Fix backward regex search ending in DOTALL

I noticed that reverse searches ending in "." stopped working in
version 2024.05.08:

	kak -n -e "exec %{%cfoobar<ret><esc>gj<a-/>foo.<ret>}'

Bisects ca7471c25 (Compute StartDesc with an offset to effective start,
2024-03-18) which updated the find_next_start() logic for the forward
case but not for backward case.  Add a symmetrical change and test
case, that seems to fix it. Not 100% sure if this is correct but
feels so.

2 files changed, 12 insertions, 1 deletions

diff --git a/src/regex_impl.cc b/src/regex_impl.cc
index 4b404cfa..d86637cd 100644
--- a/src/regex_impl.cc
+++ b/src/regex_impl.cc
@@ -1594,6 +1594,17 @@ auto test_regex = UnitTest{[]{
     }
 
     {
+        TestVM<RegexMode::Backward | RegexMode::Search> vm{"oo(.{3,4}|f)"};
+        kak_assert(vm.backward_start_desc and vm.backward_start_desc->offset == 4);
+        for (int c = 0; c < CompiledRegex::StartDesc::count; ++c)
+            kak_assert(vm.backward_start_desc->map[c] == (c == 'f' or c == 'o'));
+
+        kak_assert(vm.exec("ooxxx", RegexExecFlags::None));
+        kak_assert(vm.exec("oofx", RegexExecFlags::None));
+        kak_assert(not vm.exec("oox😄", RegexExecFlags::None));
+    }
+
+    {
         auto eq = [](const CompiledRegex::NamedCapture& lhs,
                      const CompiledRegex::NamedCapture& rhs) {
             return lhs.name == rhs.name and
diff --git a/src/regex_impl.hh b/src/regex_impl.hh
index 7f3eb495..f62c31da 100644
--- a/src/regex_impl.hh
+++ b/src/regex_impl.hh
@@ -552,7 +552,7 @@ private:
             {
                 auto prev = utf8::previous(pos, config.end);
                 if (start_desc.map[static_cast<unsigned char>(*prev)])
-                    return pos;
+                    return utf8::advance(pos, start, CharCount(start_desc.offset));
                 pos = prev;
             }
         }