diff --git a/nemu/src/monitor/sdb/addrexp.l b/nemu/src/monitor/sdb/addrexp.l
index 14deb31..4e225a0 100644
--- a/nemu/src/monitor/sdb/addrexp.l
+++ b/nemu/src/monitor/sdb/addrexp.l
@@ -7,7 +7,7 @@
 
 %%
 
-$?[a-zA-Z]{2,3}   { yylval = isa_reg_str2val(yytext + 1, &success); if(!success) { return YYerror; } }
+$[a-zA-Z]{2,3}   { yylval = isa_reg_str2val(yytext + 1, &success); if(!success) { 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; }
diff --git a/nemu/src/monitor/sdb/addrexp.y b/nemu/src/monitor/sdb/addrexp.y
index f0ba851..42b9c62 100644
--- a/nemu/src/monitor/sdb/addrexp.y
+++ b/nemu/src/monitor/sdb/addrexp.y
@@ -6,6 +6,7 @@
 }
 %{
     #include <common.h>
+    #include <isa.h>
     #include <stdio.h>
     #include <stdlib.h>
     void yyerror(word_t *result, const char *err) {
@@ -47,6 +48,6 @@ number
     | HEX_NUMBER 
 
 register
-    : '$' REGISTER { $$ = $2; }
+    : REGISTER { $$ = $1; }
 
 %%