It definitely is a pest, it's Oxalis corniculata (creeping wood sorrel). It grows from little tiny tubers in the ground and has explosive seed pods that scatter its seed far and wide. Keep pulling it up - that's the best method of control that I've found. Tried Roundup and found pulling out works better. Don't compost it though - I think it's resilient enough to come through the composting process unharmed.

Weed, weed and weed some more is the only way with this one. I agree with Beattie don't put in your compost send to the municipal composting site.

Or incinerate it on a bonfire with woody garden waste - if you're allowed bonfires where you live.

I pull out every bit I see and still it keeps coming. When I think I introduced it myself because I thought it was so pretty and dainty...16 years and two gardens on, and its still there. Not wishing to discourage you or anything...