public string MakePlural(string name)
{
    Regex plural1 = new Regex("(?<keep>[^aeiou])y$");
    Regex plural2 = new Regex("(?<keep>[aeiou]y)$");
    Regex plural3 = new Regex("(?<keep>[sxzh])$");
    Regex plural4 = new Regex("(?<keep>[^sxzhy])$");

    if(plural1.IsMatch(name))
        return plural1.Replace(name, "${keep}ies");
    else if(plural2.IsMatch(name))
        return plural2.Replace(name, "${keep}s");
    else if(plural3.IsMatch(name))
        return plural3.Replace(name, "${keep}es");
    else if(plural4.IsMatch(name))
        return plural4.Replace(name, "${keep}s");

    return name;
}

public string MakeSingle(string name)
{
    Regex single1 = new Regex("(?<keep>[^aeiou])ies$");
    Regex single2 = new Regex("(?<keep>[aeiou]y)s$");
    Regex single3 = new Regex("(?<keep>[sxzh])es$");
    Regex single4 = new Regex("(?<keep>[^sxzhy])s$");

    if(single1.IsMatch(name))
        return single1.Replace(name, "${keep}y");
    else if(single2.IsMatch(name))
        return single2.Replace(name, "${keep}");
    else if(single3.IsMatch(name))
        return single3.Replace(name, "${keep}");
    else if(single4.IsMatch(name))
        return single4.Replace(name, "${keep}");

    return name;
}