As your comment says , It has predefined set of values, right. Then try like this
NSstring *str = [filename substringFromIndex:11];
// Convert the str to char[]
Then you should try with the NSScanner
:
NSString *numberString;
NSScanner *scanner = [NSScanner scannerWithString:filename];
NSCharacterSet *numbers = [NSCharacterSet characterSetWithCharactersInString:@"0123456789"];
// Throw away characters before the first number.
[scanner scanUpToCharactersFromSet:numbers intoString:NULL];
// Collect numbers.
[scanner scanCharactersFromSet:numbers intoString:&numberString];
// Result.
int number = [numberString integerValue];
// you can play around with the set of number