[Karsh]

Можно разбить строку на слова и заменять слова поотдельности, например так:

Код:

function ReplaceStr(S, Srch, Replace: string; Delim: Char; IgnoreCase: boolean): string;
var
  i, l: integer;
  tmp: string;
  ReplaceFlags: TReplaceFlags;
begin
  Result:= '';
  l:= Length(S);
  if l < 1 then exit;
  tmp:= '';
  if IgnoreCase then ReplaceFlags:= [rfIgnoreCase] else ReplaceFlags:= [];
  for i:= 1 to l do
  begin
    tmp:= tmp + s[i];
    if (s[i] = Delim) or (i = l) then
    begin
      if Trim(tmp) = Trim(Srch + Delim) then
        tmp:= StringReplace(tmp, Srch, Replace, ReplaceFlags);
      Result:= Result + tmp;
      tmp:= '';
    end;
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
const
  x1 = 'one two one three oneone';
  x2 = 'one two one three oneone one';
  x3 = 'one,two,one,three,oneone';
  x4 = 'one, two, one, three, oneone';
var
  s: string;
begin
  s:= ReplaceStr(x1, 'one', 'zero', ' ', false);
  s:= s + #13#10 + ReplaceStr(x2, 'one', 'zero', ' ', false);
  s:= s + #13#10 + ReplaceStr(x3, 'one', 'zero', ',', false);
  s:= s + #13#10 + ReplaceStr(x4, 'one', 'zero', ',', false);
  ShowMessage(s);
end;