不用正则版本

import java.util.Scanner;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String string = in.nextLine();
        char[] charArray = string.toCharArray();
        StringBuilder stringBuilder = new StringBuilder();
        for (int i = 0; i < charArray.length; i++) {
            if (Character.isLetter(charArray[i])) {
                stringBuilder.append(charArray[i]);
            } else {
                // 非字母的全部换成空格
                stringBuilder.append(" ");
            }
        }
        // 以空格切分
        String[] split = stringBuilder.toString().split(" ");
        StringBuilder builder = new StringBuilder();
        for (int i = split.length - 1; i >= 0; i--) {
            if (split[i].equals("")) {
                continue;
            }
            builder.append(split[i] + " ");
        }
        // 注意减去一个空格
        System.out.println(builder.deleteCharAt(builder.length() - 1));
    }
}