How to validate 24 hour time using a DBEdit field

Problem/Question/Abstract:

I want to have the user to enter a valid 24 hour time into a string field using hours and minutes only. How do I set up a validation to make sure the user does not enter something like 25:00 or 23:60 ?

Answer:

In order to prevent invalid entry character by character, you can use an OnKeyPress event handler and the following as an example:

procedure TForm1.Edit1KeyPress(Sender: TObject; var Key: Char);
begin
  if not ValidEditTime(Edit1, Key) then
    Key := #0;
end;

function ValidEditTime(ed: TCustomEdit; sfx: char): boolean;
var
  pfx: string;

  function CheckVal(const s: string; lim1, lim2: integer): boolean;
  var
    v: integer;
  begin
    v := StrToIntDef(s + sfx, lim2);
    if Length(s) = 0 then
      Result := (v < lim1)
    else
      Result := (v < lim2);
  end;

var
  p: integer;
begin
  Result := not (sfx in ['0'..'9', ':']);
  if (not Result) or (sfx <> #8) then
  begin
    pfx := ed.Text;
    if ed.SelLength > 0 then
      Delete(pfx, ed.SelStart + 1, ed.SelLength);
    p := Pos(':', pfx + sfx);
    if p = 0 then
      Result := CheckVal(pfx, 3, 24)
    else
    begin
      Result := (p = 3);
      if Result then
      begin
        Result := (p > Length(pfx));
        if not Result then
          Result := CheckVal(Copy(pfx, p + 1, Length(pfx) - p), 6, 60)
      end;
    end;
  end;
end;

Although the above is quite sophisticated, you will probably need an OnValidate routine as well in order to handle pasting into the control.