Ever wanted to generate a random string with specific random char on a specific position in the string?

I needed a random string generator that would make me a string with a random char on a specific position in the string that I would specify the expression from.
I.e. I wanted the string to be 6 chars long, the first three chars to contain only A..Z and a..z chars and the last three chars to contain only 0..9 chars.
So I made this function, it's simple and fast.
You pass the parameter for which chars to include in the random state.

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function RandomString(expr: string): string;
{
  (c) 2002 Uros Gaber, PowerCom d.o.o.
  expr values that combine (*nix style)
  1: A..Z
  2: a..z
  4: 0..9
  if you want
  (A..Z, a..z) use 3;
  (A..Z, a..z, 0..9) use 7
  (A..Z, 0..9) use 5
  (a..z, 0..9) use 6
  i.e. RandomString('123'); to generate a 3 letters random string...
}
var
  i: Byte;
  s: string;
  v: Byte;
begin
  Randomize;
  SetLength(Result, Length(expr));
  for i:=1 to Length(expr) do
  begin
    s:='';
    try
      v:=StrToInt(Expr[i]);
    except
      v:=0;
    end;
    if (v-4) >= 0 then
    begin
      s:=s+'0123456789';
      dec(v, 4);
    end;
    if (v-2) >= 0 then
    begin
      s:=s+'abcdefghijklmnopqrstuvwxyz';
      dec(v, 2);
    end;
    if (v-1) >= 0 then
      s:=s+'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
    Result[i]:=s[Random(Length(s)-1)+1];
  end;
end;
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