Variant 1:
NSRange rangeStart = [title rangeOfString:@"[" options:NSLiteralSearch];
NSRange rangeEnd = [title rangeOfString:@"]" options:NSLiteralSearch];
substring = [your_string substringWithRange:NSMakeRange(rangeStart.location + 1, rangeEnd.location - rangeStart.location - 1)];
Variant 2:
NSArray *ar = componentsSeparatedByCharactersInSet:[NSCharacterSet characterSetWithCharactersInString:@"[]"];
substring = ar[1];