Interesting. Used characters which aren't allowed rather than ones that are.

Also: You misspelled forbidden :\

I have understood that the "if" statement can be simpler by do not write "{" with a simple expression. And I understood that if "index" increases only in 1, it is better to write "index++" not "index += 1" the same about decrease.

Also, I have made bad return expression because I don't have to return all expression like that "index === 0? true: false;".

I understood that i don't have to write "return element = '0'" better to write "return '0'", and I understood that "if" statement can be replaced by "condition ? return value : else value " statement.

https://docs.python.org/3/library/re.html
I have read it and I know, now you have to read it to know =).