SierpinskiTriangle.cs

// Start:
string sierpinskiTriangle = "A";

// Replacement rules:
Func<string, string> transformA = x => x.Replace("A", "B-A-B");
// Auxiliary replacement rule:
Func<string, string> markBs = x => x.Replace("B", "[B]");
Func<string, string> transformB = x => x.Replace("[B]", "A+B+A");

// Recursive stages:
int length = 6;

// Applies the transformation rules:
Enumerable.Range(1, length)
    .ToList()
    .ForEach (k => sierpinskiTriangle =
        transformB(transformA(markBs(sierpinskiTriangle))));

// Performs the replacements for LOGO commands:
sierpinskiTriangle = sierpinskiTriangle
    .Replace("A", "forward 5" + Environment.NewLine)
    .Replace("B", "forward 5" + Environment.NewLine)
    .Replace("+", "left 60" + Environment.NewLine)
    .Replace("-", "right 60" + Environment.NewLine);

// Shows the resultant LOGO commands for drawing a Sierpenski triangle:
sierpinskiTriangle.Dump("LOGO commands for drawing a Sierpinsky Triangle");