I need help with Player UUID (2024.4)

So, I've made a mod with a custom mob, using Geckolib for animations. I want to make it so you can pick up the mob if it's tamed and put it back down. I made an item, and to save data like name, health, and owner, I'm using player-persistent variables. I'm struggling with the saving of the UUID, since I need the numeric form, I can't use the designated block, so instead I use the /data command as a workaround. However, since the output contains some garbage text, I've been trying to use a substring to remove that garbage text, but it keeps crashing my game. Do you know how to fix this, or a workaround for it?