diff --git a/nemu/src/monitor/sdb/addrexp.l b/nemu/src/monitor/sdb/addrexp.l
index 6882a0b..979df5a 100644
--- a/nemu/src/monitor/sdb/addrexp.l
+++ b/nemu/src/monitor/sdb/addrexp.l
@@ -8,6 +8,7 @@
 
 %%
 
+0[xX][0-9a-fA-F]+ { yylval = strtoul(yytext, NULL, 16); return HEX_NUMBER; }
 $[a-zA-Z]{2,3}   {
     yylval = isa_reg_str2val(yytext + 1, &success);
     if(!success) {
@@ -15,7 +16,6 @@ $[a-zA-Z]{2,3}   {
         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; }
 [ \t]             {  }