Oenothera species? Here is a link that might be useful: Oenothera laciniata

Wow, sounds like a real bad character! Thought it was unusual but guess I need to get rid of it quick, thanks!

See if Raphanus sativus (the cultivated raddish) or Raphanus raphanistrum (wild raddish) fits. That should be the right genus, because of those petals with darker veins.

I find it fairly difficult to tell the difference at this stage, though the seed pods look quite different. The page linked below might help. I have Chelidonium (which has the long seed pods) and am on constant watch for it in spring in the hopes of ripping it out before it goes to seed. There is quite a bit less now than there was when we moved in. Stylophorum is native and gently seeds around here. Chelidonium is a Eurasian import that is quite weedy. Greater Celandine (Chelidonium majus) vs Celandine Poppy (Stylophorum diphyllum)