diff --git a/nemu/src/monitor/sdb/addrexp.l b/nemu/src/monitor/sdb/addrexp.l
index 2ba6302..24fd7bd 100644
--- a/nemu/src/monitor/sdb/addrexp.l
+++ b/nemu/src/monitor/sdb/addrexp.l
@@ -12,5 +12,5 @@ $$?[a-zA-Z]{2,3}  { yylval = isa_reg_str2val(yytext + 1, &success); if(!success)
 [0-9]+            { yylval = strtoul(yytext, NULL, 10); return NUMBER; }
 [+\-*/()]         { return *yytext; }
 [ \t]             {  }
-.                 { printf("Unexpected character: %s\n", yytext); }
+.                 { fprintf(stderr, "Unexpected character: %s\n", yytext); return YYerror; }
 %%
diff --git a/nemu/tests/expr_test.c b/nemu/tests/expr_test.c
index 427a59e..4399584 100644
--- a/nemu/tests/expr_test.c
+++ b/nemu/tests/expr_test.c
@@ -181,17 +181,17 @@ START_TEST(test_expr_plain_register) {
 }
 END_TEST
 
-// START_TEST(test_expr_register) {
-//   yy_scan_string(reg_exprs[_i].expr);
-//   uint32_t value;
-//   ck_assert(!yyparse(&value));
-//   yylex_destroy();
+START_TEST(test_expr_register) {
+  yy_scan_string(reg_exprs[_i].expr);
+  uint32_t value;
+  ck_assert(!yyparse(&value));
+  yylex_destroy();
 
-//   ck_assert_msg(value == reg_exprs[_i].reference,
-//                 "\n\texpr = %s\n\t(addr = %u) != (reference = %u)\n", reg_exprs[_i].expr,
-//                 value, reg_exprs[_i].reference);
-// }
-// END_TEST
+  ck_assert_msg(value == reg_exprs[_i].reference,
+                "\n\texpr = %s\n\t(addr = %u) != (reference = %u)\n", reg_exprs[_i].expr,
+                value, reg_exprs[_i].reference);
+}
+END_TEST
 
 Suite *expr_suite(void) {
   Suite *s;
@@ -203,8 +203,8 @@ Suite *expr_suite(void) {
   tcase_add_loop_test(tc_core, test_expr_random_100, 0, 20);
   tcase_add_loop_test(tc_core, test_expr_negative_operand, 0,
                       sizeof(exprs) / sizeof(exprs[0]));
-  // tcase_add_loop_test(tc_core, test_expr_register, 0,
-  //                     sizeof(reg_exprs) / sizeof(reg_exprs[0]));
+  tcase_add_loop_test(tc_core, test_expr_register, 0,
+                      sizeof(reg_exprs) / sizeof(reg_exprs[0]));
   tcase_add_test(tc_core, test_expr_plain_register);
   suite_add_tcase(s, tc_core);