Dictionary<string,string> map = new Dictionary<string,string>()
{
{"11", "успешно"},
{"22", "успешно2"},
{"33", "успешно3"},
{"44", "успешно4"},
};
string str = "44112233";
Regex regex = new Regex(string.Join("|",map.Keys));
string newStr = regex.Replace(str, m => map[m.Value]);