Laduke's right. It's Oxalis. Different people have different feelings about this weed.
I wouldn't care if my tortoise snagged a big now and again, but wouldn't intentionally feed it in large amounts.
Here's a link that talks about it pretty well. Just bare in mind the negative side effects are only if consumed in large amounts regularly...
http://www.tortoiseforum.org/threads/oxalis-or-clover-whats-in-your-garden.7580/
Looks more like Oxalis to me..which is tasty but not necessarily great for torts.