diff --git a/backend/include/database.h b/backend/include/database.h
index e1ca2aa..08107c6 100644
--- a/backend/include/database.h
+++ b/backend/include/database.h
@@ -19,11 +19,16 @@ class UserDataBase {
 class BookDataBase {
   DriveArray<BookItemClass> full_book_data;
   String2Index indexer;
-  String2Index keyword2index;
+
+ public:
+  void Open(std::string file_name);
 };
 class LogDataBase {
   DriveArray<FinanceItemClass> finance_data;
   DriveArray<OperationLogItemClass> operation_log_data;
+
+ public:
+  void Open(std::string file_name);
 };
 
 #endif  // PROTECTOR_DATABASE_HPP
\ No newline at end of file
diff --git a/backend/include/engine.h b/backend/include/engine.h
index d9f4f2e..e96b284 100644
--- a/backend/include/engine.h
+++ b/backend/include/engine.h
@@ -9,11 +9,42 @@
 class BookStoreEngineClass {
   std::string config_dir;
   UserDataBase user_data_base;
+  BookDataBase book_data_base;
+  LogDataBase log_data_base;
   bool is_server;
+
  public:
   BookStoreEngineClass() = delete;
   BookStoreEngineClass(std::string __config_dir, bool __is_server);
   std::vector<std::string> Execute(const std::string &cmd,
-                                   std::stack<std::string> &login_stack);
+                                   std::stack<std::pair<std::string, std::string>> &login_stack);
+  std::vector<std::string> ExecuteSu(const std::string &cmd,
+                                     std::stack<std::pair<std::string, std::string>> &login_stack);
+  std::vector<std::string> ExecuteLogout(const std::string &cmd,
+                                         std::stack<std::pair<std::string, std::string>> &login_stack);
+  std::vector<std::string> ExecuteRegister(
+      const std::string &cmd, std::stack<std::pair<std::string, std::string>> &login_stack);
+  std::vector<std::string> ExecutePasswd(const std::string &cmd,
+                                         std::stack<std::pair<std::string, std::string>> &login_stack);
+  std::vector<std::string> ExecuteUserAdd(const std::string &cmd,
+                                          std::stack<std::pair<std::string, std::string>> &login_stack);
+  std::vector<std::string> ExecuteDelete(const std::string &cmd,
+                                         std::stack<std::pair<std::string, std::string>> &login_stack);
+  std::vector<std::string> ExecuteShow(const std::string &cmd,
+                                       std::stack<std::pair<std::string, std::string>> &login_stack);
+  std::vector<std::string> ExecuteBuy(const std::string &cmd,
+                                      std::stack<std::pair<std::string, std::string>> &login_stack);
+  std::vector<std::string> ExecuteSelect(const std::string &cmd,
+                                         std::stack<std::pair<std::string, std::string>> &login_stack);
+  std::vector<std::string> ExecuteMOdify(const std::string &cmd,
+                                         std::stack<std::pair<std::string, std::string>> &login_stack);
+  std::vector<std::string> ExecuteImport(const std::string &cmd,
+                                         std::stack<std::pair<std::string, std::string>> &login_stack);
+  std::vector<std::string> ExecuteShowFinance(
+      const std::string &cmd, std::stack<std::pair<std::string, std::string>> &login_stack);
+  std::vector<std::string> ExecuteLog(const std::string &cmd,
+                                      std::stack<std::pair<std::string, std::string>> &login_stack);
+  std::vector<std::string> ExecuteReport(const std::string &cmd,
+                                      std::stack<std::pair<std::string, std::string>> &login_stack);
 };
 #endif  // PROTECTOR_ENGINE_H
\ No newline at end of file
diff --git a/backend/include/lexer.h b/backend/include/lexer.h
index 81ec949..235884f 100644
--- a/backend/include/lexer.h
+++ b/backend/include/lexer.h
@@ -4,7 +4,27 @@
 #include <sstream>
 #include <string>
 #include <vector>
+bool CommandSuLexer(const std::string &command, std::string &user_id,
+                    std::string &password);
+bool CommandLogoutLexer(const std::string &command);
+bool CommandRegisterLexer(const std::string &command, std::string &user_id,
+                          std::string &password, std::string &username);
+bool CommandPasswdLexer(const std::string &command, std::string &user_id,
+                        std::string &old_password, std::string &new_password);
+bool CommandUseraddLexer(const std::string &command, std::string &user_id,
+                         std::string &password, int &privilege,
+                         std::string &username);
+bool CommandDeleteLexer(const std::string &command, std::string &user_id);
 bool CommandShowLexer(const std::string &command, std::string &ISBN,
                       std::string &name, std::string &author,
                       std::string &keyword);
+bool CommandBuyLexer(const std::string &command, std::string &ISBN,
+                     int &quantity);
+bool CommandSelectLexer(const std::string &command, std::string &ISBN);
+bool CommandModifyLexer(const std::string &command, std::string &ISBN,
+                        std::string &name, std::string &author,
+                        std::string &keyword, double &price);
+bool CommandImportLexer(const std::string &command, int &quantity,
+                        double &total_cost);
+bool CommandShowfinanceLexer(const std::string &command,int &count);
 #endif  // PROTECTOR_LEXER_H
\ No newline at end of file
diff --git a/backend/src/builtin-cli.cpp b/backend/src/builtin-cli.cpp
index a681f7c..e2e5281 100644
--- a/backend/src/builtin-cli.cpp
+++ b/backend/src/builtin-cli.cpp
@@ -5,6 +5,7 @@
 #include <stack>
 #include <string>
 #include <unordered_map>
+#include <utility>
 
 #include "bs-utility.h"
 #include "engine.h"
@@ -12,7 +13,7 @@ void BookStoreMain(bool is_server, std::string config_dir) {
   BookStoreEngineClass engine(config_dir, is_server);
   std::ios::sync_with_stdio(false);
   if (!is_server) {
-    std::stack<std::string> login_stack;
+    std::stack<std::pair<std::string, std::string>> login_stack;
     std::string cmd;
     while (std::getline(std::cin, cmd)) {
       auto result = std::move(engine.Execute(cmd, login_stack));
diff --git a/backend/src/database.cpp b/backend/src/database.cpp
index 1a8d0d8..fd2bfe3 100644
--- a/backend/src/database.cpp
+++ b/backend/src/database.cpp
@@ -3,4 +3,14 @@
 void UserDataBase::Open(std::string file_name) {
   full_user_data.OpenFile(file_name + ".full");
   user_name2index.OpenFile(file_name + ".n2i");
+}
+
+void BookDataBase::Open(std::string file_name) {
+  full_book_data.OpenFile(file_name + ".full");
+  indexer.OpenFile(file_name + ".n2i");
+}
+
+void LogDataBase::Open(std::string file_name) {
+  finance_data.OpenFile(file_name + ".finance");
+  operation_log_data.OpenFile(file_name + ".log");
 }
\ No newline at end of file
diff --git a/backend/src/engine.cpp b/backend/src/engine.cpp
index 8e14942..f5c41bf 100644
--- a/backend/src/engine.cpp
+++ b/backend/src/engine.cpp
@@ -1,5 +1,6 @@
 #include "engine.h"
 
+#include <cctype>
 #include <stack>
 #include <string>
 #include <unordered_map>
@@ -10,10 +11,15 @@ BookStoreEngineClass::BookStoreEngineClass(std::string __config_dir,
                                            bool __is_server) {
   config_dir = __config_dir;
   user_data_base.Open(config_dir + "user");
+  book_data_base.Open(config_dir + "book");
+  log_data_base.Open(config_dir + "log");
   is_server = __is_server;
 }
 std::vector<std::string> BookStoreEngineClass::Execute(
-    const std::string &cmd, std::stack<std::string> &login_stack) {
+    const std::string &cmd, std::stack<std::pair<std::string, std::string>> &login_stack) {
+  for (int i = 0; i < cmd.length(); i++)
+    if (std::isspace(cmd[i]) && cmd[i] != ' ')
+      return std::vector<std::string>({"Invalid"});
   static std::unordered_map<std::string, OperationType> operation_map = {
       {"su", OperationType::__Ksu},
       {"logout", OperationType::__Klogout},
@@ -42,38 +48,123 @@ std::vector<std::string> BookStoreEngineClass::Execute(
   }
   switch (operation_map[head]) {
     case OperationType::__Ksu: {
+      return ExecuteSu(cmd, login_stack);
     }
     case OperationType::__Klogout: {
+      return ExecuteLogout(cmd, login_stack);
     }
     case OperationType::__Kuseradd: {
+      return ExecuteUserAdd(cmd, login_stack);
     }
     case OperationType::__Kregister: {
+      return ExecuteRegister(cmd, login_stack);
     }
     case OperationType::__Kdelete: {
+      return ExecuteDelete(cmd, login_stack);
     }
     case OperationType::__Kpasswd: {
+      return ExecutePasswd(cmd, login_stack);
     }
     case OperationType::__Kselect: {
+      return ExecuteSelect(cmd, login_stack);
     }
     case OperationType::__Kmodify: {
+      return ExecuteMOdify(cmd, login_stack);
     }
     case OperationType::__Kimport: {
+      return ExecuteImport(cmd, login_stack);
     }
     case OperationType::__Kshow: {
       ss >> head;
       if (head == "finance") goto dst_showfinance;
+      return ExecuteShow(cmd, login_stack);
     }
     case OperationType::__Kshowfinance: {
     dst_showfinance:;
+      return ExecuteShowFinance(cmd, login_stack);
     }
     case OperationType::__Kbuy: {
+      return ExecuteBuy(cmd, login_stack);
     }
     case OperationType::__Kreport: {
-      throw FatalError("report Not implemented", 2);
+      // throw FatalError("report Not implemented", 2);
+      return ExecuteReport(cmd, login_stack);
     }
     case OperationType::__Klog: {
-      throw FatalError("log Not implemented", 3);
+      // throw FatalError("log Not implemented", 3);
+      return ExecuteLog(cmd, login_stack);
     }
   }
+  throw FatalError("Unknown Command", 5);
   return std::vector<std::string>({cmd});
+}
+
+std::vector<std::string> BookStoreEngineClass::ExecuteSu(
+    const std::string &cmd, std::stack<std::pair<std::string, std::string>> &login_stack) {
+  return std::vector<std::string>();
+}
+
+std::vector<std::string> BookStoreEngineClass::ExecuteLogout(
+    const std::string &cmd, std::stack<std::pair<std::string, std::string>> &login_stack) {
+  return std::vector<std::string>();
+}
+
+std::vector<std::string> BookStoreEngineClass::ExecuteRegister(
+    const std::string &cmd, std::stack<std::pair<std::string, std::string>> &login_stack) {
+  return std::vector<std::string>();
+}
+
+std::vector<std::string> BookStoreEngineClass::ExecutePasswd(
+    const std::string &cmd, std::stack<std::pair<std::string, std::string>> &login_stack) {
+  return std::vector<std::string>();
+}
+
+std::vector<std::string> BookStoreEngineClass::ExecuteUserAdd(
+    const std::string &cmd, std::stack<std::pair<std::string, std::string>> &login_stack) {
+  return std::vector<std::string>();
+}
+
+std::vector<std::string> BookStoreEngineClass::ExecuteDelete(
+    const std::string &cmd, std::stack<std::pair<std::string, std::string>> &login_stack) {
+  return std::vector<std::string>();
+}
+
+std::vector<std::string> BookStoreEngineClass::ExecuteShow(
+    const std::string &cmd, std::stack<std::pair<std::string, std::string>> &login_stack) {
+  return std::vector<std::string>();
+}
+
+std::vector<std::string> BookStoreEngineClass::ExecuteBuy(
+    const std::string &cmd, std::stack<std::pair<std::string, std::string>> &login_stack) {
+  return std::vector<std::string>();
+}
+
+std::vector<std::string> BookStoreEngineClass::ExecuteSelect(
+    const std::string &cmd, std::stack<std::pair<std::string, std::string>> &login_stack) {
+  return std::vector<std::string>();
+}
+
+std::vector<std::string> BookStoreEngineClass::ExecuteMOdify(
+    const std::string &cmd, std::stack<std::pair<std::string, std::string>> &login_stack) {
+  return std::vector<std::string>();
+}
+
+std::vector<std::string> BookStoreEngineClass::ExecuteImport(
+    const std::string &cmd, std::stack<std::pair<std::string, std::string>> &login_stack) {
+  return std::vector<std::string>();
+}
+
+std::vector<std::string> BookStoreEngineClass::ExecuteShowFinance(
+    const std::string &cmd, std::stack<std::pair<std::string, std::string>> &login_stack) {
+  return std::vector<std::string>();
+}
+
+std::vector<std::string> BookStoreEngineClass::ExecuteLog(
+    const std::string &cmd, std::stack<std::pair<std::string, std::string>> &login_stack) {
+  return std::vector<std::string>();
+}
+
+std::vector<std::string> BookStoreEngineClass::ExecuteReport(
+    const std::string &cmd, std::stack<std::pair<std::string, std::string>> &login_stack) {
+  return std::vector<std::string>();
 }
\ No newline at end of file
diff --git a/backend/src/lexer.cpp b/backend/src/lexer.cpp
index dee0b79..1af67f4 100644
--- a/backend/src/lexer.cpp
+++ b/backend/src/lexer.cpp
@@ -1,4 +1,210 @@
 #include "lexer.h"
+
+// clang-format off
+/**
+ * @brief Lexer for command "su"
+ * 
+ * @param command The command to be parsed
+ * @param user_id The user_id to be logged in
+ * @param password The password to be logged in
+ * 
+ * @return true if the command is valid
+ * 
+ * @note The command is valid if and only if it follows the following grammar:
+ * su [UserID] ([Password])?
+*/
+// clang-format on
+bool CommandSuLexer(const std::string &command, std::string &user_id,
+                    std::string &password) {
+  static std::basic_regex main_pattern(
+      R"(^ *su +(?:[0-9a-zA-Z_]{1,30}) +(?:[0-9a-zA-Z_]{1,30})? *$)",
+      std::regex_constants::optimize);
+  if (std::regex_match(command, main_pattern)) {
+    std::stringstream ss(command);
+    std::string token;
+    ss >> token;
+    user_id = "";
+    password = "";
+    ss >> user_id;
+    ss >> password;
+    return true;
+  } else
+    return false;
+}
+
+// clang-format off
+/**
+ * @brief Lexer for command "logout"
+ * 
+ * @param command The command to be parsed
+ * 
+ * @return true if the command is valid
+ * 
+ * @note The command is valid if and only if it follows the following grammar:
+ * logout
+*/
+// clang-format on
+bool CommandLogoutLexer(const std::string &command) {
+  static std::basic_regex main_pattern(R"(^ *logout *$)",
+                                       std::regex_constants::optimize);
+  return std::regex_match(command, main_pattern);
+}
+
+// clang-format off
+/**
+ * @brief Lexer for command "register"
+ * 
+ * @param command The command to be parsed
+ * @param user_id The user_id to be registered
+ * @param password The password to be registered
+ * 
+ * @return true if the command is valid
+ * 
+ * @note The command is valid if and only if it follows the following grammar:
+ * register [UserID] [Password] [Username]
+*/
+// clang-format on
+bool CommandRegisterLexer(const std::string &command, std::string &user_id,
+                          std::string &password, std::string &username) {
+  static std::basic_regex main_pattern(
+      R"(^ *register +(?:[0-9a-zA-Z_]{1,30}) +(?:[0-9a-zA-Z_]{1,30}) +(?:[^\s]{1,30}) *$)",
+      std::regex_constants::optimize);
+  if (std::regex_match(command, main_pattern)) {
+    std::stringstream ss(command);
+    std::string token;
+    ss >> token;
+    user_id = "";
+    password = "";
+    username = "";
+    ss >> user_id;
+    ss >> password;
+    ss >> username;
+    return true;
+  } else
+    return false;
+}
+
+// clang-format off
+/**
+ * @brief Lexer for command "passwd"
+ * 
+ * @param command The command to be parsed
+ * @param user_id The user_id to be changed password
+ * @param old_password The old password
+ * @param new_password The new password
+ * 
+ * @return true if the command is valid
+ * 
+ * @note The command is valid if and only if it follows the following grammar:
+ * passwd [UserID] ([CurrentPassword])? [NewPassword]
+*/
+// clang-format on
+bool CommandPasswdLexer(const std::string &command, std::string &user_id,
+                        std::string &old_password, std::string &new_password) {
+  static std::basic_regex main_pattern(
+      R"(^ *passwd +(?:[0-9a-zA-Z_]{1,30}) +(?:[0-9a-zA-Z_]{1,30})? +(?:[0-9a-zA-Z_]{1,30}) *$)",
+      std::regex_constants::optimize);
+  if (std::regex_match(command, main_pattern)) {
+    std::stringstream ss(command);
+    std::string token;
+    ss >> token;
+    user_id = "";
+    old_password = "";
+    new_password = "";
+    ss >> user_id;
+    ss >> old_password;
+    ss >> new_password;
+    if (new_password == "") {
+      new_password = old_password;
+      old_password = "";
+    }
+    return true;
+  } else
+    return false;
+}
+
+// clang-format off
+/**
+ * @brief Lexer for command "useradd"
+ * 
+ * @param command The command to be parsed
+ * @param user_id The user_id to be added
+ * @param password The password to be added
+ * @param privilege The privilege to be added
+ * 
+ * @return true if the command is valid
+ * 
+ * @note The command is valid if and only if it follows the following grammar:
+ * useradd [UserID] [Password] [Privilege] [Username]
+*/
+// clang-format on
+bool CommandUseraddLexer(const std::string &command, std::string &user_id,
+                         std::string &password, int &privilege,
+                         std::string &username) {
+  static std::basic_regex main_pattern(
+      R"(^ *useradd +(?:[0-9a-zA-Z_]{1,30}) +(?:[0-9a-zA-Z_]{1,30}) +[0-9] +(?:[^\s]{1,30}) *$)",
+      std::regex_constants::optimize);
+  if (std::regex_match(command, main_pattern)) {
+    std::stringstream ss(command);
+    std::string token;
+    ss >> token;
+    user_id = "";
+    password = "";
+    privilege = 0;
+    username = "";
+    ss >> user_id;
+    ss >> password;
+    ss >> privilege;
+    ss >> username;
+    return true;
+  } else
+    return false;
+}
+
+// clang-format off
+/**
+ * @brief Lexer for command "delete"
+ * 
+ * @param command The command to be parsed
+ * @param user_id The user_id to be deleted
+ * 
+ * @return true if the command is valid
+ * 
+ * @note The command is valid if and only if it follows the following grammar:
+ * delete [UserID]
+*/
+// clang-format on
+bool CommandDeleteLexer(const std::string &command, std::string &user_id) {
+  static std::basic_regex main_pattern(
+      R"(^ *delete +(?:[0-9a-zA-Z_]{1,30}) *$)",
+      std::regex_constants::optimize);
+  if (std::regex_match(command, main_pattern)) {
+    std::stringstream ss(command);
+    std::string token;
+    ss >> token;
+    user_id = "";
+    ss >> user_id;
+    return true;
+  } else
+    return false;
+}
+
+// clang-format off
+/**
+ * @brief Lexer for command "show"
+ * 
+ * @param command The command to be parsed
+ * @param ISBN The ISBN of the book to be shown
+ * @param name The name of the book to be shown
+ * @param author The author of the book to be shown
+ * @param keyword The keyword of the book to be shown
+ * 
+ * @return true if the command is valid
+ * 
+ * @note The command is valid if and only if it follows the following grammar:
+ * show (-ISBN=[ISBN] | -name="[BookName]" | -author="[Author]" | -keyword="[Keyword]")?
+*/
+// clang-format on
 bool CommandShowLexer(const std::string &command, std::string &ISBN,
                       std::string &name, std::string &author,
                       std::string &keyword) {
@@ -6,7 +212,6 @@ bool CommandShowLexer(const std::string &command, std::string &ISBN,
       R"(^ *show(?: +-ISBN=(?:\S{1,20})| +-name=\"(?:[^\s"]{1,60})\"| +-author=\"(?:[^\s"]{1,60})\"| +-keyword=\"(?:[^\s"]{1,60})\")* *$)",
       std::regex_constants::optimize);
 
-  std::smatch results;
   bool has_ISBN = false;
   bool has_name = false;
   bool has_author = false;
@@ -15,7 +220,7 @@ bool CommandShowLexer(const std::string &command, std::string &ISBN,
   name = "";
   author = "";
   keyword = "";
-  if (std::regex_match(command, results, main_pattern)) {
+  if (std::regex_match(command, main_pattern)) {
     std::stringstream ss(command);
     std::string token;
     ss >> token;
@@ -39,6 +244,180 @@ bool CommandShowLexer(const std::string &command, std::string &ISBN,
       } else
         return false;
     }
+    for (int i = 0; i < keyword.length(); i++)
+      if (keyword[i] == '|') return false;
+    return true;
+  } else
+    return false;
+}
+
+// clang-format off
+/**
+ * @brief Lexer for command "buy"
+ * 
+ * @param command The command to be parsed
+ * @param ISBN The ISBN of the book to be bought
+ * @param quantity The quantity of the book to be bought
+ * 
+ * @return true if the command is valid
+ * 
+ * @note The command is valid if and only if it follows the following grammar:
+ * buy [ISBN] [Quantity]
+*/
+// clang-format on
+bool CommandBuyLexer(const std::string &command, std::string &ISBN,
+                     int &quantity) {
+  static std::basic_regex main_pattern(
+      R"(^ *buy +(?:\S{1,20}) +[0-9]{1,10} *$)",
+      std::regex_constants::optimize);
+  if (std::regex_match(command, main_pattern)) {
+    std::stringstream ss(command);
+    std::string token;
+    ss >> token;
+    ISBN = "";
+    quantity = 0;
+    ss >> ISBN;
+    ss >> quantity;
+    return true;
+  } else
+    return false;
+}
+
+// clang-format off
+/**
+ * @brief Lexer for command "select"
+ * 
+ * @param command The command to be parsed
+ * @param ISBN The ISBN of the book to be selected
+ * 
+ * @return true if the command is valid
+ * 
+ * @note The command is valid if and only if it follows the following grammar:
+ * select [ISBN]
+*/
+// clang-format on
+bool CommandSelectLexer(const std::string &command, std::string &ISBN) {
+  static std::basic_regex main_pattern(R"(^ *select +(?:\S{1,20}) *$)",
+                                       std::regex_constants::optimize);
+  if (std::regex_match(command, main_pattern)) {
+    std::stringstream ss(command);
+    std::string token;
+    ss >> token;
+    ISBN = "";
+    ss >> ISBN;
+    return true;
+  } else
+    return false;
+}
+
+// clang-format off
+/**
+ * @brief Lexer for command "modify"
+ * 
+ * @param command The command to be parsed
+ * @param ISBN The ISBN of the book to be modified
+ * @param name The name of the book to be modified
+ * @param author The author of the book to be modified
+ * @param keyword The keyword of the book to be modified
+ * @param price The price of the book to be modified
+ * 
+ * @return true if the command is valid
+ * 
+ * @note The command is valid if and only if it follows the following grammar:
+ * modify (-ISBN=[ISBN] | -name="[BookName]" | -author="[Author]" | -keyword="[Keyword]" | -price=[Price])+
+*/
+// clang-format on
+bool CommandModifyLexer(const std::string &command, std::string &ISBN,
+                        std::string &name, std::string &author,
+                        std::string &keyword, double &price) {
+  static std::basic_regex main_pattern(
+      R"(^ *modify(?: +-ISBN=(?:\S{1,20})| +-name=\"(?:[^\s"]{1,60})\"| +-author=\"(?:[^\s"]{1,60})\"| +-keyword=\"(?:[^\s"]{1,60})\"| +-price=[0-9]{1,10}\.[0-9]{2})+ *$)",
+      std::regex_constants::optimize);
+  if (std::regex_match(command, main_pattern)) {
+    std::stringstream ss(command);
+    std::string token;
+    ss >> token;
+    ISBN = "";
+    name = "";
+    author = "";
+    keyword = "";
+    price = 0;
+    while (ss >> token) {
+      if (token[1] == 'I') {
+        ISBN = token.substr(6);
+      } else if (token[1] == 'n') {
+        name = token.substr(6 + 1, token.size() - 7 - 1);
+      } else if (token[1] == 'a') {
+        author = token.substr(8 + 1, token.size() - 9 - 1);
+      } else if (token[1] == 'k') {
+        keyword = token.substr(9 + 1, token.size() - 10 - 1);
+      } else if (token[1] == 'p') {
+        price = std::stod(token.substr(7));
+      } else
+        return false;
+    }
+    for (int i = 0; i < keyword.length(); i++)
+      if (keyword[i] == '|') return false;
+    return true;
+  } else
+    return false;
+}
+
+// clang-format off
+/**
+ * @brief Lexer for command "import"
+ * 
+ * @param command The command to be parsed
+ * @param quantity The quantity of the book to be imported
+ * @param total_cost The total cost of the book to be imported
+ * 
+ * @return true if the command is valid
+ * 
+ * @note The command is valid if and only if it follows the following grammar:
+ * import [Quantity] [TotalCost]
+*/
+// clang-format on
+bool CommandImportLexer(const std::string &command, int &quantity,
+                        double &total_cost) {
+  static std::basic_regex main_pattern(
+      R"(^ *import +[0-9]{1,10} +[0-9]{1,10}\.[0-9]{2} *$)",
+      std::regex_constants::optimize);
+  if (std::regex_match(command, main_pattern)) {
+    std::stringstream ss(command);
+    std::string token;
+    ss >> token;
+    quantity = 0;
+    total_cost = 0;
+    ss >> quantity;
+    ss >> total_cost;
+    return true;
+  } else
+    return false;
+}
+
+// clang-format off
+/**
+ * @brief Lexer for command "showfinance"
+ * 
+ * @param command The command to be parsed
+ * @param count The count of the finance to be shown
+ * 
+ * @return true if the command is valid
+ * 
+ * @note The command is valid if and only if it follows the following grammar:
+ * show finance ([Count])?
+*/
+// clang-format on
+bool CommandShowfinanceLexer(const std::string &command, int &count) {
+  static std::basic_regex main_pattern(
+      R"(^ *show +finance(?: +[0-9]{1,10})? *$)",
+      std::regex_constants::optimize);
+  if (std::regex_match(command, main_pattern)) {
+    std::stringstream ss(command);
+    std::string token;
+    ss >> token;
+    count = -1;
+    ss >> count;
     return true;
   } else
     return false;