Test replace single symbol using regex approach

tests/nft/strings.cc

@@ -808,7 +808,7 @@ TEST_CASE("mata::nft::strings::replace_reluctant_regex()") {
     Nft expected{};
     EnumAlphabet alphabet{ 'a', 'b', 'c' };
 
-    SECTION("'a' replace with 'b' replace all") {
+    SECTION("'a+b+c' replace with 'dd' replace all") {
         // Use replace symbol with symbol.
         nft = nft::strings::replace_reluctant_regex("a+b+c", Word{ 'd', 'd' }, &alphabet, ReplaceMode::All);
         nft.print_to_DOT(std::cout);
@@ -824,6 +824,19 @@ TEST_CASE("mata::nft::strings::replace_reluctant_regex()") {
                                      { 'd', 'd', 'a', 'a', 'a', 'b', 'b', 'd', 'd', 'c', 'a', 'b', 'a', 'c' } }));
 //        expected = nft::builder::parse_from_mata(std::string(
 //        ));
+//        CHECK(nft::are_equivalent(nft, expected));
+    }
+
+    SECTION("'a' replace with 'd' replace all") {
+        // Use replace symbol with symbol.
+        nft = nft::strings::replace_reluctant_regex("a", Word{ 'd' }, &alphabet, ReplaceMode::All);
+        nft.print_to_DOT(std::cout);
+        CHECK(nft.is_tuple_in_lang({ { 'a', 'a', 'a', 'b', 'a', 'a', 'a', 'b', 'a', 'c', 'a' },
+                                     { 'd', 'd', 'd', 'b', 'd', 'd', 'd', 'b', 'd', 'c', 'd' } }));
+        CHECK(nft.is_tuple_in_lang({ {},
+                                     {} }));
+//        expected = nft::builder::parse_from_mata(std::string(
+//        ));
 //        CHECK(nft::are_equivalent(nft, expected));
     }
     // TODO(nft): Test dropping regex, correctly replacing shortest/longest match, ...