affected by the order of letters of paresis using Regex

There is no sensible regex which will do that - it is a job for a sort routine.
The easiest way is to convert the string to a character array, sort it, and convert back to a string again.

string s = "bcade";
char[] chars = s.ToCharArray();
Array.Sort(chars);
s = new String(chars);
Console.WriteLine(s);