-(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string{

if (textField ==自己定义的textField) {

        NSUInteger lengthOfString = string.length;  //lengthOfString的值始终为1

        for (NSInteger loopIndex = ; loopIndex < lengthOfString; loopIndex++) {

            unichar character = [string characterAtIndex:loopIndex]; //将输入的值转化为ASCII值（即内部索引值），可以参考ASCII表

           // 48-57;{0,9};65-90;{A..Z};97-122:{a..z}

            if (character < ) return NO; // 48 unichar for 0

            if (character >  && character < ) return NO; //

            if (character >  && character < ) return NO;

            if (character > ) return NO;

        }

        // Check for total length

        NSUInteger proposedNewLength = textField.text.length - range.length + string.length;

        if (proposedNewLength > ) {

            return NO;//限制长度

        }

        return YES;

    }

    return YES;

}