NSString *newstr = [string stringByReplacingOccurrencesOfString:@"[^0-9]" withString:@"" options:NSRegularExpressionSearch range:NSMakeRange(0, [string length])];

NSString* str = @"//test.test.com/test/1234/";
NSString* newstr = [str lastPathComponent];     //  Returns "1234"

NSString* str = @"//test.test.com/test/1234/";
NSString* lastPart = @"";

if ([str rangeOfString:@"//test.test.com/test"].location != NSNotFound)
{
    lastPart = [str lastPathComponent];
}

NSString *pattern = @"//test.test.com/test/(\d+)/";
            NSRegularExpression *regex = [NSRegularExpression
                                          regularExpressionWithPattern:pattern
                                          options:NSRegularExpressionCaseInsensitive
                                          error:nil];
            NSTextCheckingResult *textCheckingResult = [regex firstMatchInString:string options:0 range:NSMakeRange(0, string.length)];

            NSRange matchRange = [textCheckingResult rangeAtIndex:1];
            NSString *match = [string substringWithRange:matchRange];
            NSLog(@"Found string '%@'", match);