code: @VisibleForTesting
      protected int stringToInt(final String exp) {
          final Integer value = fieldConstraints.getStringMappingValue(exp);
          if (value != null) {
              return value;
          } else {
              try {
                  return Integer.parseInt(exp);
              } catch (final NumberFormatException e) {
                  final String invalidChars = new StringValidations(fieldConstraints).removeValidChars(exp);
                  throw new IllegalArgumentException(String.format("Invalid chars in expression! Expression: %s Invalid chars: %s", exp, invalidChars));
              }
          }
      }
docs: Maps string expression to integer. If no mapping is found, will try to parse String as Integer
  
  @param exp - expression to be mapped
  @return integer value for string expression