Java regex search & replace using backreference

RegexBackref.java

Strig input = "_ab__f_";

//Pattern p = Pattern.compile("_+(.)", Pattern.DOTALL);

// pattern with named backreference i.e. (?<firstElement>)
Pattern p = Pattern.compile("_+(?<firstElement>.)", Pattern.DOTALL);

Matcher m = p.matcher(input);

StringBuffer buff = new StringBuffer();
while(m.find()) {
  // m.appendReplacement(buff, m.group(1).toUpperCase());
  m.appendReplacement(buff, m.group("firstElement").toUpperCase());
}

System.out.printf("Result: %s --> %s", input, buff); 
// Result: _ab__f_ --> AbF