Oxalis acetosella is the most common type of wood sorrel in much of Europe and parts of Asia. In the UK is it often known as shamrock, although it is not technically a type of clover. It is also sometimes called Alleluia because it commonly blooms between Easter and Pentecost, the time during which Christian responsories include calling Hallelujah.
The common wood sorrel has traditional clover-shaped leaves, and it is hard to tell from clover until it blooms; it has wide, upward-facing bells consisting of five white petals, veined with violet and a burst of yellow in the center. Unlike clover, wood sorrel does not improve the soil, although it is not generally considered a weed in the way that American woodsorrels often are.
Like most woodsorrels, the leaves, stem, and flowers are edible with a tangy sour flavor. While it is sometimes used in salads, most people eat it as children and then largely forget about it. Children are also drawn to these plants because ripe seedpods will burst to hurtle a seed over a meter away, making hikes more exciting. It prefers woodlands and hedgerows, and does best in moist and shaded habitats.
These plants are interesting to amateur naturalists for a few other reasons. The leaves are more active than those of many plants, folding up neatly during the night, during bright sun or heavy rain, and when touched. After the first bloom -- which is earlier than most flowers, making them a draw for pollinators -- a second summer cleistogamous flower will bud but never open, functioning exclusively to self-pollinate.
It should be noted that in North America the most common woodsorrel is Common Yellow Woodsorrel, but the woodsorrel that looks most like Oxalis acetosella, the Oxalis montana, is also often called common wood sorrel.