regex_executor.tcc

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// class template regex -*- C++ -*-
 
// Copyright (C) 2013-2026 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
 
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
 
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.
 
// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.
 
/**
 *  @file bits/regex_executor.tcc
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{regex}
 */
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
 
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr
namespace __detail
{
  template<typename _BiIter, typename _Alloc, typename _TraitsT>
    bool _Executor<_BiIter, _Alloc, _TraitsT>::
    _M_search()
    {
      if (_M_search_from_first())
        return true;
      if (_M_flags & regex_constants::match_continuous)
        return false;
      _M_flags |= regex_constants::match_prev_avail;
      while (_M_begin != _M_end)
        {
          ++_M_begin;
          if (_M_search_from_first())
            return true;
        }
      return false;
    }
 
  enum _ExecutorFrameOpcode : unsigned char
  {
    _S_fopcode_next,
    _S_fopcode_fallback_next,
    _S_fopcode_rep_once_more,
    _S_fopcode_fallback_rep_once_more,
    _S_fopcode_posix_alternative,
    _S_fopcode_merge_sol,
    _S_fopcode_restore_cur_results,
    _S_fopcode_restore_rep_count,
    _S_fopcode_decrement_rep_count,
  };
 
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpedantic" // anon struct
  struct _ExecutorFrameBase
  {
    _ExecutorFrameBase(_ExecutorFrameOpcode __op, _StateIdT __i)
    : _M_op(__op), _M_state_id(__i)
    { }
 
    _ExecutorFrameOpcode _M_op;
    union {
      unsigned char _M_byte0 = 0;
      struct { // Used by restore_rep_count frame
        unsigned char _M_count : 2;
      };
      struct { // Used by restore_cur_results frame
        unsigned char _M_subexpr_end : 1;
        unsigned char _M_matched : 1;
      };
    };
    unsigned char _M_bytes[6];
    _StateIdT _M_state_id;
  };
#pragma GCC diagnostic pop
 
  template<typename _BiIter, bool _Trivial /* = is_trivially_copyable<_BiIter>::value */>
    struct _ExecutorFrame : _ExecutorFrameBase
    {
      _ExecutorFrame(_ExecutorFrameOpcode __op, _StateIdT __i)
      : _ExecutorFrameBase(__op, __i)
      { }
 
      _ExecutorFrame(_ExecutorFrameOpcode __op, _StateIdT __i, _BiIter __p)
      : _ExecutorFrameBase(__op, __i), _M_pos(__p)
      { }
 
      _ExecutorFrame(_ExecutorFrameOpcode __op, _StateIdT __i, long __v)
      : _ExecutorFrameBase(__op, __i), _M_val(__v)
      { }
 
      // _M_pos and _M_val are mutually exclusive, which the optimized
      // partial specialization below depends on.
      _BiIter _M_pos = _BiIter();
      long _M_val = 0;
    };
 
  // Space-optimized partial specialization for when the input iterator is
  // trivially copyable.
  template<typename _BiIter>
    struct _ExecutorFrame<_BiIter, true> : _ExecutorFrameBase
    {
      _ExecutorFrame(_ExecutorFrameOpcode __op, _StateIdT __i)
      : _ExecutorFrameBase(__op, __i)
      { }
 
      _ExecutorFrame(_ExecutorFrameOpcode __op, _StateIdT __i, _BiIter __p)
      : _ExecutorFrameBase(__op, __i), _M_pos(__p)
      { }
 
      _ExecutorFrame(_ExecutorFrameOpcode __op, _StateIdT __i, long __v)
      : _ExecutorFrameBase(__op, __i), _M_val(__v)
      { }
 
      union {
        _BiIter _M_pos;
        long _M_val;
      };
    };
 
  // The _M_main function operates in different modes, DFS mode or BFS mode,
  // indicated by _M_search_mode, and dispatches to either _M_main_dfs or
  // _M_main_bfs.
  //
  // ------------------------------------------------------------
  //
  // DFS mode:
  //
  // It applies a Depth-First-Search (aka backtracking) on given NFA and input
  // string.
  // At the very beginning the executor stands in the start state, then it
  // tries every possible state transition in current state recursively. Some
  // state transitions consume input string, say, a single-char-matcher or a
  // back-reference matcher; some don't, like assertion or other anchor nodes.
  // When the input is exhausted and/or the current state is an accepting
  // state, the whole executor returns true.
  //
  // TODO: This approach is exponentially slow for certain input.
  //       Try to compile the NFA to a DFA.
  //
  // Time complexity: \Omega(match_length), O(2^(_M_nfa.size()))
  // Space complexity: \theta(match_results.size() + match_length)
  //
  template<typename _BiIter, typename _Alloc, typename _TraitsT>
    bool _Executor<_BiIter, _Alloc, _TraitsT>::
    _M_main_dfs(_Match_mode __match_mode)
    {
      _M_has_sol = false;
      *_M_get_sol_pos() = _BiIter();
      _M_cur_results = _M_results;
      _M_dfs<_Search_mode::_Dfs>(__match_mode, _M_start);
      return _M_has_sol;
    }
 
  // Return whether a prefix search at _M_current might still match after
  // looking only through the non-consuming front of the NFA.
  //
  // This is not a general implementation.  It is deliberately small and
  // conservative: when it reaches a construct whose first consuming character
  // is hard to know cheaply, it returns true and lets the normal executor run.
  // The important fast paths are the common negative cases.
  //
  // Examples:
  // * Pattern "[0-9]+" at input 'x': the first _S_opcode_match rejects 'x',
  //   so a full DFS search would only allocate/pop frames to fail.  Return
  //   false and let regex_search advance the starting position.
  //
  // * Pattern "[01]?[0-9]" at input '9': the optional [01] branch rejects,
  //   but the skip branch can consume '9'.  Return true and let DFS decide
  //   the full match.
  //
  // * Pattern "foo|bar" at input 'b': one alternative rejects, the other can
  //   start with 'b'.  Return true.
  template<typename _BiIter, typename _Alloc, typename _TraitsT>
    bool _Executor<_BiIter, _Alloc, _TraitsT>::
    _M_maybe_start_match(_StateIdT __i, size_t __depth)
    {
      // Depth is bounded by the NFA size so epsilon cycles cannot make the
      // precheck recurse forever.  Hitting the bound means "unknown", not
      // "no match", so stay conservative and run the real executor.  This is
      // important for patterns such as "(a*)*" where epsilon paths can cycle
      // before a consuming state is reached.
      if (__depth > _M_nfa.size())
        return true;
 
      // An invalid edge is a real dead end for the explored path.
      if (__i == _S_invalid_state_id)
        return false;
 
      const auto& __state = _M_nfa[__i];
      switch (__state._M_opcode())
        {
        case _S_opcode_match:
          return __state._M_matches(*_M_current);
 
        case _S_opcode_accept:
          // Empty matches are possible, so the full executor must decide.
          return true;
 
        case _S_opcode_subexpr_begin:
        case _S_opcode_subexpr_end:
        case _S_opcode_dummy:
          // Captures and dummy states do not consume input, so they cannot
          // affect the first-character decision.  Continue along the only
          // successor.
          return _M_maybe_start_match(__state._M_next, __depth + 1);
 
        case _S_opcode_line_begin_assertion:
          // Assertions do not consume characters, but they can reject the
          // current position.  For "^abc" at a non-begin position, there is no
          // need to run DFS merely to discover that ^ fails.
          return _M_at_begin()
                 && _M_maybe_start_match(__state._M_next, __depth + 1);
 
        case _S_opcode_line_end_assertion:
          // Same idea for "$": if the assertion does not hold here, this
          // starting position cannot match via this path.
          return _M_at_end()
                 && _M_maybe_start_match(__state._M_next, __depth + 1);
 
        case _S_opcode_word_boundary:
          // Word-boundary assertions are also checked before the first
          // consuming state.  For "\bfoo" in the middle of "xfoo", this path
          // rejects before testing 'f'.
          return _M_word_boundary() == !__state._M_neg
                 && _M_maybe_start_match(__state._M_next, __depth + 1);
 
        case _S_opcode_alternative:
          // A branch might match if either arm can start with *_M_current.
          // Example: "foo|bar" at 'b' rejects the "foo" arm but keeps the
          // search because the "bar" arm is viable.
          return _M_maybe_start_match(__state._M_alt, __depth + 1)
                 || _M_maybe_start_match(__state._M_next, __depth + 1);
 
        case _S_opcode_repeat:
          // Repeats can either enter the body or skip to the exit, so inspect
          // both paths.  This matters for constructs such as "[01]?[0-9]": at
          // '9' the optional first digit can be skipped, while at 'x' both
          // paths reject.
          return _M_maybe_start_match(__state._M_alt, __depth + 1)
                 || _M_maybe_start_match(__state._M_next, __depth + 1);
 
        case _S_opcode_backref:
        case _S_opcode_subexpr_lookahead:
        default:
          return true;
        }
    }
 
  // ------------------------------------------------------------
  //
  // BFS mode:
  //
  // Russ Cox's article (http://swtch.com/~rsc/regexp/regexp1.html)
  // explained this algorithm clearly.
  //
  // It first computes epsilon closure (states that can be achieved without
  // consuming characters) for every state that's still matching,
  // using the same DFS algorithm, but doesn't re-enter states (using
  // _M_visited to check), nor follow _S_opcode_match.
  //
  // Then apply DFS using every _S_opcode_match (in _M_match_queue)
  // as the start state.
  //
  // It significantly reduces potential duplicate states, so has a better
  // upper bound; but it requires more overhead.
  //
  // Time complexity: \Omega(match_length * match_results.size())
  //                  O(match_length * _M_nfa.size() * match_results.size())
  // Space complexity: \Omega(_M_nfa.size() + match_results.size())
  //                   O(_M_nfa.size() * match_results.size())
  template<typename _BiIter, typename _Alloc, typename _TraitsT>
    bool _Executor<_BiIter, _Alloc, _TraitsT>::
    _M_main_bfs(_Match_mode __match_mode)
    {
      _M_match_queue.emplace_back(_M_start, _M_results);
      bool __ret = false;
      while (1)
        {
          _M_has_sol = false;
          if (_M_match_queue.empty())
            break;
          std::fill_n(_M_visited_states, _M_nfa.size(), false);
          auto __old_queue = std::move(_M_match_queue);
          auto __alloc = _M_cur_results.get_allocator();
          for (auto& __task : __old_queue)
            {
              _M_cur_results = _ResultsVec(std::move(__task.second), __alloc);
              _M_dfs<_Search_mode::_Bfs>(__match_mode, __task.first);
            }
          if (__match_mode == _Match_mode::_Prefix)
            __ret |= _M_has_sol;
          if (_M_current == _M_end)
            break;
          ++_M_current;
        }
      if (__match_mode == _Match_mode::_Exact)
        __ret = _M_has_sol;
      _M_match_queue.clear();
      return __ret;
    }
 
  // Return whether now match the given sub-NFA.
  template<typename _BiIter, typename _Alloc, typename _TraitsT>
    bool _Executor<_BiIter, _Alloc, _TraitsT>::
    _M_lookahead(_StateIdT __next)
    {
      // Backreferences may refer to captured content.
      // We may want to make this faster by not copying,
      // but let's not be clever prematurely.
      _ResultsVec __what(_M_cur_results);
      _Executor __sub(_M_current, _M_end, __what, _M_re, _M_flags,
                      bool(_M_search_mode));
      __sub._M_start = __next;
      if (__sub._M_search_from_first())
        {
          for (size_t __i = 0; __i < __what.size(); __i++)
            if (__what[__i].matched)
              _M_cur_results[__i] = __what[__i];
          return true;
        }
      return false;
    }
 
  // __rep_count records how many times (__rep_count.second)
  // this node is visited under certain input iterator
  // (__rep_count.first). This prevent the executor from entering
  // infinite loop by refusing to continue when it's already been
  // visited more than twice. It's `twice` instead of `once` because
  // we need to spare one more time for potential group capture.
  //
  // If the node cannot be re-entered anymore from the current state then return
  // _S_invalid_state_id otherwise return the current state without going
  // through a vector, allowing the caller to decide what to do with the state
  // This is beneficial for DFS since DFS can continue with the next state
  // immediately
  template<typename _BiIter, typename _Alloc, typename _TraitsT>
    _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
    _M_rep_once_more(_Match_mode, _StateIdT __i)
    {
      const auto& __state = _M_nfa[__i];
      auto& __rep_count = _M_rep_count[__i];
      if (__rep_count.second == 0 || __rep_count.first != _M_current)
        {
          _M_frames.emplace_back(_S_fopcode_restore_rep_count,
                                 __i, __rep_count.first);
          _M_frames.back()._M_count = __rep_count.second;
          __rep_count.first = _M_current;
          __rep_count.second = 1;
          return __state._M_alt;
        }
      else
        {
          if (__rep_count.second < 2)
            {
              __rep_count.second++;
              _M_frames.emplace_back(_S_fopcode_decrement_rep_count, __i);
              return __state._M_alt;
            }
        }
      return _S_invalid_state_id;
    }
 
  // Try to consume the common repeat body shape
  //   repeat -> match -> repeat
  // without going through the generic state dispatch again.
  template<typename _BiIter, typename _Alloc, typename _TraitsT>
#ifdef __OPTIMIZE__
    [[__gnu__::__always_inline__]]
#endif
    inline _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
    _M_match_simple_repeat_body(_StateIdT __next, _StateIdT __repeat)
    {
      if (__next == _S_invalid_state_id)
        return _S_invalid_state_id;
 
      const auto& __state = _M_nfa[__next];
      if (__state._M_opcode() != _S_opcode_match
          || __state._M_next != __repeat)
        return __next;
 
      if (_M_current == _M_end || !__state._M_matches(*_M_current))
        return _S_invalid_state_id;
 
      ++_M_current;
      return __repeat;
    }
 
  // _M_alt branch is "match once more", while _M_next is "get me out
  // of this quantifier". Executing _M_next first or _M_alt first don't
  // mean the same thing, and we need to choose the correct order under
  // given greedy mode.
  template<typename _BiIter, typename _Alloc, typename _TraitsT>
  template<_Search_mode __search_mode>
#ifdef __OPTIMIZE__
    [[__gnu__::__always_inline__]]
#endif
    inline _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
    _M_handle_repeat(_Match_mode __match_mode, _StateIdT __i)
    {
      const auto& __state = _M_nfa[__i];
      // Greedy.
      if (!__state._M_neg)
        {
          if constexpr (__search_mode == _Search_mode::_Dfs)
            // If it's DFS executor and already accepted, we're done.
            _M_frames.emplace_back(_S_fopcode_fallback_next, __state._M_next,
                                   _M_current);
          else
            _M_frames.emplace_back(_S_fopcode_next, __state._M_next);
          _StateIdT __next = _M_rep_once_more(__match_mode, __i);
          if constexpr (__search_mode == _Search_mode::_Dfs)
            return _M_match_simple_repeat_body(__next, __i);
          else
            return __next;
        }
      else // Non-greedy mode
        {
          if constexpr (__search_mode == _Search_mode::_Dfs)
            {
              // vice-versa.
              _M_frames.emplace_back(_S_fopcode_fallback_rep_once_more, __i,
                                     _M_current);
              return __state._M_next;
            }
          else
            {
              // DON'T attempt anything, because there's already another
              // state with higher priority accepted. This state cannot
              // be better by attempting its next node.
              if (!_M_has_sol)
                {
                  // DON'T attempt anything if it's already accepted. An
                  // accepted state *must* be better than a solution that
                  // matches a non-greedy quantifier one more time.
                  _M_frames.emplace_back(_S_fopcode_fallback_rep_once_more, __i);
                  return __state._M_next;
                }
            }
        }
      return _S_invalid_state_id;
    }
 
  template<typename _BiIter, typename _Alloc, typename _TraitsT>
  template<_Search_mode __search_mode>
#ifdef __OPTIMIZE__
    [[__gnu__::__always_inline__]]
#endif
    inline _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
    _M_handle_subexpr_begin(_Match_mode, _StateIdT __i)
    {
      const auto& __state = _M_nfa[__i];
      auto& __res = _M_cur_results[__state._M_subexpr];
      if (_M_nfa._M_has_backref
          || __state._M_subexpr != 0
          || __search_mode != _Search_mode::_Dfs)
        _M_frames.emplace_back(_S_fopcode_restore_cur_results,
                               static_cast<_StateIdT>(__state._M_subexpr),
                               __res.first);
      __res.first = _M_current;
      return __state._M_next;
    }
 
  template<typename _BiIter, typename _Alloc, typename _TraitsT>
  template<_Search_mode __search_mode>
#ifdef __OPTIMIZE__
    [[__gnu__::__always_inline__]]
#endif
    inline _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
    _M_handle_subexpr_end(_Match_mode, _StateIdT __i)
    {
      const auto& __state = _M_nfa[__i];
      auto& __res = _M_cur_results[__state._M_subexpr];
      if (_M_nfa._M_has_backref
          || __state._M_subexpr != 0
          || __search_mode != _Search_mode::_Dfs)
        {
          _M_frames.emplace_back(_S_fopcode_restore_cur_results,
                                 static_cast<_StateIdT>(__state._M_subexpr),
                                 __res.second);
          _M_frames.back()._M_subexpr_end = true;
          _M_frames.back()._M_matched = __res.matched;
        }
 
      __res.second = _M_current;
      __res.matched = true;
      return __state._M_next;
    }
 
  template<typename _BiIter, typename _Alloc, typename _TraitsT>
    inline _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
    _M_handle_line_begin_assertion(_Match_mode, _StateIdT __i)
    {
      const auto& __state = _M_nfa[__i];
      if (_M_at_begin())
        return __state._M_next;
      return _S_invalid_state_id;
    }
 
  template<typename _BiIter, typename _Alloc, typename _TraitsT>
    inline _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
    _M_handle_line_end_assertion(_Match_mode, _StateIdT __i)
    {
      const auto& __state = _M_nfa[__i];
      if (_M_at_end())
        return __state._M_next;
      return _S_invalid_state_id;
    }
 
  template<typename _BiIter, typename _Alloc, typename _TraitsT>
    inline _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
    _M_handle_word_boundary(_Match_mode, _StateIdT __i)
    {
      const auto& __state = _M_nfa[__i];
      if (_M_word_boundary() == !__state._M_neg)
        return __state._M_next;
      return _S_invalid_state_id;
    }
 
  // Here __state._M_alt offers a single start node for a sub-NFA.
  // We recursively invoke our algorithm to match the sub-NFA.
  template<typename _BiIter, typename _Alloc, typename _TraitsT>
    _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
    _M_handle_subexpr_lookahead(_Match_mode, _StateIdT __i)
    {
      const auto& __state = _M_nfa[__i];
      if (_M_lookahead(__state._M_alt) == !__state._M_neg)
        return __state._M_next;
      return _S_invalid_state_id;
    }
 
  template<typename _BiIter, typename _Alloc, typename _TraitsT>
  template<_Search_mode __search_mode>
#ifdef __OPTIMIZE__
    [[__gnu__::__always_inline__]]
#endif
    inline _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
    _M_handle_match(_Match_mode, _StateIdT __i)
    {
      const auto& __state = _M_nfa[__i];
      if (_M_current == _M_end)
        return _S_invalid_state_id;
      if constexpr (__search_mode == _Search_mode::_Dfs)
        {
          if (__state._M_matches(*_M_current))
            {
              ++_M_current;
              return __state._M_next;
            }
        }
      else
        if (__state._M_matches(*_M_current))
          _M_match_queue.emplace_back(__state._M_next, _M_cur_results);
 
      return _S_invalid_state_id;
    }
 
  template<typename _BiIter, typename _TraitsT>
    struct _Backref_matcher
    {
      _Backref_matcher(bool /* __icase */, const _TraitsT& __traits)
      : _M_traits(__traits) { }
 
      bool
      _M_apply(_BiIter __expected_begin,
               _BiIter __expected_end, _BiIter __actual_begin,
               _BiIter __actual_end)
      {
        return _M_traits.transform(__expected_begin, __expected_end)
            == _M_traits.transform(__actual_begin, __actual_end);
      }
 
      const _TraitsT& _M_traits;
    };
 
  template<typename _BiIter, typename _CharT>
    struct _Backref_matcher<_BiIter, std::regex_traits<_CharT>>
    {
      using _TraitsT = std::regex_traits<_CharT>;
      _Backref_matcher(bool __icase, const _TraitsT& __traits)
      : _M_icase(__icase), _M_traits(__traits) { }
 
      bool
      _M_apply(_BiIter __expected_begin,
               _BiIter __expected_end, _BiIter __actual_begin,
               _BiIter __actual_end)
      {
        if (!_M_icase)
          return _GLIBCXX_STD_A::__equal4(__expected_begin, __expected_end,
                               __actual_begin, __actual_end);
        typedef std::ctype<_CharT> __ctype_type;
        const auto& __fctyp = use_facet<__ctype_type>(_M_traits.getloc());
        return _GLIBCXX_STD_A::__equal4(__expected_begin, __expected_end,
                             __actual_begin, __actual_end,
                             [this, &__fctyp](_CharT __lhs, _CharT __rhs)
                             {
                               return __fctyp.tolower(__lhs)
                                 == __fctyp.tolower(__rhs);
                             });
      }
 
      bool _M_icase;
      const _TraitsT& _M_traits;
    };
 
  // First fetch the matched result from _M_cur_results as __submatch;
  // then compare it with
  // (_M_current, _M_current + (__submatch.second - __submatch.first)).
  // If matched, keep going; else just return and try another state.
  template<typename _BiIter, typename _Alloc, typename _TraitsT>
    _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
    _M_handle_backref(_Match_mode, _StateIdT __i)
    {
      __glibcxx_assert(_M_search_mode == _Search_mode::_Dfs);
 
      const auto& __state = _M_nfa[__i];
      auto& __submatch = _M_cur_results[__state._M_backref_index];
      if (!__submatch.matched)
        return _S_invalid_state_id;
      auto __last = _M_current;
      for (auto __tmp = __submatch.first;
           __last != _M_end && __tmp != __submatch.second;
           ++__tmp)
        ++__last;
      if (_Backref_matcher<_BiIter, _TraitsT>(
              _M_re.flags() & regex_constants::icase,
              _M_re._M_automaton->_M_traits)._M_apply(
                  __submatch.first, __submatch.second, _M_current, __last))
        {
          _M_current = __last;
          return __state._M_next;
        }
 
      return _S_invalid_state_id;
    }
 
  template<typename _BiIter, typename _Alloc, typename _TraitsT>
  template<_Search_mode __search_mode>
#ifdef __OPTIMIZE__
    [[__gnu__::__always_inline__]]
#endif
    inline _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
    _M_handle_accept(_Match_mode __match_mode, _StateIdT)
    {
      if constexpr (__search_mode == _Search_mode::_Dfs)
        {
          __glibcxx_assert(!_M_has_sol);
          if (__match_mode == _Match_mode::_Exact)
            _M_has_sol = _M_current == _M_end;
          else
            _M_has_sol = true;
          if (_M_current == _M_begin
              && (_M_flags & regex_constants::match_not_null))
            _M_has_sol = false;
          if (_M_has_sol)
            {
              if (_M_nfa._M_flags & regex_constants::ECMAScript)
                _M_results = _M_cur_results;
              else // POSIX
                {
                  __glibcxx_assert(_M_get_sol_pos());
                  // Here's POSIX's logic: match the longest one. However
                  // we never know which one (lhs or rhs of "|") is longer
                  // unless we try both of them and compare the results.
                  // The member variable _M_sol_pos records the end
                  // position of the last successful match. It's better
                  // to be larger, because POSIX regex is always greedy.
                  // TODO: This could be slow.
                  if (*_M_get_sol_pos() == _BiIter()
                      || std::distance(_M_begin, *_M_get_sol_pos())
                         < std::distance(_M_begin, _M_current))
                    {
                      *_M_get_sol_pos() = _M_current;
                      _M_results = _M_cur_results;
                    }
                }
            }
        }
      else
        {
          if (_M_current == _M_begin
              && (_M_flags & regex_constants::match_not_null))
            return _S_invalid_state_id;
          if (__match_mode == _Match_mode::_Prefix || _M_current == _M_end)
            if (!_M_has_sol)
              {
                _M_has_sol = true;
                _M_results = _M_cur_results;
              }
        }
      return _S_invalid_state_id;
    }
 
  template<typename _BiIter, typename _Alloc, typename _TraitsT>
#ifdef __OPTIMIZE__
    [[__gnu__::__always_inline__]]
#endif
    inline _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
    _M_handle_alternative(_Match_mode, _StateIdT __i)
    {
      const auto& __state = _M_nfa[__i];
      if (_M_nfa._M_flags & regex_constants::ECMAScript)
        {
          // TODO: Fix BFS support. It is wrong.
          // Pick lhs if it matches. Only try rhs if it doesn't.
          _M_frames.emplace_back(_S_fopcode_fallback_next, __state._M_next,
                                 _M_current);
          return __state._M_alt;
        }
      else
        {
          // Try both and compare the result.
          // See "case _S_opcode_accept:" handling above.
          _M_frames.emplace_back(_S_fopcode_posix_alternative, __state._M_next,
                                 _M_current);
          return __state._M_alt;
        }
    }
 
  template<typename _BiIter, typename _Alloc, typename _TraitsT>
  template<_Search_mode __search_mode>
#ifdef __OPTIMIZE__
    [[__gnu__::__always_inline__]]
#endif
    inline _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
    _M_node(_Match_mode __match_mode, _StateIdT __i)
    {
      // DFS has no _M_visited implementation as such don't even have the branch
      // or the check in the call graph.
      if constexpr (__search_mode == _Search_mode::_Bfs)
        if (_M_visited(__i))
          return _S_invalid_state_id;
 
      _StateIdT __next = _S_invalid_state_id;
      switch (_M_nfa[__i]._M_opcode())
        {
        case _S_opcode_repeat:
          __next = _M_handle_repeat<__search_mode>(__match_mode, __i); break;
        case _S_opcode_subexpr_begin:
          __next = _M_handle_subexpr_begin<__search_mode>(__match_mode, __i);
          break;
        case _S_opcode_subexpr_end:
          __next = _M_handle_subexpr_end<__search_mode>(__match_mode, __i);
          break;
        case _S_opcode_line_begin_assertion:
          __next = _M_handle_line_begin_assertion(__match_mode, __i); break;
        case _S_opcode_line_end_assertion:
          __next = _M_handle_line_end_assertion(__match_mode, __i); break;
        case _S_opcode_word_boundary:
          __next = _M_handle_word_boundary(__match_mode, __i); break;
        case _S_opcode_subexpr_lookahead:
          __next = _M_handle_subexpr_lookahead(__match_mode, __i); break;
        case _S_opcode_match:
          __next = _M_handle_match<__search_mode>(__match_mode, __i); break;
        case _S_opcode_backref:
          if constexpr (__search_mode == _Search_mode::_Dfs)
            __next = _M_handle_backref(__match_mode, __i);
          else
            __builtin_unreachable();
          break;
        case _S_opcode_accept:
          __next = _M_handle_accept<__search_mode>(__match_mode, __i); break;
        case _S_opcode_alternative:
          __next = _M_handle_alternative(__match_mode, __i); break;
        default:
          __glibcxx_assert(false);
        }
      return __next;
    }
 
  template<typename _BiIter, typename _Alloc, typename _TraitsT>
  template<_Search_mode __search_mode>
    void _Executor<_BiIter, _Alloc, _TraitsT>::
    _M_dfs(_Match_mode __match_mode, _StateIdT __start)
    {
      _StateIdT __next = __start;
 
      while (true)
        {
          // Follow immediate successors without re-entering the frame
          // loop until we fail.  This avoids the needless state save and
          // restore through memory.
          while (__next != _S_invalid_state_id)
            __next = _M_node<__search_mode>(__match_mode, __next);
 
          if (_M_frames.empty())
            break;
 
          _ExecutorFrame<_BiIter> __frame = std::move(_M_frames.back());
          _M_frames.pop_back();
 
          switch (__frame._M_op)
            {
            case _S_fopcode_fallback_next:
              if (_M_has_sol)
                break;
              if constexpr (__search_mode == _Search_mode::_Dfs)
                _M_current = __frame._M_pos;
              [[__fallthrough__]];
            case _S_fopcode_next:
              __next = __frame._M_state_id;
              break;
 
            case _S_fopcode_fallback_rep_once_more:
              if (_M_has_sol)
                break;
              if constexpr (__search_mode == _Search_mode::_Dfs)
                _M_current = __frame._M_pos;
              [[__fallthrough__]];
            case _S_fopcode_rep_once_more:
              __next = _M_rep_once_more(__match_mode, __frame._M_state_id);
              break;
 
            case _S_fopcode_posix_alternative:
              _M_frames.emplace_back(_S_fopcode_merge_sol, 0, _M_has_sol);
              __next = __frame._M_state_id;
              if constexpr (__search_mode == _Search_mode::_Dfs)
                _M_current = __frame._M_pos;
              _M_has_sol = false;
              break;
 
            case _S_fopcode_merge_sol:
              _M_has_sol |= __frame._M_val;
              break;
 
            case _S_fopcode_restore_cur_results:
              if (!__frame._M_subexpr_end)
                _M_cur_results[__frame._M_state_id].first = __frame._M_pos;
              else
                {
                  _M_cur_results[__frame._M_state_id].second = __frame._M_pos;
                  _M_cur_results[__frame._M_state_id].matched = __frame._M_matched;
                }
              break;
 
            case _S_fopcode_restore_rep_count:
              _M_rep_count[__frame._M_state_id].first = __frame._M_pos;
              _M_rep_count[__frame._M_state_id].second = __frame._M_count;
              break;
 
            case _S_fopcode_decrement_rep_count:
              _M_rep_count[__frame._M_state_id].second--;
              break;
            }
        }
    }
 
  // Return whether now is at some word boundary.
  template<typename _BiIter, typename _Alloc, typename _TraitsT>
    bool _Executor<_BiIter, _Alloc, _TraitsT>::
    _M_word_boundary() const
    {
      if (_M_current == _M_begin && (_M_flags & regex_constants::match_not_bow))
        return false;
      if (_M_current == _M_end && (_M_flags & regex_constants::match_not_eow))
        return false;
 
      bool __left_is_word = false;
      if (_M_current != _M_begin
          || (_M_flags & regex_constants::match_prev_avail))
        {
          auto __prev = _M_current;
          if (_M_is_word(*std::prev(__prev)))
            __left_is_word = true;
        }
      bool __right_is_word =
        _M_current != _M_end && _M_is_word(*_M_current);
 
      return __left_is_word != __right_is_word;
    }
} // namespace __detail
#pragma GCC diagnostic pop
 
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace