How to pass a string variable as a TSysCharSet

Question

Is it possible to pass a string as a TSysCharSet variable?

This does not compile of course:

var
  AValidChars: SysUtils.TSysCharSet;
  AResult: string;
begin
  // Edit1.Text can contain 0..9 or a..z
  AValidChars := SysUtils.TSysCharSet( [Edit1.Text] );
end;

Thanks, Bill

Answer 1

No, it is not possible to simply pass a string as a TSysCharSet.

What you can do however is to create a TSysCharSet which contains all the chars in the string. This code would do this:

var
  AValidChars: SysUtils.TSysCharSet;
  s: AnsiString;
  i: integer;
begin
  // Edit1.Text can contain 0..9 or a..z
  AValidChars := [];
  s := Edit1.Text;
  for i := 1 to Length(s) do
    Include(AValidChars, s[i]);
end;

If you are not using an earlier Delphi version you could also make use of "for ... in" instead of the loop above:

var
  AValidChars: SysUtils.TSysCharSet;
  c: AnsiChar;
begin
  // Edit1.Text can contain 0..9 or a..z
  AValidChars := [];
  for c in Edit1.Text do
    Include(AValidChars, c);
end;

Note that in both code snippets AnsiString / AnsiChar is used, as this technique will not work with WideString or the Unicode string type introduced with Delphi 2009.

Many thanks to Craig Stuntz, Ken White and Rob Kennedy for their very valuable comments, I have edited this answer to address all of their points.

Answer 2

If in your Edit1.Text you have the string:

'0..9'

Then the following code should help you:

var 
  AValidChars: SysUtils.TSysCharSet;
  StartChar, EndChar: char;
  c: char;
begin
  StartChar := Edit1.Text[1]; // some validation should be done
  EndChar := Edit1.Text[4];
  AValidChars := [];
  for c := StartChar to EndChar do
    Include(AValidChars, c);
end;

A Delphi/Pascal parser can be used to validate the input.

Update:

More elaborated function supporting set constructors:

function StrToSysCharSet(const S: string): TSysCharSet;
var
  Elements: TStringList;
  CurrentElement: string;
  StartChar, EndChar: char;
  c: char;
  i: Integer;
  p: Integer;
  function ReadChar: Char;
  begin
    Result := CurrentElement[p];
    Inc(p);
  end;
  function NextIsDotDot: Boolean;
  begin
    Result := '..' = Copy(CurrentElement, p, 2);
  end;
begin
  Elements := TStringList.Create;
  try
    Elements.CommaText := S;
    Result := [];
    for i := 0 to Elements.Count - 1 do
    begin
      CurrentElement := Trim(Elements[i]);
      p := 1;
      StartChar := ReadChar;
      if NextIsDotDot then
      begin
        Inc(p, 2);
        EndChar := ReadChar;
        for c := StartChar to EndChar do
          Include(Result, c);
      end
      else
        Include(Result, StartChar);
    end;
  finally
    Elements.Free;
  end;
end;

It can be used like this:

S := '0..9, a..z';
AValidChars := StrToSysCharSet(S);

or

S := '0..9 and a..z';
AValidChars := StrToSysCharSet(AnsiReplaceText(S, ' and ', ', '));

Adapting to support

S := '''0''..''9'' and ''a''..''z'''

is simple.