Regex for all legal regular expressions

I'm curious if there is such a regular expression that defines all possible regular expressions. Since RE contains escape characters, it would be difficult to denote such characters in another, say, RE validator, since REs primarily describe sequences of alphanumeric characters.

My question can be similarly interpreted as if there were a finite state machine that could decide whether a candidate with a finite self model is FA or not. This is because we know that FA can be designed in such a way as to exclude a given input string matching a pattern that FA defines or not. So, if we can somehow define all (FA candidates) as strings, we can define FA, which checks if the input is FA or not. However, I do not know how I can prove this statement, I would be happy if you could help me, how can I prove it.

Thank you in advance