gistfile1.txt

    NSString *pattern = @"[A-Z][a-z]?\\d*|(?<!\\([^)]*)\\(.*\\)\\d+(?![^(]*\\))";
    NSRegularExpression *regex = [NSRegularExpression
                                  regularExpressionWithPattern:pattern
                                  options:0
                                  error:nil];
    NSArray *tests = [[NSArray alloc ] initWithObjects:@"Ca3(PO4)2", @"HCl", @"CaCO3", @"ZnCl2", @"C7H6O2", @"PtCl2(NH3)2", @"Co3(Fe(CN)6)2", nil];
    for (NSString *testString in tests)
    {
        NSLog(@"Testing: %@", testString);
        NSArray *myArray = [regex matchesInString:testString options:0 range:NSMakeRange(0, [testString length])] ;
        NSMutableArray *matches = [NSMutableArray arrayWithCapacity:[myArray count]];

        for (NSTextCheckingResult *match in myArray) {
            NSRange matchRange = [match rangeAtIndex:0];
            [matches addObject:[testString substringWithRange:matchRange]];
            NSLog(@"%@", [matches lastObject]);
        }
    }