Here's why:
* Regis are late-game encounters: The Regis (Regirock, Regice, Registeel) are legendary Pokemon that are designed to be challenging encounters. They are placed in the game after you've had a chance to build up a strong team and progress through the main storyline.
* Specific requirements: To encounter and catch the Regis, you typically need to complete the Pokemon League and meet certain other criteria, such as collecting specific items or completing other side quests.
So, no, you can't catch the Regis before completing the Pokemon League in any mainline Pokemon game.