Only allow certain characters in a string

Question

I am trying to validate a string, where by it can contain all alphebetical and numerical characters, aswell as the underline ( _ ) symbol.

This is what I tried so far:

var
  S: string;
const
  Allowed = ['A'..'Z', 'a'..'z', '0'..'9', '_'];
begin
  S := 'This_is_my_string_0123456789';

  if Length(S) > 0 then
  begin
    if (Pos(Allowed, S) > 0 then
      ShowMessage('Ok')
    else
      ShowMessage('string contains invalid symbols');
  end;
end;

In Lazarus this errors with:

Error: Incompatible type for arg no. 1: Got "Set Of Char", expected "Variant"

Clearly my use of Pos is all wrong and I am not sure if my approach is even the correct way of going about it or not?

Thanks.

Answer 1

You will have to check every single character of the string, if it's contained in Allowed

e.g.:

var
  S: string;
const
  Allowed = ['A' .. 'Z', 'a' .. 'z', '0' .. '9', '_'];

  Function Valid: Boolean;
  var
    i: Integer;
  begin
    Result := Length(s) > 0;
    i := 1;
    while Result and (i <= Length(S)) do
    begin
      Result := Result AND (S[i] in Allowed);
      inc(i);
    end;
    if  Length(s) = 0 then Result := true;
  end;

begin
  S := 'This_is_my_string_0123456789';
  if Valid then
    ShowMessage('Ok')
  else
    ShowMessage('string contains invalid symbols');
end;