Instead, you will need to have all of the following:
- Regirock, Regice, Registeel, and their respective Regi-Stones
- TM10 Hidden Power
- A Pokémon that knows Dive
Once you have these items and Pokémon, follow the steps below:
1. Go to the Great Marsh and use Dive.
2. Look for the Regi Statue that corresponds to the Regi that you want to encounter.
3. Approach the statue, and interact with it.
4. Choose the option to Use Regi-Stone.
5. Select the correct Regi-Stone for the statue that you are interacting with.
6. The Regi will then appear.
7. Battle the Regi and capture it.
Regirock - Use the Regirock-Stone
Regice - Use the Regice-Stone
Registeel - Use the Registeel-Stone
If you have all of the necessary items and Pokémon, you will be able to get the Regis in Pokémon Platinum without the need for an Action Replay.