diff --git a/nemu/src/monitor/sdb/addrexp.l b/nemu/src/monitor/sdb/addrexp.l
index 274f3ce..255c4ce 100644
--- a/nemu/src/monitor/sdb/addrexp.l
+++ b/nemu/src/monitor/sdb/addrexp.l
@@ -10,7 +10,7 @@
 
 0[xX][0-9a-fA-F]+ { yylval = strtoul(yytext, NULL, 16); return HEX_NUMBER; }
 [0-9]+            { yylval = strtoul(yytext, NULL, 10); return NUMBER; }
-$[a-zA-Z]{2,3}    {
+${1,2}[a-zA-Z]{2,3}    {
     yylval = isa_reg_str2val(yytext + 1, &success);
     if(!success) {
         yyerror(NULL, "Failed to convert reg to value");