NSStringの全角文字のみ□に変換する

.m

NSMutableString *convertedText = [[NSMutableString alloc] initWithCapacity:[text length]];

for (NSUInteger i = 0; i < [text length]; i++) {
    NSString *character = [text substringWithRange:NSMakeRange(i, 1)];
    if (![character canBeConvertedToEncoding:NSASCIIStringEncoding]) {
        character = @"□";
    }
    [convertedText appendString:character];
}

NSLog(@"%@", convertedText);