Apr 11, 2026
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58 changes: 25 additions & 33 deletions stl/inc/regex
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Original file line number Diff line number Diff line change
Expand Up @@ -1618,7 +1618,7 @@ public:
_Node_base* _Begin_capture_group(unsigned int _Idx);
void _Add_backreference(unsigned int _Idx);
_Node_base* _Begin_if(_Node_base* _Start);
void _Else_if(_Node_base*, _Node_base*);
void _Else_if2(_Node_base*, _Node_base*);
void _Add_rep(int _Min, int _Max, bool _Greedy);
void _Negate();
_Root_node* _End_pattern();
Expand Down Expand Up @@ -2238,7 +2238,7 @@ private:
void _Do_assert_group(bool);
bool _Wrapped_disjunction();
void _Quantifier();
bool _Alternative();
void _Alternative2();
void _Disjunction();
void _Calculate_loop_simplicity(_Node_base* _Nx, _Node_base* _Ne, _Node_rep* _Outer_rep, bool _Nonreentrant);

Expand Down Expand Up @@ -3661,21 +3661,25 @@ _Node_base* _Builder2<_FwdIt, _Elem, _RxTraits>::_Begin_if(_Node_base* _Start) {
}

template <class _FwdIt, class _Elem, class _RxTraits>
void _Builder2<_FwdIt, _Elem, _RxTraits>::_Else_if(_Node_base* _Start, _Node_base* _End) { // add else node
_Node_if* _Parent = static_cast<_Node_if*>(_Start->_Next);
_Node_base* _First = _End->_Next;
_End->_Next = nullptr;
_Node_base* _Last = _Current;
_Current = _End;
_End->_Next = nullptr;
_Last->_Next = _End;
void _Builder2<_FwdIt, _Elem, _RxTraits>::_Else_if2(_Node_base* const _Start, _Node_base* const _End) { // add else node
auto _Parent = static_cast<_Node_if*>(_Start->_Next);
while (_Parent->_Child) {
_Parent = _Parent->_Child;
}

_Parent->_Child = new _Node_if(_End);
_Parent->_Child->_Next = _First;
_First->_Prev = _Parent->_Child;
const auto _Elseif_node = new _Node_if(_End);
_Parent->_Child = _Elseif_node;
const auto _First = _End->_Next;
if (_First) {
_End->_Next = nullptr;
_Elseif_node->_Next = _First;
_First->_Prev = _Elseif_node;
_Current->_Next = _End;
} else { // empty alternative
_Elseif_node->_Next = _End;
}

_Current = _End;
}

template <class _FwdIt, class _Elem, class _RxTraits>
Expand Down Expand Up @@ -5812,13 +5816,12 @@ void _Parser2<_FwdIt, _Elem, _RxTraits>::_Quantifier() { // check for quantifier
}

template <class _FwdIt, class _Elem, class _RxTraits>
bool _Parser2<_FwdIt, _Elem, _RxTraits>::_Alternative() { // check for valid alternative
void _Parser2<_FwdIt, _Elem, _RxTraits>::_Alternative2() { // check for valid alternative
bool _Found = false;
for (;;) { // concatenate valid elements
while (_Mchar != _Meta_eos && _Mchar != _Meta_bar
&& (_Mchar != _Meta_rpar || _Disj_count == 0)) { // concatenate valid elements
bool _Quant = true;
if (_Mchar == _Meta_eos || _Mchar == _Meta_bar || (_Mchar == _Meta_rpar && _Disj_count != 0)) {
return _Found;
} else if (_Mchar == _Meta_rpar && !(_L_flags & _L_paren_bal)) {
if (_Mchar == _Meta_rpar && !(_L_flags & _L_paren_bal)) {
_Error(regex_constants::error_paren);
} else if (_Mchar == _Meta_dot) { // add dot node
_Nfa._Add_dot();
Expand Down Expand Up @@ -5883,26 +5886,15 @@ bool _Parser2<_FwdIt, _Elem, _RxTraits>::_Alternative() { // check for valid alt
template <class _FwdIt, class _Elem, class _RxTraits>
void _Parser2<_FwdIt, _Elem, _RxTraits>::_Disjunction() { // check for valid disjunction
_Node_base* _Pos1 = _Nfa._Getmark();
if (!_Alternative()) {
if (_Mchar != _Meta_bar) {
return; // zero-length alternative not followed by '|'
}

// zero-length leading alternative
_Node_base* _Pos3 = _Nfa._Begin_group();
_Nfa._End_group(_Pos3);
}
_Alternative2();

if (_Mchar == _Meta_bar) {
if (_Mchar == _Meta_bar) { // at least one more alternative
_Node_base* _Pos2 = _Nfa._Begin_if(_Pos1);
do { // append terms as long as we keep finding | characters
_Next();
if (!_Alternative()) { // zero-length trailing alternative
_Node_base* _Pos3 = _Nfa._Begin_group();
_Nfa._End_group(_Pos3);
}

_Nfa._Else_if(_Pos1, _Pos2);
_Alternative2();
_Nfa._Else_if2(_Pos1, _Pos2);
} while (_Mchar == _Meta_bar);
}
}
Expand Down
22 changes: 22 additions & 0 deletions tests/std/tests/VSO_0000000_regex_use/test.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2579,6 +2579,27 @@ void test_gh_6191() {
}
}

void test_gh_6249() {
// GH-6249: Avoid generating empty groups when parsing empty alternatives
for (const string re : {"|a|b", "a||b", "a|b|"}) {
g_regexTester.should_match("a", re);
g_regexTester.should_match("b", re);
g_regexTester.should_match("", re);
g_regexTester.should_not_match("c", re);
g_regexTester.should_not_match("ab", re);
}

for (const string re : {"(?:|a|b)c", "(?:a||b)c", "(?:a|b|)c"}) {
g_regexTester.should_match("ac", re);
g_regexTester.should_match("bc", re);
g_regexTester.should_match("c", re);
g_regexTester.should_not_match("", re);
g_regexTester.should_not_match("a", re);
g_regexTester.should_not_match("b", re);
g_regexTester.should_not_match("abc", re);
}
}

int main() {
test_dev10_449367_case_insensitivity_should_work();
test_dev11_462743_regex_collate_should_not_disable_regex_icase();
Expand Down Expand Up @@ -2646,6 +2667,7 @@ int main() {
test_gh_6181();
test_gh_6189();
test_gh_6191();
test_gh_6249();

return g_regexTester.result();
}