[Bug libstdc++/124004] Positive lookahead (?=^) matches incorrectly with std::regex

[ppalka at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124004

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2026-02-06
                 CC|                            |ppalka at gcc dot gnu.org

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Perhaps just

diff --git a/libstdc++-v3/include/bits/regex_executor.tcc
b/libstdc++-v3/include/bits/regex_executor.tcc
index 19b5ad27df40..083bd9f4f6d0 100644
--- a/libstdc++-v3/include/bits/regex_executor.tcc
+++ b/libstdc++-v3/include/bits/regex_executor.tcc
@@ -238,6 +238,8 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
       _ResultsVec __what(_M_cur_results);
       _Executor __sub(_M_current, _M_end, __what, _M_re, _M_flags);
       __sub._M_states._M_start = __next;
+      if (_M_current != _M_begin)
+       __sub._M_flags |= regex_constants::match_prev_avail;
       if (__sub._M_search_from_first())
        {
          for (size_t __i = 0; __i < __what.size(); __i++)