String s = "${abc.xzy}/demo/${ttt.bbb}\ntest${kkk.mmm}";

Map<String, String> map = new HashMap<String, String>();

map.put("abc.xzy", "World");

map.put("ttt.bbb", "Hello");

map.put("kkk.mmm", "System");

StringBuffer result = new StringBuffer();

Matcher m = Pattern.compile("\\$\\{([^{}]+)\\}").matcher(s);

while (m.find()) {

    String value = map.get(m.group(1));

    m.appendReplacement(result, value != null ? value : m.group());

}

m.appendTail(result);

System.out.println(result.toString());

String input = "abcd";

Pattern p = Pattern.compile("regexp");

String output = p.matcher(input).replaceAll("replacement");