需要引用strUtils

unit Unit1;

interface

uses
  Windows, StrUtils,Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
var
  s,s1,s2: string;
begin
  s:='0001000200030004';
  s1:= LeftStr(s,4);
  s2:= RightStr(s,4);
  ShowMessage('s1 leftstr'+ s1 + '   '+ ' s2 rightstr' + s2);
end;

结果显示

LEFTSTR('中国1',4)  结果为'中国'
MIDSTR('12314',2,4)  结果为'2314'
一点问题也没有,非常正确.