Regex replace the substitution string

\\U is not implemented in the java regex AFAIK and you can't do it with a regex as such (.NET has it IIRC). It's a bit verbose, but one way to do it would be:

    String test = "abc";
    Pattern p = Pattern.compile("(a)");
    Matcher m = p.matcher(test);

    StringBuilder sb = new StringBuilder();
    if (m.find()) {
        String match = test.substring(m.start(1), m.end(1));
        m.appendReplacement(sb, match.toUpperCase());
    }

    m.appendTail(sb);
    System.out.println(sb.toString());