Oxalis pes-caprae (Bermuda buttercup)?

I agree; it is an awful, awful weed here in Northern California.

Thanks saltcedar!

Susanc, it only lasts until May here in the pheonix area. It adds a small splash of color to the garden.

'...it only lasts until May....' Although the foliage may have died down the bulbils will be sitting in the ground and will increase continually year after year. It is an introduced potential invasive that can take over huge areas. I'd be very wary of it if I were you.