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String text = "<div>内容1</div><div>内容2</div>";
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// 贪婪：匹配整个字符串
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text.replaceAll("<div>.*</div>", "");
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// 结果：""（全部被替换掉了）
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// 非贪婪：只匹配第一个div
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text.replaceAll("<div>.*?</div>", "");
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// 结果：<div>内容2</div>

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String regex = "\\d{3}-\\d{3}-\\d{4}"; // 匹配 xxx-xxx-xxxx
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String text = "我的电话是123-456-7890，他的是987-654-3210。";
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Pattern pattern = Pattern.compile(regex);
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Matcher matcher = pattern.matcher(text);
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while (matcher.find()) {
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    System.out.println("找到: " + matcher.group()); // 匹配到的内容
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    System.out.println("位置: " + matcher.start() + "-" + matcher.end());
10
}

1
// matches()：完全匹配
2
"user@example.com".matches("[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}");
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// split()：分割
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"apple,banana,orange".split(","); // → ["apple", "banana", "orange"]
6

7
// replaceAll()：全部替换
8
"Hello 123 World 456".replaceAll("\\d+", "*"); // → "Hello * World *"
9

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// replaceFirst()：替换第一个
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"Hello 123 World 456".replaceFirst("\\d+", "*"); // → "Hello * World 456"

1
public static boolean isValidEmail(String email) {
2
    String regex = "[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}";
3
    return email.matches(regex);
4
}

1
public static boolean isValidPhone(String phone) {
2
    return phone.matches("\\d{3}-\\d{3}-\\d{4}");
3
}

1
public static List<String> extractNumbers(String text) {
2
    List<String> numbers = new ArrayList<>();
3
    Matcher matcher = Pattern.compile("\\d+").matcher(text);
4
    while (matcher.find()) {
5
        numbers.add(matcher.group());
6
    }
7
    return numbers;
8
}

1
public static String camelToSnake(String camelCase) {
2
    return camelCase.replaceAll("([a-z0-9])([A-Z])", "$1_$2").toLowerCase();
3
}
4
// camelToSnake("userName") → "user_name"

1
public static String snakeToCamel(String snakeCase) {
2
    return Pattern.compile("_([a-z])").matcher(snakeCase)
3
        .replaceAll(m -> m.group(1).toUpperCase());
4
}
5
// snakeToCamel("user_name") → "userName"

1
public static boolean isStrongPassword(String password) {
2
    // 至少8位，包含大小写字母、数字、特殊字符
3
    String regex = "^(?=.*[a-z])(?=.*[A-Z])(?=.*\\d)(?=.*[@$!%*?&])[A-Za-z\\d@$!%*?&]{8,}$";
4
    return password.matches(regex);
5
}

1
// 好：缓存Pattern
2
private static final Pattern EMAIL_PATTERN =
3
    Pattern.compile("[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}");
4

5
public static boolean isValidEmail(String email) {
6
    return EMAIL_PATTERN.matcher(email).matches();
7
}

1
// 捕获组（会保存匹配结果，额外开销）
2
Pattern.compile("(abc|def)");
3

4
// 非捕获组（不保存，性能更好）
5
Pattern.compile("(?:abc|def)");

1
// 危险！这个正则可能导致指数级回溯
2
String regex = "(a+)+b";
3
String text = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
4
// 匹配失败时，会尝试所有可能的分组组合，CPU直接爆炸
5

6
// 修复：用更具体的模式
7
String regex = "a+b";

1
// 错误
2
String regex = "\d+"; // Java编译错误：\d不是有效的转义字符
3

4
// 正确
5
String regex = "\\d+";