https://stackoverflow.com/questions/41667525/regex-capturing-indefinitely-repeating-patterns-with-specific-backreferences

example.cs

var input = "Example task section 1 section tomorrow section tagone tagtwo tagthree tagfour";

// regex altered slightly from question
var r = new Regex(@"^([\w ]+) section (\d) section (\w+) section(?: (\w+))+$");
var g = r.Match(input).Groups;
var output = $"{g[1]} !{g[2]} ^{g[3]} " +
    string.Join(" ", g[4].Captures.Cast<Capture>().Select(x => "#" + x));

// output: "Example task !1 ^tomorrow #tagone #tagtwo #tagthree #tagfour"