SitecoreContentSearch.PredicateBuilder.cs

using (var context = ContentSearchManager.GetIndex("sitecore_master_index").CreateSearchContext())  
{
  // Create an initial predicate - use .True<T> since we'll be AND'ing this clause together
  var filterPredicate = PredicateBuilder.True<SearchResultItem>();
  
  // Ensure correct parent
  filterPredicate = filterPredicate.And(x => x.Parent == ID.Parse("TheParentId"));
  
  // Ensure correct path
  filterPredicate = filterPredicate.And(x => x.Path.Contains("/Path/To/SearchResults"));
  
  // Ensure correct naming conventions - use .False<T> since we'll be OR'ing this clause together
  var namingPredicate = PredicateBuilder.False<SearchResultItem>();  
  namingPredicate = namingPredicate.Or(x => x.Name.StartsWith("A")  
                                   .Or(x => x.Name.StartsWith("B")
                                   .Or(x => x.Name.StartsWith("C");
  
  // Append the naming predicate to the overall filter predicate
  filterPredicate = filterPredicate.And(namingPredicate);
  
  // Fetch results from the search index using the predicate
  var results = context.GetQueryable<SearchItemResult().Filter(filterPredicate); 
}