参考资料：

http://segmentfault.com/q/1010000002705645/a-1020000002708433

//设置键盘类型
self.textField.keyboardType = UIKeyboardTypeASCIICapable;

define kAlphaNum @"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"

//判断是否是数字，不是的话就输入失败
- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
NSCharacterSet *cs;

cs = [[NSCharacterSet characterSetWithCharactersInString:kAlphaNum] invertedSet];

NSString *filtered = [[string componentsSeparatedByCharactersInSet:cs] componentsJoinedByString:@""]; //按cs分离出数组,数组按@""分离出字符串

BOOL canChange = [string isEqualToString:filtered];

return self.textField.text.length>=5?NO: canChange;

}