common/peg : suppress incomplete escape sequences (#26780)

tests/peg-parser/test-json-parser.cpp

@@ -77,6 +77,30 @@ void test_json_parser(testing &t) {
         t.assert_equal("result_is_need_more_input", true, result.need_more_input());
     });
 
+    // Test need_more_input() parsing - incomplete escape sequence in a string value
+    t.test("need_more_input() parsing - incomplete escape sequence", [](testing &t) {
+        auto json = build_peg_parser([](common_peg_parser_builder & p) { return p.json(); });
+
+        std::vector<std::string> inputs {
+            R"({"text": "hello\)",     // dangling backslash
+            R"({"text": "hello\u)",    // incomplete unicode escape sequence
+            R"({"text": "hello\u00)",
+        };
+
+        for (const auto & input : inputs) {
+            t.test(input, [&](testing &t) {
+                common_peg_parse_context ctx(input, COMMON_PEG_PARSE_FLAG_LENIENT);
+
+                auto result = json.parse(ctx);
+
+                t.assert_equal("result_is_need_more_input", true, result.need_more_input());
+
+                // the incomplete escape sequence is not part of the partial value
+                t.assert_equal("result_end", input.find('\\'), result.end);
+            });
+        }
+    });
+
     t.test("object member", [](testing &t) {
         auto parser = build_peg_parser([](common_peg_parser_builder & p) {
             return p.json_member("name", "\"" + p.chars("[a-z]") + "\"");