@Override
            public void filtersWillBeAdded(CriteriaBuilder criteriaBuilder, CriteriaQuery<?> query, List<Predicate> predicates) {
                    JpaCriteriaBuilder cb = (JpaCriteriaBuilder) criteriaBuilder;
                    Root<?> r = query.getRoots().iterator().next();
                    Expression exp = cb.fromExpression(cb.toExpression(r.get("field")).regexp(REGEXP));
                    // how can I cast the previous expression to a predicate so that I can add it to the list of predicates??
            }

  Metamodel model =  em.getMetamodel();
  Predicate predicate  =  cb.isTrue(new ExpressionImpl(model, ClassConstants.BOOLEAN, exp) );