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Only() function to check if a string Only has these characters

Function code:

// The Only() function will check a string to see if it only has something.
// Made by Ron. :)
function Only(TheString, OnlyWhat: string; CheckBlank: Boolean): Boolean;
var
  i: integer;
begin
  TheString := Trim(Lowercase(TheString));
  Result := True;
  if (CheckBlank) and (Length(TheString) < 1) then
  begin
    Result := False;
  end else
  begin
    for i := 1 to Length(TheString) do
    begin
      if (pos(TheString[i], OnlyWhat) < 1) then
      begin
        Result := False;
        Break;
      end;
    end;
  end;
end;

Test the function:

program New;

const
  AbcString = 'abcdefg';
  NumString = '8273894';

// The Only() function will check a string to see if it only has something.
// Made by Ron. :)
function Only(TheString, OnlyWhat: string; CheckBlank: Boolean): Boolean;
var
  i: integer;
begin
  TheString := Trim(Lowercase(TheString));
  Result := True;
  if (CheckBlank) and (Length(TheString) < 1) then
  begin
    Result := False;
  end else
  begin
    for i := 1 to Length(TheString) do
    begin
      if (pos(TheString[i], OnlyWhat) < 1) then
      begin
        Result := False;
        Break;
      end;
    end;
  end;
end;

begin
  ClearDebug;
  if(Only(AbcString,'abcdefghijklmnopqrstuvwxyz',True))then
  begin
    WriteLn('"'+AbcString+'" has only letters between A and Z');
  end;
  if(Only(NumString,'1234567890',True))then
  begin
    WriteLn('"'+NumString+'" has only numbers between 0 and 9');
  end;
end.

~Ron