diff --git a/.gitignore b/.gitignore
index c9a224c..74779ba 100644
--- a/.gitignore
+++ b/.gitignore
@@ -86,9 +86,10 @@ Module.symvers
 Mkfile.old
 dkms.conf
 
-# 自定义
+# 自定义过滤
 */output
 *.i
 *.s
 *.o
-*.swp
\ No newline at end of file
+*.swp
+.vscode
\ No newline at end of file
diff --git a/day4/d12.cpp b/day4/d12.cpp
new file mode 100644
index 0000000..22f59ac
--- /dev/null
+++ b/day4/d12.cpp
@@ -0,0 +1,24 @@
+#include <iostream>
+#include <regex> // 引入正则表达式库
+
+using namespace std;
+
+int main()
+{
+    string x;
+    cout << "输入一个字符:" << endl;
+    cin >> x;
+
+    regex regex_lower("[a-z]");
+    regex regex_upper("[A-Z]");
+    cout << regex_match(x, regex_lower) << regex_match(x, regex_upper) << endl;
+    // 正则表达式
+    if (regex_match(x, regex_lower)) // regex_match 匹配正则表达式
+    {
+        cout << (char)(x.at(0) - 32) << endl; // .at(0) 获取第一个字符
+    }
+    else if (regex_match(x, regex_upper))
+    {
+        cout << (char)(x.at(0) + 32) << endl;
+    }
+}
\ No newline at end of file
diff --git a/day4/d13.cpp b/day4/d13.cpp
new file mode 100644
index 0000000..561b3c8
--- /dev/null
+++ b/day4/d13.cpp
@@ -0,0 +1,24 @@
+#include <string>
+#include <iostream>
+
+using namespace std;
+
+int main()
+{
+    // 字符串打印遇到\0则结束打印
+    string str1 = "abc\0good"; // abc\0good\0
+    cout << str1 << str1 << endl;
+    cout << "sizeof str1 = " << sizeof(str1) << endl; // 32B 32*8=256位
+
+    char str2[] = "disen666"; // 字符数组, 实际上也是一个字符串
+    cout << "str2 = " << str2 << endl;
+
+    char str3[] = {'d', 'i', 's', 'e', 'n'}; // 自动添加 '\0'
+    cout << "str3 = " << str3 << endl;
+    cout << "sizeof str3 = " << sizeof(str3) << endl; // 5B, 没算 '\0'
+
+    string str4 = "a";
+    cout << "sizeof str4 = " << sizeof(str4) << endl; // 32B 32*8=256位
+
+    return 0;
+}
diff --git a/day4/exercise_2.4.5.2.cpp b/day4/exercise_2.4.5.2.cpp
new file mode 100644
index 0000000..241c3a9
--- /dev/null
+++ b/day4/exercise_2.4.5.2.cpp
@@ -0,0 +1,12 @@
+#include <iostream>
+
+using namespace std;
+
+int main()
+{
+    int a, b;
+    cout << "输入两个数:" << endl;
+    cin >> a >> b;
+    cout << "最大值为: " << (a > b ? a : b) << endl;
+    return 0;
+}
\ No newline at end of file