upd: ready to lex the command

backend/src/lexer.cpp

@@ -0,0 +1,45 @@
+#include "lexer.h"
+bool CommandShowLexer(const std::string &command, std::string &ISBN,
+                      std::string &name, std::string &author,
+                      std::string &keyword) {
+  static std::basic_regex main_pattern(
+      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;
+  bool has_keyword = false;
+  ISBN = "";
+  name = "";
+  author = "";
+  keyword = "";
+  if (std::regex_match(command, results, main_pattern)) {
+    std::stringstream ss(command);
+    std::string token;
+    ss >> token;
+    while (ss >> token) {
+      if (token[1] == 'I') {
+        if (has_ISBN) return false;
+        has_ISBN = true;
+        ISBN = token.substr(6);
+      } else if (token[1] == 'n') {
+        if (has_name) return false;
+        has_name = true;
+        name = token.substr(6 + 1, token.size() - 7 - 1);
+      } else if (token[1] == 'a') {
+        if (has_author) return false;
+        has_author = true;
+        author = token.substr(8 + 1, token.size() - 9 - 1);
+      } else if (token[1] == 'k') {
+        if (has_keyword) return false;
+        has_keyword = true;
+        keyword = token.substr(9 + 1, token.size() - 10 - 1);
+      } else
+        return false;
+    }
+    return true;
+  } else
+    return false;
+}