Mime Encoded-Word parsing in Objective-C

Question

Do OS X or iOS provide an API to parse MIME Encoded-Word? These lovely strings:

=?iso-8859-1?Q?=A1Hola,_se=F1or!?=

Alternatively, is there a known open-source library that does this?

Answer 1

I took @NickolayO.'s answer, added base-64 support with QSStrings and shortened the code by using componentsSeparatedByString: and stringByReplacingOccurrencesOfString:withString:.

I've made the code available on GitHub. Here's a snippet for convenience:

@implementation NSString (MimeEncodedWord)

- (BOOL) isMimeEncodedWord
{
    return [self hasPrefix:@"=?"]  && [self hasSuffix:@"?="];
}

+ (NSString*) stringWithMimeEncodedWord:(NSString*)word
{ // Example: =?iso-8859-1?Q?=A1Hola,_se=F1or!?=
    NSArray *components = [word componentsSeparatedByString:@"?"];
    if (components.count < 5) return nil;

    NSString *charset = [components objectAtIndex:1];
    NSStringEncoding encoding = CFStringConvertEncodingToNSStringEncoding(CFStringConvertIANACharSetNameToEncoding((CFStringRef)charset)); // TODO: What happens if the encoding is invalid?

    NSString *encodingType = [components objectAtIndex:2];
    NSString *encodedText = [components objectAtIndex:3];
    if ([encodingType isEqualToString:@"Q"])
    { // quoted-printable
        encodedText = [encodedText stringByReplacingOccurrencesOfString:@"_" withString:@" "];
        encodedText = [encodedText stringByReplacingOccurrencesOfString:@"=" withString:@"%"];
        NSString *decoded = [encodedText stringByReplacingPercentEscapesUsingEncoding:encoding];
        return decoded;
    } else if ([encodingType isEqualToString:@"B"])
    { // base64
        NSData *data = [QSStrings decodeBase64WithString:encodedText];
        NSString *decoded = [[NSString alloc] initWithData:data encoding:encoding];
        return decoded;
    } else {
        NSLog(@"%@ is not a valid encoding (must be Q or B)", encodingType);
        return nil;
    }    
}

@end