unit Unit1;

interface

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

type
  TForm1 = class(TForm)
    RichEdit1: TRichEdit;
    procedure RichEdit1MouseDown(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.RichEdit1MouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
var
    a:integer;
begin
  a:=RichEdit1.Perform(EM_LINELENGTH,RichEdit1.SelStart,0);
  RichEdit1.SelStart:=RichEdit1.Perform(EM_LINEINDEX,RichEdit1.CaretPos.Y,0);
  RichEdit1.SelLength:=a;
end;

end.