Following method will encode and decode NSString. Characters to leave unescaped (NULL = all escaped sequences are replaced).

- (NSString *)URLEncodedString {
    CFStringRef url = CFURLCreateStringByAddingPercentEscapes(kCFAllocatorDefault, (CFStringRef)self, NULL, CFSTR("!*'();:@&=+$,/?%#[]"), kCFStringEncodingUTF8); // for some reason, releasing this is disasterous
    NSString *result = (NSString *)url;
    [result autorelease];
    return result;
}

- (NSString *)URLDecodedString {
    CFStringRef url = CFURLCreateStringByReplacingPercentEscapesUsingEncoding(kCFAllocatorDefault, (CFStringRef)self, CFSTR(""), kCFStringEncodingUTF8);
    NSString *result = (NSString *)url;
    [result autorelease];
    return result;
}