unit Unit1;

interface

uses
  SysUtils, WinTypes, WinProcs, Messages, Classes, Graphics, Controls,
  StdCtrls, ExtCtrls, Forms;

type
  TForm1 = class(TForm)
    Memo1: TMemo;
    procedure FormCreate(Sender: TObject);
    procedure Memo1KeyPress(Sender: TObject; var Key: Char);
  public
    MaxCharsPerLine, MaxLines: Integer;
    function MemoLine: Integer;
    function LineLen(r: Integer): Integer;
    function NRows: Integer;
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

function TForm1.NRows: Integer;
begin

  with Memo1 do
    Result := 1 + SendMessage(Handle, EM_LINEFROMCHAR, GetTextLen - 1, 0);
end;

function TForm1.LineLen(r: Integer): Integer;
var
  r1, r2: Integer;
begin

  with Memo1 do
  begin
    r1 := SendMessage(Handle, EM_LINEINDEX, r, 0);
    if (r > NRows - 1) then
      r2 := SendMessage(Handle, EM_LINEINDEX, r + 1, 0) - 2 {-CR/LF}
    else
      r2 := GetTextLen;
  end;
  Result := r2 - r1;
end;

function TForm1.MemoLine: Integer;
begin

  with Memo1 do
    Result := SendMessage(Handle, EM_LINEFROMCHAR, SelStart, 0);
end;

procedure TForm1.FormCreate(Sender: TObject);
begin

  MaxCharsPerLine := 8;
  MaxLines := 4;
end;

procedure TForm1.Memo1KeyPress(Sender: TObject; var Key: Char);
begin

  with Memo1 do
  begin
    case Key of
      ' '..#255: if (LineLen(MemoLine) >= MaxCharsPerLine) then
          Key := #0;
      #10, #13: if (NRows >= MaxLines) then
          Key := #0;
      #8: if (SelStart = SendMessage(Handle, EM_LINEINDEX, MemoLine, 0)) then
          Key := #0;
    end;
  end;
end;

end.