java - 为什么 [\\s*] 不等同于\\s*？

Question

我刚刚接触 Java 中的正则表达式，我正在阅读一本书和 Java 文档 - 我不明白为什么 - 给定以下程序 - "[\\s*]" 用作分隔符时不等同于 "\\s*"。似乎 "[\\s*]" 等同于 "\\s+"，有人可以从逻辑上告诉我为什么会这样吗？

import java.util.Scanner;
import java.util.regex.Pattern;
public class ScanString {
    public static void main(String[] args) {
        String str = "Smith , where Jones had had 'had', had had 'had had'.";
        String regex = "had";
        System.out.println("String is:\n" + str + "\nToken sought is " + regex);

        Pattern had = Pattern.compile(regex);
        Scanner strScan = new Scanner(str);
        strScan.useDelimiter("\\s*");
        int hadCount = 0;
        while(strScan.hasNext()) {
            if(strScan.hasNext(had)) {
                ++hadCount;
                System.out.println("Token found!: " + strScan.next(had));

            } else {
                System.out.println("Token is    : " + strScan.next());
            }
        }
        System.out.println("Count is: " + hadCount);
    }
}

对我来说有意义的输出是每个非空白字符作为一个单独的标记。当分隔符更改为 "\\s+" 或 "[\\s*]" 时，输出为:

String is:
Smith , where Jones had had 'had', had had 'had had'.
Token sought is had
Token is    : Smith
Token is    : ,
Token is    : where
Token is    : Jones
Token found!: had
Token found!: had
Token is    : 'had',
Token found!: had
Token found!: had
Token is    : 'had
Token is    : had'.
Count is: 4

Answer 1

方括号 [] 包含一个字符类。在它们内部，关于特殊字符的规则是不同的。唯一的特殊字符是“右括号 (])、反斜杠 (\)、插入符 (^) 和连字符 ( -)。” (取自this page)

所以在这种情况下 [\\s*] 表示“空格或 *”。

处理正则表达式时，可以使用像RegexPlanet 这样的网站。 (测试您的代码)或 Regexper (以图形方式可视化正则表达式)。