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function Search_And_Replace(RichEdit: TRichEdit; SearchText, ReplaceText: string): Boolean;
var
  startpos, Position, endpos: integer;
begin
  startpos := 0;
  with RichEdit do
  begin
    endpos := Length(RichEdit.Text);
    Lines.BeginUpdate;
    while FindText(SearchText, startpos, endpos, [stMatchCase])<>-1 do
    begin
      endpos   := Length(RichEdit.Text) - startpos;
      Position := FindText(SearchText, startpos, endpos, [stMatchCase]);
      Inc(startpos, Length(SearchText));
      SetFocus;
      SelStart  := Position;
      SelLength := Length(SearchText);
      RichEdit.clearselection;
      RichEdit.SelAttributes.Style := [fsBold];
      SelText := ReplaceText;
      RichEdit.SelAttributes.Style := [];
    end;
    Lines.EndUpdate;
  end;
end;                    

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procedure TForm1.Button1Click(Sender: TObject);
begin
  Search_And_Replace(Richedit1, 'OldText', 'NewText');
end;