需求：限制UITextField只能输入中文，并且最大长度为4；
解决方案：直接上干货！
1，先声明一个UITextfield 变量；
@property (nonatomic, strong) UITextField *textField;
2，添加通知；

• (void)viewDidLoad {
  [super viewDidLoad];
  // Do any additional setup after loading the view
  [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(textFiledEditChanged:)name:UITextFieldTextDidChangeNotification object:self.textField];
  }
  3，在监听中，实现过滤非中文字符，并限制字符数量；

• (BOOL)textFieldShouldReturn:(UITextField *)textField{
  [textField resignFirstResponder];

  //过滤非汉字字符
  textField.text = [self filterCharactor:textField.text withRegex:@"[^\u4e00-\u9fa5]"];

  if (textField.text.length >= 4) {
  textField.text = [textField.text substringToIndex:4];

  }
  return NO;
  }

• (void)textFiledEditChanged:(id)notification{

  UITextRange *selectedRange = self.textField.markedTextRange;
  UITextPosition *position = [self.textField positionFromPosition:selectedRange.start offset:0];

  if (!position) { //// 没有高亮选择的字
  //过滤非汉字字符
  self.textField.text = [self filterCharactor:self.textField.text withRegex:@"[^\u4e00-\u9fa5]"];

    if (self.textField.text.length >= 4) {
        self.textField.text = [self.textField.text substringToIndex:4];
    }
  

  }else { //有高亮文字
  //do nothing
  }
  }

//根据正则，过滤特殊字符

• (NSString *)filterCharactor:(NSString *)string withRegex:(NSString *)regexStr{
  NSString *searchText = string;
  NSError *error = NULL;
  NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:regexStr options:NSRegularExpressionCaseInsensitive error:&error];
  NSString *result = [regex stringByReplacingMatchesInString:searchText options:NSMatchingReportCompletion range:NSMakeRange(0, searchText.length) withTemplate:@""];
  return result;
  }
  4，Game over.