code: private String interpolate(MarcField marcField, String value) {
          if (value == null) {
              return null;
          }
          Matcher m = REP.matcher(value);
          if (m.find()) {
              return m.replaceAll(Integer.toString(repeatCounter));
          }
          m = NREP.matcher(value);
          if (m.find()) {
              if (repeatCounter > 99) {
                  repeatCounter = 99;
                  logger.log(Level.WARNING, "counter > 99, overflow in %s", marcField);
              }
              return m.replaceAll(String.format("%02d", repeatCounter));
          }
          return value;
      }
docs: Interpolate variables.
  
  @param marcField MARC field
  @param value the input value
  @return the interpolated string