Diamond polyomino
A diamond is a polyomino that is orthogonally directed from all four directions and thus has no outside concavities. They form a rough diamond or lozenge shape, giving them their name. When a diamond has no punctures, is is called convex. Convex polyominoes have a perimeter equal to their boundary box. By definition, they are crescents.
Symbol
Regex
ru(ru)*(lu)*ld(ld)*(rd)*