code: public String filter(String input) {
  
          StringBuilder output = new StringBuilder();
          Matcher tokenMatcher = tokenPattern.matcher(input);
  
          // Track last regex match
          int endOfLastMatch = 0;
  
          // For each possible token
          while (tokenMatcher.find()) {
  
              // Pull possible leading text and first char before possible token
              String literal = tokenMatcher.group(LEADING_TEXT_GROUP);
              String escape = tokenMatcher.group(ESCAPE_CHAR_GROUP);
  
              // Append leading non-token text
              output.append(literal);
  
              // If char before token is '$', the token itself is escaped
              if ("$".equals(escape)) {
                  String notToken = tokenMatcher.group(TOKEN_GROUP);
                  output.append(notToken);
              }
  
              // If char is not '$', interpret as a token
              else {
  
                  // The char before the token, if any, is a literal
                  output.append(escape);
  
                  // Pull token value
                  String tokenName = tokenMatcher.group(TOKEN_NAME_GROUP);
                  String tokenValue = getToken(tokenName);
  
                  // If token is unknown, interpret as literal
                  if (tokenValue == null) {
                      String notToken = tokenMatcher.group(TOKEN_GROUP);
                      output.append(notToken);
                  }
  
                  // Otherwise, substitute value
                  else
                      output.append(tokenValue);
  
              }
  
              // Update last regex match
              endOfLastMatch = tokenMatcher.end();
              
          }
  
          // Append any remaining non-token text
          output.append(input.substring(endOfLastMatch));
          
          return output.toString();
         
      }
docs: Filters the given string, replacing any tokens with their corresponding
  values.
  
  @param input
  The string to filter.
  
  @return
  A copy of the input string, with any tokens replaced with their
  corresponding values.