Delphi programming
filtering characters for names and passwords

download filter demo program
download Delphi-7 project


This article describes how to program character filters,
to allow only a selected set of characters to enter an Edit component.
Below is a form with two Edit boxes, one (nameEdit) for entering a name, a second (pwEdit) for entering passwords

Below the Edit components, the allowed characters are listed.

For the pwEdit, the property passwordChar is set to #149 during design time.
So each character entered here will be displayed as a big dot.

Both Edit components are placed on form1.
To be able to inspect a typed character, the keyPreview property of the form must be set true.

A typed character generates a TForm1.FormKeyPress(Sender: TObject; var Key: Char) event.
If the character key is not part of a predefined set, key is set to #0, character code 0, which is ignored.

Make sure the character #8 (backspace) is also allowed, else a backspace in the Edit box is not possisble.

Below is the complete Delphi-7 unit 
unit Unit1;
interface

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

type
  TForm1 = class(TForm)
    nameEdit: TEdit;
    pwEdit: TEdit;
    Label1: TLabel;
    Label2: TLabel;
    Label3: TLabel;
    Label4: TLabel;
    Label5: TLabel;
    procedure FormKeyPress(Sender: TObject; var Key: Char);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure namefilter(var ch : char);
begin
 if not (ch in ['0'..'9','A'..'Z','a'..'z',#8]) then ch := #0;
end;

procedure pwfilter(var ch : char);
begin
 if not (ch in ['!','#'..'&','*','-','/','0'..'9','A'..'Z','a'..'z',#8]) then
  ch := #0;
end;

procedure TForm1.FormKeyPress(Sender: TObject; var Key: Char);
begin
 if activecontrol = nameEdit then namefilter(key);
 if activecontrol = pwEdit then pwfilter(key);
end;

end.