Detailed introduction to single character matching and predefined characters of Java regular expressions

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一、需求

现有一个字符串，需满足如下要求：

• [6, 18] 个字符

• 只能包含字母、数字、下划线

• 需以字母开头

如果字符串满足上述要求，返回 true，否则返回 false

   public static boolean validString(String s) {
       return s.matches("[a-zA-Z][a-zA-Z0-9_]{5,17}");
   }

正则表达式用极简的规则取代了复杂的验证逻辑

Regex Expression

正则表达式是一种通用的技术，适用于多种编程语言

二、单字符匹配（6个）

<span style="font-family:Microsoft Yahei, Hiragino Sans GB, Helvetica, Helvetica Neue, 微软雅黑, Tahoma, Arial, sans-serif">1. </span>[abc]：字符串的某个位置（某一个字符）满足 a、b、c 中的一个

某个位置：该【单字符匹配】放的位置

public class TestDemo {
    public static void main(String[] args) {
        String regex = "[zgq]";
        System.out.println("z".matches(regex)); // true
        System.out.println("g".matches(regex)); // true
        System.out.println("q".matches(regex)); // true
        System.out.println("zgq".matches(regex)); // false
    }}

public class TestDemo {
    public static void main(String[] args) {
        String regex = "26[abc]3q";
        System.out.println("26a3q".matches(regex)); // true
        System.out.println("26abc".matches(regex)); // false
        System.out.println("26b3q".matches(regex)); // true 
    }}

2. [^abc]：除了 a、b、c 之外的任意单个字符

public class TestDemo {
    public static void main(String[] args) {
        String regex = "[^cat]666";
        System.out.println("c666".matches(regex)); // false
        System.out.println("a666".matches(regex)); // false
        System.out.println("t666".matches(regex)); // false
        System.out.println("bb666".matches(regex)); // false
        System.out.println("b666".matches(regex)); // true
    }}

public class TestDemo {
    public static void main(String[] args) {
        String regex1 = "[12345]666";
        String regex2 = "[^1-5]666";
        System.out.println("1666".matches(regex1)); // true
        System.out.println("3666".matches(regex1)); // true
        System.out.println("5666".matches(regex1)); // true
        System.out.println("6666".matches(regex1)); // false

        System.out.println("1666".matches(regex2)); // false
        System.out.println("3666".matches(regex2)); // false
        System.out.println("5666".matches(regex2)); // false

        System.out.println("6666".matches(regex2)); // true
    }}

3. [a-zA-z]：匹配单个英文字母

public class TestDemo {
    public static void main(String[] args) {
        String regex = "[a-zA-Z]666";
        System.out.println("6666".matches(regex)); // false
        System.out.println("b666".matches(regex)); // true
    }}

4. [a-d[1-6]]：和 [a-d1-6] 一样（并集）

public class TestDemo {
    public static void main(String[] args) {
        String regex1 = "[a-d[1-6]]";
        String regex2 = "[a-d1-6]";
        System.out.println("a".matches(regex1)); // true
        System.out.println("e".matches(regex1)); // false
        System.out.println("1".matches(regex1)); // true
        System.out.println("7".matches(regex1)); // false

        System.out.println("a".matches(regex2)); // true
        System.out.println("e".matches(regex2)); // false
        System.out.println("1".matches(regex2)); // true
        System.out.println("7".matches(regex2)); // false
    }}

5. [zgq&&[god]]：交集

public class TestDemo {
    public static void main(String[] args) {
        String regex1 = "[zgq&&[god]]";
        System.out.println("q".matches(regex1)); // false
        System.out.println("d".matches(regex1)); // false
        System.out.println("g".matches(regex1)); // true
    }}

6. [zgq&&[god]]：取差集

public class TestDemo {
    public static void main(String[] args) {
        String regex1 = "[zgq&&[^god]]";
        System.out.println("q".matches(regex1)); // true
        System.out.println("d".matches(regex1)); // false
        System.out.println("g".matches(regex1)); // false
        System.out.println("z".matches(regex1)); // true

        // 取差集, 从字母 a 到字母 z 中去除字母 b 和 d
        String regex2 = "[a-z&&[^bd]]";
        System.out.println("d".matches(regex2)); // false
        System.out.println("a".matches(regex2)); // true
    }}

三、预定义字符（7个）

预定义字符匹配的仍然是单个字符

【.】：任意单个字符
【\d】：数字
【\D】：非数字
【\s】：空白
【\S】：非空白
【\w】：字母（英文字母、下划线、数字）
【\W】：非英文字母

Java 中需以两个【\】开头表示预定义字符

public class TestDemo {
    public static void main(String[] args) {
        String r1 = ".";
        System.out.println("@".matches(r1)); // true
        System.out.println("庆".matches(r1)); // true
        System.out.println("I".matches(r1)); // true
        System.out.println(" ".matches(r1)); // true
        System.out.println(".".matches(r1)); // true
    }}

public class TestDemo {
    public static void main(String[] args) {
        // 匹配 java 文件
        String r1 = ".\\.java";
        System.out.println("a.java".matches(r1)); // true
        System.out.println("xjava".matches(r1)); // false
        System.out.println("5java".matches(r1)); // false
    }}

public class TestDemo {
    public static void main(String[] args) {
        String r1 = "[abc]";
        String r2 = "\\[abc\\]";
        System.out.println("a".matches(r1)); // true
        System.out.println("c".matches(r1)); // true
        System.out.println("[abc]".matches(r1)); // false

        System.out.println("a".matches(r2)); // false
        System.out.println("c".matches(r2)); // false
        System.out.println("[abc]".matches(r2)); // true
    }}

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