У меня два варианта и оба ему не по вкусу .
package com.javarush.task.task22.task2207;
import java.io.*;
import java.util.LinkedList;
import java.util.List;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
/*
Обращенные слова
*/
public class Solution {
public static List<Pair> result = new LinkedList<>();
public static void main(String[] args) throws IOException {
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
BufferedReader br = new BufferedReader(new FileReader(reader.readLine()));
reader.close();
StringBuilder s = new StringBuilder();
String st;
while ((st = br.readLine()) != null) {
s.append(st + " ");
}
Pattern p = Pattern.compile("(\\b[^\\w ^\\s]+\\b)");//
Matcher m = p.matcher(s.toString());
while (m.find()) {
StringBuilder sb = new StringBuilder(m.group());
String sg = sb.toString();
Pattern pattern = Pattern.compile(sb.reverse().toString());
s = new StringBuilder(s.toString().replaceFirst("\\b"+sg+"\\b", ""));//replaceAll("\\b"+sg+"\\b", "") 2 вариант
Matcher matcher = pattern.matcher(s.toString());
if (matcher.find()) {
result.add(new Pair(sg, sb.toString()));
} s = new StringBuilder(s.toString().replaceAll("\\b" + sb.toString() + "\\b", ""));
m.reset(s.toString());
}
System.out.println(result.toString());
}
public static class Pair {
String first;
String second;
public Pair() {
}
public Pair(String first, String second) {
this.first = first;
this.second = second;
}
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Pair pair = (Pair) o;
if (first != null ? !first.equals(pair.first) : pair.first != null) return false;
return second != null ? second.equals(pair.second) : pair.second == null;
}
@Override
public int hashCode() {
int result = first != null ? first.hashCode() : 0;
result = 31 * result + (second != null ? second.hashCode() : 0);
return result;
}
@Override
public String toString() {
return first == null && second == null ? "" :
first == null && second != null ? second :
second == null && first != null ? first :
first.compareTo(second) < 0 ? first + " " + second : second + " " + first;
}
}
}