Here's how I grab the matching #topics in Threadnote, based on the substring of text that's been typed so far.

gistfile1.m

-(void)showTagTableWithString:(NSString *)partialTagString {
    [self.noteTextView setSpellCheckingType:UITextSpellCheckingTypeNo];
    //If there isn't any string
    if (partialTagString.length == 0) {
        //Display all of the tags, sorted by most recently used.
        NSFetchRequest *hashtagRequest = [[NSFetchRequest alloc] init];
        NSEntityDescription *tagEntityDescription = [NSEntityDescription entityForName:@"Tags" 
                                                                inManagedObjectContext:self.note.managedObjectContext];
        [hashtagRequest setEntity:tagEntityDescription];
        NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"dateLastUsed" 
                                                                         ascending:NO];
        NSArray *sortDescriptors = [NSArray arrayWithObject:sortDescriptor];
        [hashtagRequest setSortDescriptors:sortDescriptors];
        
        tagsToDisplay = (NSMutableArray *)[self.note.managedObjectContext executeFetchRequest:hashtagRequest error:nil];
        
        if ([tagsToDisplay count] >0)
        {
            [self showTagTable];
            
            [tagListTable reloadData];  
        }

    } else {
        //Display all of the tags that match the partial tag string
        NSFetchRequest *hashtagRequest = [[NSFetchRequest alloc] init];
        NSEntityDescription *tagEntityDescription = [NSEntityDescription entityForName:@"Tags" 
                                                                inManagedObjectContext:self.note.managedObjectContext];
        [hashtagRequest setEntity:tagEntityDescription];
        NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"dateLastUsed" 
                                                                         ascending:NO];
        NSArray *sortDescriptors = [NSArray arrayWithObject:sortDescriptor];
        [hashtagRequest setSortDescriptors:sortDescriptors];
        
        NSPredicate *tagPredicate = [NSPredicate predicateWithFormat:@"name contains[c] %@", partialTagString];
        [hashtagRequest setPredicate:tagPredicate];
        
        tagsToDisplay = (NSMutableArray *)[self.note.managedObjectContext executeFetchRequest:hashtagRequest error:nil];
        [self showTagTable];
        [tagListTable reloadData];
        
        //If there are no matching hashtags, then let's hide the tag table.
        if ([tagsToDisplay count] == 0) 
        {
            [self hideTagTable];
            return;
        }
    }
}