When passing data to a web site, especially via "GET" requests (e.g. calling + stringWithContentsOfURL:encoding:error:) it's important to ensure content is properly escaped. This is also true on the iPhone when you are attempting to use a "mailto:" URL to invoke the native e-mail client from your app.

One obvious way to do this is by calling NSString's "- (NSString *)stringByAddingPercentEscapesUsingEncoding:(NSStringEncoding)encoding" method. While this does a good job, it is not perfect and will miss things like "/", which may make some web services cry. The best way to do this encoding is to use:

CFStringRef CFURLCreateStringByAddingPercentEscapes (
   CFAllocatorRef allocator,
   CFStringRef originalString,
   CFStringRef charactersToLeaveUnescaped,
   CFStringRef legalURLCharactersToBeEscaped,
   CFStringEncoding encoding
);

This method lets you specify what you want encoded and what you want left raw. I threw together a really quick sample command line tool to show you the difference. Here are the salient lies:

NSString *url = @"<b>some HTML content</b> <a href=\"http://theresurgence.com/search/node/bible\">study materials</a>" ;

NSLog(@"Original text: [%@]", url) ;

NSLog(@"NSString percent escapes: [%@]",
      [ url stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding]) ;

NSLog(@"CFURL percent escapes: [%@]",
      CFURLCreateStringByAddingPercentEscapes(kCFAllocatorDefault,
          (CFStringRef)url, NULL, CFSTR("!*'();:@&=+$,/?%#[]"), kCFStringEncodingUTF8));

And, here is the output:

Original text: [<b>some HTML content</b> <a href="http://theresurgence.com/search/node/bible">study materials</a>]
NSString percent escapes: [%3Cb%3Esome%20HTML%20content%3C/b%3E%20%3Ca%20href=%22http://theresurgence.com/search/node/bible%22%3Estudy%20materials%3C/a%3E]
CFURL percent escapes: [%3Cb%3Esome%20HTML%20content%3C%2Fb%3E%20%3Ca%20href%3D%22http%3A%2F%2Ftheresurgence.com%2Fsearch%2Fnode%2Fbible%22%3Estudy%20materials%3C%2Fa%3E]

Big difference without tons of additional code. Plus, you can use CFURLCreateStringByReplacingPercentEscapesUsingEncoding just as easily to unescape any string.