diff --git a/nemu/src/monitor/sdb/addrexp.l b/nemu/src/monitor/sdb/addrexp.l
index 53cf666..9d6b263 100644
--- a/nemu/src/monitor/sdb/addrexp.l
+++ b/nemu/src/monitor/sdb/addrexp.l
@@ -7,7 +7,13 @@
 
 %%
 
-\$[a-zA-Z]{2,3}   { yylval = isa_reg_str2val(yytext + 1, &success); if(!success) { return YYerror; } }
+$[a-zA-Z]{2,3}   {
+    yylval = isa_reg_str2val(yytext + 1, &success);
+    if(!success) {
+        printf("YYTEXT: %s", yytext);
+        return YYerror;
+    }
+}
 0[xX][0-9a-fA-F]+ { yylval = strtoul(yytext, NULL, 16); return HEX_NUMBER; }
 [0-9]+            { yylval = strtoul(yytext, NULL, 10); return NUMBER; }
 [+\-*/()]         { return *yytext; }