Один из способов сделать это - использовать NSScanner . Смотрите этот пример:
NSString *url = nil;
NSString *htmlString = ...
NSScanner *theScanner = [NSScanner scannerWithString:htmlString];
// find start of IMG tag
[theScanner scanUpToString:@"<img" intoString:nil];
if (![theScanner isAtEnd]) {
[theScanner scanUpToString:@"src" intoString:nil];
NSCharacterSet *charset = [NSCharacterSet characterSetWithCharactersInString:@"\"'"];
[theScanner scanUpToCharactersFromSet:charset intoString:nil];
[theScanner scanCharactersFromSet:charset intoString:nil];
[theScanner scanUpToCharactersFromSet:charset intoString:&url];
// "url" now contains the URL of the img
}