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Oct 22, 2025
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<regex>: Process greedy simple loops non-recursively #5790

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e2c6bc1
`<regex>`: Process greedy simple loops non-recursively
muellerj2 Oct 18, 2025
e341706
Comment cleanups.
StephanTLavavej Oct 20, 2025
f0f2943
Add missing `options`.
StephanTLavavej Oct 20, 2025
142b9c1
Drop duplicated test.
StephanTLavavej Oct 20, 2025
84dc9ae
Add test cases.
StephanTLavavej Oct 20, 2025
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134 changes: 43 additions & 91 deletions stl/inc/regex
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Original file line number Diff line number Diff line change
Expand Up @@ -1681,6 +1681,7 @@ enum class _Rx_unwind_ops {
_Disjunction_eval_alt_always,
_Do_nothing,
_Loop_simple_nongreedy,
_Loop_simple_greedy,
};

template <class _BidIt>
Expand Down Expand Up @@ -1815,7 +1816,6 @@ private:
void _Decrease_stack_usage_count();
void _Increase_complexity_count();

bool _Do_rep0(_Node_rep*);
bool _Do_rep(_Node_rep*, bool, int);
void _Prepare_rep(_Node_rep*);
bool _Find_first_inner_capture_group(_Node_base*, _Loop_vals_v2_t*);
Expand Down Expand Up @@ -3413,72 +3413,6 @@ void _Matcher3<_BidIt, _Elem, _RxTraits, _It, _Alloc>::_Increase_complexity_coun
}
}

template <class _BidIt, class _Elem, class _RxTraits, class _It, class _Alloc>
bool _Matcher3<_BidIt, _Elem, _RxTraits, _It, _Alloc>::_Do_rep0(_Node_rep* _Node) {
// apply repetition to loop with no nested if/do
int _Ix = _Node->_Min;
const size_t _Frame_idx = _Loop_vals[_Node->_Loop_number]._Loop_frame_idx;
_Loop_vals[_Node->_Loop_number]._Loop_idx = _Ix + 2;

_Tgt_state_t<_It> _Final;
bool _Matched0 = false;
_It _Saved_pos = _Tgt_state._Cur;
bool _Done = false;

if (_Match_pat(_Node->_End_rep->_Next)) {
// record an acceptable match and continue
_Final = _Tgt_state;
_Matched0 = true;
}

if (_Ix == 0 && _Node->_Max != 0) {
_Tgt_state._Cur = _Saved_pos;
_Tgt_state._Grp_valid = _Frames[_Frame_idx]._Match_state._Grp_valid;

if (!_Match_pat(_Node->_Next)) { // rep match failed, we are done
_Done = true;
} else if (_Saved_pos == _Tgt_state._Cur) { // match empty, try no more repetitions
_Done = true;
// we only potentially accept/try tail for POSIX
if ((_Sflags & regex_constants::_Any_posix) && _Match_pat(_Node->_End_rep->_Next)) {
return true; // go with current match
}
} else {
_Saved_pos = _Tgt_state._Cur;
if (_Match_pat(_Node->_End_rep->_Next)) {
// record match and continue
_Final = _Tgt_state;
_Matched0 = true;
}
}
_Ix = 1;
}

if (!_Done) {
while (_Node->_Max == -1 || _Ix++ < _Node->_Max) { // try another rep/tail match
_Tgt_state._Cur = _Saved_pos;
_Tgt_state._Grp_valid = _Frames[_Frame_idx]._Match_state._Grp_valid;
if (!_Match_pat(_Node->_Next) || _Tgt_state._Cur == _Saved_pos) {
break; // rep match failed, quit loop
}

// since loop is branchless, empty rep match is not possible at this point
_Saved_pos = _Tgt_state._Cur;
if (_Match_pat(_Node->_End_rep->_Next)) {
// record match and continue
_Final = _Tgt_state;
_Matched0 = true;
}
}
}

if (_Matched0) { // record final match
_Tgt_state = _Final;
}

return _Matched0;
}

template <class _BidIt, class _Elem, class _RxTraits, class _It, class _Alloc>
bool _Matcher3<_BidIt, _Elem, _RxTraits, _It, _Alloc>::_Do_rep(_Node_rep* _Node, bool _Greedy, int _Init_idx) {
// apply repetition
Expand Down Expand Up @@ -4117,32 +4051,31 @@ bool _Matcher3<_BidIt, _Elem, _RxTraits, _It, _Alloc>::_Match_pat(_Node_base* _N
break;

case _N_rep:
{
{ // handle start of loop
auto _Node = static_cast<_Node_rep*>(_Nx);
_Prepare_rep(_Node);
bool _Greedy = (_Node->_Flags & _Fl_greedy) != 0;

if (_Node->_Simple_loop == 1) {
auto& _Sav = _Loop_vals[_Node->_Loop_number];
_Sav._Loop_frame_idx = _Push_frame(_Rx_unwind_ops::_Do_nothing);
if (_Node->_Min > 0) { // try to match a rep
_Increase_complexity_count();
_Increase_complexity_count();
if (_Node->_Min > 0 || (_Greedy && !_Longest && _Node->_Max != 0)) { // try a rep first
_Sav._Loop_idx = 1;
// _Next is already assigned correctly for matching a rep
} else if (!_Greedy || _Longest) { // non-greedy matching
_Increase_complexity_count();

// try tail first
// set up stack unwinding for greedy matching if no rep is allowed
if (_Node->_Min == 0) {
_Push_frame(_Rx_unwind_ops::_Loop_simple_greedy, _Node);
}
} else { // try tail first
_Sav._Loop_idx = 0;
_Next = _Node->_End_rep->_Next;

// set up stack unwinding for non-greedy matching if at least one rep is allowed
if (_Node->_Max != 0) {
_Push_frame(_Rx_unwind_ops::_Loop_simple_nongreedy, _Node);
}
} else {
_Failed = !_Do_rep0(_Node);
_Next = nullptr;
}
} else {
_Failed = !_Do_rep(_Node, _Greedy, 0);
Expand All @@ -4153,7 +4086,7 @@ bool _Matcher3<_BidIt, _Elem, _RxTraits, _It, _Alloc>::_Match_pat(_Node_base* _N
break;

case _N_end_rep:
{
{ // handle end of loop
_Node_rep* _Nr = static_cast<_Node_end_rep*>(_Nx)->_Begin_rep;
auto& _Sav = _Loop_vals[_Nr->_Loop_number];
bool _Greedy = (_Nr->_Flags & _Fl_greedy) != 0;
Expand All @@ -4163,31 +4096,36 @@ bool _Matcher3<_BidIt, _Elem, _RxTraits, _It, _Alloc>::_Match_pat(_Node_base* _N
== _Frames[_Sav._Loop_frame_idx]._Match_state._Cur) { // initial match empty
// loop is branchless, so it will only ever match empty strings
// -> we only try tail for POSIX or if minimum number of reps is non-zero
if ((_Sflags & regex_constants::_Any_posix) || _Nr->_Min > 0) {
_Increase_complexity_count();
// _Next is already assigned correctly for matching tail
} else {
// _Next is already assigned correctly for matching tail

if (!(_Sflags & regex_constants::_Any_posix) && _Nr->_Min == 0) {
_Failed = true;
}
} else if (_Sav._Loop_idx < _Nr->_Min) { // at least one more rep to reach minimum
_Increase_complexity_count();

_Next = _Nr->_Next;
// GH-5365: We have to reset the capture groups from the second iteration on.
_Tgt_state._Grp_valid = _Frames[_Sav._Loop_frame_idx]._Match_state._Grp_valid;
++_Sav._Loop_idx;
} else if (_Longest || !_Greedy) {
_Increase_complexity_count();
} else if (_Greedy && !_Longest && _Sav._Loop_idx != _Nr->_Max) { // one more rep to try next
// set up stack unwinding for greedy matching
_Push_frame(_Rx_unwind_ops::_Loop_simple_greedy, _Nr);

_Next = _Nr->_Next;
// GH-5365: We have to reset the capture groups from the second iteration on.
_Tgt_state._Grp_valid = _Frames[_Sav._Loop_frame_idx]._Match_state._Grp_valid;
if (_Sav._Loop_idx < INT_MAX) { // avoid overflowing _Loop_idx
++_Sav._Loop_idx;
}
} else { // non-greedy matching or greedy matching with maximum reached
// set up stack unwinding for non-greedy matching if one more rep is allowed
if (_Sav._Loop_idx != _Nr->_Max) {
_Push_frame(_Rx_unwind_ops::_Loop_simple_nongreedy, _Nr);
}
// _Next is already assigned correctly for matching tail
} else if (_Sav._Loop_idx == _Nr->_Min) { // greedy and minimum number of reps reached
_Failed = !_Do_rep0(_Nr);
_Next = nullptr;
} else { // internal _Match_pat(_Node->_Next) call in _Do_rep0()
_Next = nullptr;
}

if (!_Failed) {
_Increase_complexity_count();
}
} else {
_Failed = !_Do_rep(_Nr, _Greedy, _Sav._Loop_idx);
Expand Down Expand Up @@ -4297,6 +4235,20 @@ bool _Matcher3<_BidIt, _Elem, _RxTraits, _It, _Alloc>::_Match_pat(_Node_base* _N
}
break;

case _Rx_unwind_ops::_Loop_simple_greedy:
// try tail if matching one more rep failed
if (_Failed) {
auto _Node = static_cast<_Node_rep*>(_Frame._Node);
auto& _Sav = _Loop_vals[_Node->_Loop_number];

_Increase_complexity_count();
_Nx = _Node->_End_rep->_Next;
_Tgt_state._Cur = _Frame._Match_state._Cur;
_Tgt_state._Grp_valid = _Frames[_Sav._Loop_frame_idx]._Match_state._Grp_valid;
_Failed = false;
}
break;

default:
#if _ITERATOR_DEBUG_LEVEL != 0
_STL_REPORT_ERROR("internal stack of regex matcher corrupted");
Expand Down Expand Up @@ -5299,7 +5251,7 @@ void _Parser2<_FwdIt, _Elem, _RxTraits>::_Calculate_loop_simplicity(
break;
case _N_rep:
// _Node_rep inside another _Node_rep makes both not simple if _Outer_rep can be repeated more than once
// because _Matcher3::_Do_rep0() does not reset capture group boundaries when control is returned to it.
// because the matcher does not reset capture group boundaries when handling simple loops.
// If _Outer_rep can repeat at most once, we have to analyze the structure of the inner loop.
if (_Outer_rep) {
_Outer_rep->_Simple_loop = 0;
Expand Down
81 changes: 81 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 @@ -2133,6 +2133,7 @@ void test_gh_5774() {
// GH-5774: Process non-greedy and longest-mode simple loops non-recursively.
// This extends our test coverage on non-greedy simple loops with bounded number of repetitions.
g_regexTester.should_not_match("", "a+?");
g_regexTester.should_match("b", "a{0}?b");
g_regexTester.should_not_match("ab", "a{0}?b");
g_regexTester.should_match("ab", "a{0,1}?b");
g_regexTester.should_not_match("aab", "a{0,1}?b");
Expand All @@ -2143,6 +2144,85 @@ void test_gh_5774() {
g_regexTester.should_match("aaab", "a{1,3}?b");
}

void test_gh_5790() {
// GH-5790: Process greedy simple loops non-recursively.
// This extends our test coverage on (mainly greedy) simple loops.
g_regexTester.should_not_match("", "a+");
g_regexTester.should_match("b", "a{0}b");
g_regexTester.should_not_match("ab", "a{0}b");
g_regexTester.should_match("ab", "a{0,1}b");
g_regexTester.should_not_match("aab", "a{0,1}b");
g_regexTester.should_match("aab", "a{0,2}b");
g_regexTester.should_match("aab", "a{1,2}b");
g_regexTester.should_not_match("aab", "a{1}b");
g_regexTester.should_not_match("aaab", "a{1,2}b");
g_regexTester.should_match("aaab", "a{1,3}b");

// Check that greedy and non-greedy search find the appropriate match.
// For the following regexes, greedy and leftmost-longest search yield the same matches.
for (syntax_option_type options : {ECMAScript, extended}) {
{
test_regex greedy_a_star(&g_regexTester, "a*", options);
greedy_a_star.should_search_match("aaaaaaaaaa", "aaaaaaaaaa");
}

{
test_regex bounded_greedy_a_rep(&g_regexTester, "a{5}", options);
bounded_greedy_a_rep.should_search_match("aaaaaaaaaa", "aaaaa");
}

{
test_regex upper_bounded_greedy_a_rep(&g_regexTester, "a{0,5}", options);
upper_bounded_greedy_a_rep.should_search_match("aaaaaaaaaa", "aaaaa");
}

{
test_regex lower_bounded_greedy_a_rep(&g_regexTester, "a{4,1000}", options);
lower_bounded_greedy_a_rep.should_search_match("aaaaaaaaaa", "aaaaaaaaaa");
}

{
test_regex lower_and_upper_bounded_greedy_a_rep(&g_regexTester, "a{2,5}", options);
lower_and_upper_bounded_greedy_a_rep.should_search_match("aaaaaaaaaa", "aaaaa");
}

{
test_regex too_large_min_greedy_a_rep(&g_regexTester, "a{11,1000}", options);
too_large_min_greedy_a_rep.should_search_fail("aaaaaaaaaa");
}
}

{
test_regex nongreedy_a_star(&g_regexTester, "a*?");
nongreedy_a_star.should_search_match("aaaaaaaaaa", "");
}

{
test_regex bounded_nongreedy_a_rep(&g_regexTester, "a{5}?");
bounded_nongreedy_a_rep.should_search_match("aaaaaaaaaa", "aaaaa");
}

{
test_regex upper_bounded_nongreedy_a_rep(&g_regexTester, "a{0,5}?");
upper_bounded_nongreedy_a_rep.should_search_match("aaaaaaaaaa", "");
}

{
test_regex lower_bounded_nongreedy_a_rep(&g_regexTester, "a{4,1000}?");
lower_bounded_nongreedy_a_rep.should_search_match("aaaaaaaaaa", "aaaa");
}

{
test_regex lower_and_upper_bounded_nongreedy_a_rep(&g_regexTester, "a{2,5}?");
lower_and_upper_bounded_nongreedy_a_rep.should_search_match("aaaaaaaaaa", "aa");
}

{
test_regex too_large_min_nongreedy_a_rep(&g_regexTester, "a{11,1000}?");
too_large_min_nongreedy_a_rep.should_search_fail("aaaaaaaaaa");
}
}

int main() {
test_dev10_449367_case_insensitivity_should_work();
test_dev11_462743_regex_collate_should_not_disable_regex_icase();
Expand Down Expand Up @@ -2195,6 +2275,7 @@ int main() {
test_gh_5576();
test_gh_5672();
test_gh_5774();
test_gh_5790();

return g_regexTester.result();
}