Перечислять подстроки символов NSString с блоком
NSString *characters = @"abcdefghijklmnopqrstuvwxyz";
[characters enumerateSubstringsInRange:NSMakeRange(0, characters.length) options:NSStringEnumerationByComposedCharacterSequences usingBlock:^(NSString * _Nullable substring, NSRange substringRange, NSRange enclosingRange, BOOL * _Nonnull stop) {
NSLog(@"substring: %@ substringRange: %@, enclosingRange %@", substring, NSStringFromRange(substringRange), NSStringFromRange(enclosingRange));
}];