code: public String validate(String pAttributeName,
                             String pAttributeValue) {
          try {
              sEvaluator.setBypassCache(true);
              sEvaluator.parseExpressionString(pAttributeValue);
              sEvaluator.setBypassCache(false);
              return null;
          }
          catch (ELException exc) {
              return
                      MessageFormat.format
                              (Constants.ATTRIBUTE_PARSE_EXCEPTION,
                                              "" + pAttributeName,
                                              "" + pAttributeValue,
                                      exc.getMessage());
          }
      }
docs: Translation time validation of an attribute value.  This method
  will return a null String if the attribute value is valid;
  otherwise an error message.