Fix incorrect use of subject end/begin in regex execution

This could lead to reading past subject string end in certain
conditions

Fixes #4794

src/regex.hh

@@ -220,6 +220,7 @@ private:
         m_results.values().clear();
         std::move(m_vm.captures().begin(), m_vm.captures().end(), std::back_inserter(m_results.values()));
         m_next_pos = forward ? m_results[0].second : m_results[0].first;
+        kak_assert(forward ? (m_next_pos <= m_end) : (m_next_pos >= m_begin));
         return true;
     }
 

src/regex_impl.cc

@@ -1204,6 +1204,8 @@ struct TestVM : CompiledRegex, ThreadedRegexVM<const char*, mode>
     {
         return TestVM::ThreadedRegexVM::exec(re.begin(), re.end(), re.begin(), re.end(), flags);
     }
+
+    using TestVM::ThreadedRegexVM::exec;
 };
 }
 
@@ -1564,6 +1566,12 @@ auto test_regex = UnitTest{[]{
         kak_assert(vm.exec("д", RegexExecFlags::None));
     }
 
+    {
+        TestVM<RegexMode::Forward | RegexMode::Search> vm{"ab"};
+        const char str[] = "fa😄ab";
+        kak_assert(not vm.exec(str, str+4, str, str + sizeof(str)-1, RegexExecFlags::None));
+    }
+
     {
         TestVM<> vm{R"(\0\x0A\u00260e\u00260F)"};
         const char str[] = "\0\n☎☏"; // work around the null byte in the literal

src/regex_impl.hh

@@ -492,8 +492,8 @@ private:
                 return m_found_match;
             }
 
-            forward ? utf8::to_next(pos, config.subject_end)
+            forward ? utf8::to_next(pos, config.end)
-                    : utf8::to_previous(pos, config.subject_begin);
+                    : utf8::to_previous(pos, config.end);
 
             if (search and not m_found_match)
             {