diff --git a/nemu/src/monitor/sdb/addrexp.l b/nemu/src/monitor/sdb/addrexp.l
index bd490c3..3d5c359 100644
--- a/nemu/src/monitor/sdb/addrexp.l
+++ b/nemu/src/monitor/sdb/addrexp.l
@@ -7,7 +7,7 @@
 
 %%
 
-${0,1}[a-zA-Z0-9]{2,3}  { yylval = isa_reg_str2val(yytext + 1, &success); if(!success) { return YYerror; } }
+$?[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; }