using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        string input = "STEAM_1:1:66138017";

        //string input= "";

        var id = TranslateSteamID(input);

        //Correct result according to https://developer.valvesoftware.com/wiki/SteamID :

        // 76561198092541763

        Console.WriteLine(id);

    }

    public static Int64 TranslateSteamID(string steamID)

    {

        Int64 result = 0;

        var template = new Regex(@"STEAM_(\d):([0-1]):(\d+)");

        var matches = template.Matches(steamID);

        if (matches.Count <= 0) return 0;

        var parts = matches[0].Groups;

        if (parts.Count != 4) return 0;

        Int64 x = Int64.Parse(parts[1].Value) << 24;

        Int64 y = Int64.Parse(parts[2].Value);

        Int64 z = Int64.Parse(parts[3].Value) << 1;

        result =  ((1 + (1 << 20) + x) << 32) | (y + z);        

        return result;

    }

}