Simple template replacer

Often I find I need a simple template engine to replace named regions.

public abstract class Replace {

   public static void main(String[] args) {
      Replace r = new Replace() {
         public String lookup(String key) {
            return "*newvalue*";
         }
      };
      String s =
    "Welcome ${name}, you have just won a new ${prize}.";
      System.out.println(r.replace(s));
   }

   public String replace(String s) {
      return replace(s, 0, new StringBuilder()).toString();
   }

   private StringBuilder replace(String s, int offset, StringBuilder b) {
      int startKey = s.indexOf("${", offset);
      if (startKey == -1)
         return b.append(s.substring(offset));
      else {
         b.append(s.substring(offset, startKey));
         int keyEnd = s.indexOf('}', startKey);
         String key = s.substring(startKey + 2, keyEnd);
         b.append(lookup(key));
         return replace(s, keyEnd + 1, b);
      }
   }

   abstract public String lookup(String key);
}