function valFn(v, ctx) {
            // do not invalidate if empty; use a separate required test
            if (v == null || v === '') return true;
            if (typeof (v) !== 'string') return false;
            try {
                var re = new RegExp(ctx.expression);
            } catch (e) {
                throw new Error('Missing or invalid expression parameter to regExp validator');
            }
            return re.test(v);
        }
Returns a regular expression validator; the expression must be specified
    @example
        // Add validator to a property. Assume em is a preexisting EntityManager.
        var customerType = em.metadataStore.getEntityType("Customer");
        var regionProperty = customerType.getProperty("Region");
        // Validates that the value of Customer.Region is 2 char uppercase alpha.
        regionProperty.validators.push(Validator.regularExpression( {expression: '^[A-Z]{2}$'} );
    @method regularExpression
    @static
    @param context {Object} 
    @param context.expression {String} String form of the regular expression to apply
    @return {Validator} A new Validator