[ao] is the same as (a | o)? Regular expressions

I'm studying regular expressions, I've noticed that perr[ao] and perr(a|o) are equivalent.

Here comes my doubt (which surely is a bullshit), is there no difference in this regard? That is, by writing less, could I use perr[ao] ? I suppose that doing this, however short, is neither semantic nor good practice.

What do you think?

I already know that it is different to apply (a | o) to [ao] (for different cases).