Name this Weed

This plant sprouted up in the front lawn. I've searched numerous web sites trying to identify it but cannot find anything close. It has a very unusual flower stem - it is long and ribbed or squared. The blooms are yellow about the size of a dime with four separate petals. The leaves are narrow and about 4 - 6 inches in length. The plant stems have a purplish hew.
Just joined today and this is my first post. I thought I would be able to attach a snapshot of the plant, but how?

I uploaded a snapshot but not sure I did it correctly. I hope someone stumbles across it and will be able to identify it for me.

Your image:

Thanks.
Does anyone know what this plant is?

Finally found it - anglestem primrose (scientific name Ludwigia leptocarpa )
http://swbiodiversity.org/seinet/taxa/index.php?taxon=48395