diff --git a/lab9/Makefile b/lab9/Makefile
new file mode 100644
index 0000000..8201bd0
--- /dev/null
+++ b/lab9/Makefile
@@ -0,0 +1,11 @@
+expr_bison:
+	flex lex.l &&  bison -t -d syntax.y
+	gcc syntax.tab.c -lfl -ly -shared -fPIC -o libexpr.so
+	@echo "\e[92mTesting BISON implementation...\e[0m"
+	@python3 expr_test.py ||:
+expr_stack:
+	gcc stack.c -shared -fPIC -o libexpr.so
+	@echo "\e[92mTesting STACK implementation...\e[0m"
+	@python3 expr_test.py ||:
+clean:
+	@rm -f lex.yy.c syntax.tab.* *.out *.so
diff --git a/lab9/expr_test.py b/lab9/expr_test.py
new file mode 100644
index 0000000..1000b2e
--- /dev/null
+++ b/lab9/expr_test.py
@@ -0,0 +1,25 @@
+#!/usr/bin/env python3
+# encoding: utf-8
+
+import ctypes
+import os
+
+
+cwd = os.getcwd()
+lib_path = os.path.join(cwd, 'libexpr.so')
+lib = ctypes.cdll.LoadLibrary(lib_path)
+
+def convert(expr):
+    func = lib.convert
+    func.restype = ctypes.c_char_p
+    expr_b = expr.encode()
+    expr_buf = ctypes.c_char_p(expr_b)
+    return func(expr_buf).decode()
+
+
+assert convert('5')=='5', convert('5')
+
+assert convert('9-5+2')=='95-2+', convert('9-5+2')
+
+print("Testing completed.")
+
diff --git a/lab9/lex.l b/lab9/lex.l
new file mode 100644
index 0000000..9c92044
--- /dev/null
+++ b/lab9/lex.l
@@ -0,0 +1,12 @@
+%{
+    #include"syntax.tab.h"
+%}
+%%
+[0-9] {
+    yylval = (char*)malloc(2);
+    memcpy(yylval, yytext, 2);
+    return TERM;
+}
+"+" { yylval = "+"; return ADD; }
+"-" { yylval = "-"; return SUB; }
+. { /* do nothing */ }
diff --git a/lab9/stack.c b/lab9/stack.c
new file mode 100644
index 0000000..759b535
--- /dev/null
+++ b/lab9/stack.c
@@ -0,0 +1,39 @@
+#include<stdlib.h>
+#include<stdio.h>
+#include<string.h>
+#define STACK_LEN 32
+
+char *convert(char *expr){
+    char opstack[STACK_LEN];
+    char stack[STACK_LEN];
+    int optop, top;
+    char *c, *ret;
+
+    memset(opstack, '\0', STACK_LEN);
+    memset(stack, '\0', STACK_LEN);
+    optop = top = 0;
+    c = expr;
+
+    while(*c != '\0'){
+        if((*c >= '0') && (*c <= '9')){
+
+        }
+        else if((*c == '+') || (*c == '-')){
+
+        }
+        else{
+            // lexical errors
+        }
+        c++;
+    }
+
+    ret = (char*)malloc(strlen(stack)+1);
+    sprintf(ret, "%s", stack);
+    return ret;
+}
+
+int main(){
+    puts( convert("3-7+4-5") );
+    return 0;
+}
+
diff --git a/lab9/syntax.y b/lab9/syntax.y
new file mode 100644
index 0000000..730d0df
--- /dev/null
+++ b/lab9/syntax.y
@@ -0,0 +1,32 @@
+%{
+    #include"lex.yy.c"
+    void yyerror(char*s){}
+
+    /*
+     * We make assumptions on the input:
+     * there are no syntax errors, and
+     * only consist two operators, plus
+     * and minus, and all numbers contain
+     * exactly one digit, no spaces.
+     */
+
+    // where reverse Polish notation goes
+    char *result;
+%}
+%define api.value.type { char* }
+%token TERM ADD SUB
+
+%%
+Eval: Expr { result = $1; }
+Expr: TERM              /* write the   */
+    | Expr ADD TERM     /* SDT actions */
+    | Expr SUB TERM     /* at HERE     */
+%%
+
+char *convert(char *expr){
+    YY_BUFFER_STATE buf;
+    buf = yy_scan_string(expr);
+    yyparse();
+    yy_delete_buffer(buf);
+    return result;
+}