string ReplaceVars(string str)
{
    bool insideQuotes = false;
    Dictionary<string, string> dict = new Dictionary<string, string>();
    Random r = new Random();
    Regex rx = new Regex(@"""|[A-Za-z_][0-9A-Za-z_]*(?!\s*\()(?=([^0-9A-Za-z_]|$))");
    return rx.Replace(str, (MatchEvaluator)delegate(Match m)
    {
        if (m.Value == "\"") {
            insideQuotes = !insideQuotes;
            return m.Value;
        }
        if (insideQuotes)
        {
            return m.Value;
        }
        string v;
        if (!dict.TryGetValue(m.Value, out v))
        {
            v = r.Next().ToString();
            dict[m.Value] = v;
        }
        return v;
    });
}

//-------
string str = "z - min(z123, 456) + (k2) / sqrt(x) * (SomeFunc(\"Sunshine z 34\",f_var) + 45 - t) + z";
string result = ReplaceVars(str);