Regex Filter aggregation with NEST

gistfile1.txt

.....

//this filters result of the query, then apply aggregations on terms. 
//if you want to filter result of the aggregation you will need reducers
//https://github.com/elasticsearch/elasticsearch/issues/8110
 .Aggregations(a1 => a1
     .Filter("filter", fd => fd.Filter(f => 
         f.Regexp(f3 => f3
             .OnField(d => d.MdFacets)
             .Value(parameters.FacetName + @"\|.*"))
         )
         .Aggregations(a2 => a2
             .Terms("mdFacets", st => st
                 .Field(x => x.MdFacets)
                 .Size(9999)
             )
         )
     )
 );
 
 var filter = result.Aggs.Filter("filter");
 var metadataFacets = filter.Terms("mdFacets").Items........ ;