One of my friend asked for a clean solution to an interview question he was asked i.e., "reverse alphabetical sequences in a string" in Java. Clean would ofcourse be using StringBuilder but I'm not if using too many built-ins (we using RegExp already) is allowed during tech interviews.

StrRev.java

package com.vaibhavpandey;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

class StrRev {

    static Pattern AZ = Pattern.compile("[a-zA-Z]+");

    static String reverse(String string) {
        char[] characters = string.toCharArray();
        char[] reversed = new char[characters.length];
        for (int i = characters.length - 1, j = 0; i >= 0; i--, j++) {
            reversed[j] = characters[i];
        }

        return new String(reversed);
    }

    public static void main(String[] args) {
        String input = "1234abcd5678efgh";
        String output = "<mismatch>"; // expected 1234dcba5678hgfe

        Matcher matcher = AZ.matcher(input);
        if (matcher.find()) {
            output = matcher.replaceAll(s -> reverse(s.group()));
            // or simply: output = matcher.replaceAll(s -> new StringBuilder(s.group()).reverse().toString());
        }

        System.out.println(output);
    }
}