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SendMessage(RichEdit1.Handle, EM_SETEVENTMASK, 0, mask or ENM_LINK); 
  SendMessage(RichEdit1.Handle, EM_AUTOURLDETECT, Integer(True), 0);                    

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// -------------------------------------------------------------------------- //

 Type
   
    TNotifyEventMsg = procedure (Sender: TObject; Msg: TMessage ) of object;

    TMyRichEdit = class (TRichEdit)

     Private

      FOnMouseLeave  : TNotifyEventMsg;
      FOnMouseEnter  : TNotifyEventMsg;

      procedure CMMouseEnter(var Message: TMessage); message CM_MOUSEENTER;
      procedure CMMouseLeave(var Message: TMessage); message CM_MOUSELEAVE;

    Public

      property OnMouseEnter   : TNotifyEventMsg read FOnMouseEnter write FOnMouseEnter;
      property OnMouseLeave   : TNotifyEventMsg read FOnMouseLeave write FOnMouseLeave;

end;

// -------------------------------------------------------------------------- //

 [...]

// -------------------------------------------------------------------------- //

procedure TMyRichEdit.CMMouseEnter(var Message: TMessage);
 begin
  inherited;
  if assigned(FOnMouseEnter) then FOnMouseEnter(self, Message);
end;

// -------------------------------------------------------------------------- //

procedure TMyRichEdit.CMMouseLeave(var Message: TMessage);
 begin
  inherited;
  if assigned(FOnMouseLeave) then FOnMouseLeave(self, Message);
end;

// -------------------------------------------------------------------------- //