JavaでJavaScriptのように正規表現で置換する ref: http://qiita.com/alucky0707/items/ab1babc0bcde9f3e4ef6

file0.js

var
str = 'Trick or Treat',

rep = str.replace(/[a-zA-Z]/g, function (c, i, str) {
  console.log("%s:%d:%s", c, i, str);
  return 'a' <= c && c <= 'z' && c.toUpperCase() ||
         'A' <= c && c <= 'Z' && c.toLowerCase();
});

console.log(rep);

file1.txt

T:0:Trick or Treat
r:1:Trick or Treat
i:2:Trick or Treat
c:3:Trick or Treat
k:4:Trick or Treat
o:6:Trick or Treat
r:7:Trick or Treat
T:9:Trick or Treat
r:10:Trick or Treat
e:11:Trick or Treat
a:12:Trick or Treat
t:13:Trick or Treat
tRICK OR tREAT

RegexExample.java

import java.util.List;
import java.util.regex.MatchResult;
import java.util.regex.Pattern;

public class RegexSample {

    static public void main(String... args) {
        String str = "Trick or Treat";
        Pattern pat = Pattern.compile("[a-zA-Z]");
        String rep = Replacer.replace(str, pat, new ReplaceCallback() {
            @Override
            public String replaceString(String matched, List<String> groups,
                    String str, Pattern pat, MatchResult matchResult) {
                System.out.printf("%s:%d:%s\n", matched, matchResult.start(), str);
                char c = matched.charAt(0);
                c = (char)('a' <= c && c <= 'z' ? c - 0x20 :
                           'A' <= c && c <= 'Z' ? c + 0x20 : c);
                return Character.toString(c);
            }
        });

        System.out.println(rep);
    }
}

RegexSample.java

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

public class RegexSample {

    static public void main(String... args) {
        String str = "Trick or Treat";
        Pattern pat = Pattern.compile("[a-zA-Z]");
        Matcher m = pat.matcher(str);

        StringBuilder sb = new StringBuilder();
        int start = 0;
        while(m.find()) {
            System.out.printf("%s:%d:%s\n", m.group(), m.start(), str);
            sb.append(str.substring(start, m.start()));
            start = m.end();
            char c = m.group().charAt(0);
            if ('a' <= c && c <= 'z') sb.append((char)(c - 0x20));
            else if ('A' <= c && c <= 'Z') sb.append((char)(c + 0x20));
        }
        sb.append(str.substring(start));

        System.out.println(sb.toString());
    }

}

ReplaceCallback.java

import java.util.List;
import java.util.regex.MatchResult;
import java.util.regex.Pattern;

public interface ReplaceCallback {
    String replaceString(String matched, List<String> groups, String str, Pattern pat, MatchResult matchResult);
}

Replacer.java

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

public class Replacer {

    static String replace(String str, Pattern pat, ReplaceCallback callback) {
        StringBuilder sb = new StringBuilder();

        Matcher m = pat.matcher(str);
        int start = 0;
        while (m.find()) {
            sb.append(str.substring(start, m.start()));
            String matched = str.substring(m.start(), m.end());
            start = m.end();
            ArrayList<String> groups = new ArrayList<>();
            for (int j = 0; j <= m.groupCount(); j++) {
                groups.add(m.group(j));
            }
            sb.append(callback.replaceString(matched, groups, str, pat, m.toMatchResult()));
        }
        sb.append(str.substring(start));

        return sb.toString();
    }

}