I have a weed in my lawn which is prolific where the grass is of low height. It is a very small clover with yellow flowers.
I live in the Sydney, Australia region.
Here's a photo with a ruler to scale:
1 inch = 2.54 centimeters
Gardening & Landscaping Stack Exchange is a question and answer site for gardeners and landscapers. It only takes a minute to sign up.Sign up to join this community
I believe what you have there is Yellow Woodsorrel (Oxalis stricta).
If it isn't, I'm 90 to 95% certain it's in the Oxalis genus, which Yellow Woodsorrel belongs to...
Yellow Wood Sorrel via HerbiGuide (Australia)
Yellow woodsorrel via Midwest Sod Council
Yellow Woodsorrel via Virginia Tech Weed Identification Guide
Where I come from (SA) it's called "Soursob" and as far as I know its binomial name is Oxalis pes-caprae. It is an extremely well known weed throughout the agricultural regions of Australia and some studies identify it as a health risk for grazing livestock. You can use it as a herb apparently, though most people spend countless hours of their lives trying to kill it!
From 891's Talkback Gardening with Jon Lamb and Ashley Walsh podcast, listen to the below show as the Aussie gardening superstars talk about dealing with soursobs (and hopefully similar species in Oxalis genus). Listen at 22:30 and 32:23.