using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        const string originalPath = @"file:\\mail\attach\2015_02\random file name";

        var newPath = Regex.Replace(originalPath, @"file:\\{2}(.+)", @"file:\$1");

        Console.WriteLine(newPath);

    }

}