diff --git a/nemu/src/monitor/sdb/addrexp.l b/nemu/src/monitor/sdb/addrexp.l
index 3d5c359..095014f 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 730c497..f0ba851 100644
--- a/nemu/src/monitor/sdb/addrexp.y
+++ b/nemu/src/monitor/sdb/addrexp.y
@@ -47,6 +47,6 @@ number
     | HEX_NUMBER 
 
 register
-    : REGISTER
+    : '$' REGISTER { $$ = $2; }
 
 %%