private List<Product2> performSearch(string searchString) {

    String soql = 'Select Id, Name, PriceBook2Id, Product2Id, ProductCode from Pricebookentry where PriceBook2Id =: Qid';

    if(searchString != '' && searchString != null){
         soql = soql +  ' AND Product2.name LIKE '%' + searchString +'%' ';
    }
    if(BrandsearchString != '' && BrandsearchString != null){
         soql = soql +  ' AND Product2.brand1__c LIKE '%' + BrandsearchString +'%' ';
    }
    if(GroupsearchString != '' && GroupsearchString != null){
         soql = soql +  ' AND Product2.Group1__c LIKE '%' + GroupsearchString +'%'  ';
    }

    return database.query(soql);

}