diff --git a/nemu/src/monitor/sdb/addrexp.l b/nemu/src/monitor/sdb/addrexp.l
index da00e36..8c01bee 100644
--- a/nemu/src/monitor/sdb/addrexp.l
+++ b/nemu/src/monitor/sdb/addrexp.l
@@ -1,11 +1,13 @@
 %{
+    #include <isa.h>
     #include <addrexp.h>
+    static bool success = false;
 %}
 %option noyywrap
 
 %%
 
-$[a-zA-Z]         { }
+$?[a-zA-Z]{2,3}   { yylval = isa_reg_str2val(yytext, &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 1d72129..730c497 100644
--- a/nemu/src/monitor/sdb/addrexp.y
+++ b/nemu/src/monitor/sdb/addrexp.y
@@ -28,6 +28,7 @@ input
 
 expression
     : number { $$ = $1; }
+    | register { $$ = $1; }
     | expression '+' expression { $$ = $1 + $3; }
     | expression '-' expression { $$ = $1 - $3; }
     | expression '*' expression { $$ = $1 * $3; } 
@@ -45,4 +46,7 @@ number
     : NUMBER 
     | HEX_NUMBER 
 
+register
+    : REGISTER
+
 %%
diff --git a/nemu/src/monitor/sdb/sdb.h b/nemu/src/monitor/sdb/sdb.h
index c6fa9a2..b1c87de 100644
--- a/nemu/src/monitor/sdb/sdb.h
+++ b/nemu/src/monitor/sdb/sdb.h
@@ -18,6 +18,7 @@
 
 #include <common.h>
 
+// TODO: Cache expression evalutation result in ExprResult
 enum ExprType {
   EXPR_TYPE_MEM_ADDR,
   EXPR_TYPE_REG
diff --git a/nemu/tests/expr_test.c b/nemu/tests/expr_test.c
index 87c1882..1bef171 100644
--- a/nemu/tests/expr_test.c
+++ b/nemu/tests/expr_test.c
@@ -1,5 +1,5 @@
 #include "sys/types.h"
-#include "unistd.h"
+#include <unistd.h>
 #include <assert.h>
 #include <check.h>
 #include <math.h>