removing characters

gistfile1.m

    // making a string
    NSMutableString *s = [[NSMutableString alloc] initWithCapacity:256];
    for (char c = 0x20; c < 0x7f; c++) {
        [s appendFormat:@"%c", c];
    }
    [s appendString:@"日本語"];
    NSString *original = [NSString stringWithString:s];
    NSLog(@"original:[%@]", original);
    
    // setting a character set
    NSMutableCharacterSet *workingSet = [[NSCharacterSet alphanumericCharacterSet] mutableCopy];
    [workingSet addCharactersInString:@"-._"];
    
    // removing characters
    NSMutableString *output = [[NSMutableString alloc] initWithCapacity:256];
    NSRange range = NSMakeRange(0, original.length);
    while (true) {
        NSRange r = [original rangeOfCharacterFromSet:workingSet options:NSCaseInsensitiveSearch range:range];
        if (r.location == NSNotFound) {
            break;
        }
        [output appendString:[original substringWithRange:r]];
        range.location = r.location + r.length;
        range.length = original.length - range.location;
    }
    
    NSLog(@"output:  [%@]", output);


    // NSLog
    // original:[ !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~日本語]
    // output:  [-.0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz日本語] <= why?