如果您不想为这一项任务引入正则表达式库...

If you don't want to bring in a regex library for this one task...

NSString *str = @"aA09";
NSCharacterSet *alphaSet = [NSCharacterSet alphanumericCharacterSet];
BOOL valid = [[str stringByTrimmingCharactersInSet:alphaSet] isEqualToString:@""];