> The rest is just shorthands.

If you really want to get technical: ., ?, and + are also shorthands. The formal definition of a regular expression is very minimal on purpose.

`.` is just `(a|b|c|...)` - List of all literal characters

`x?` is just `(x|)` - "x or empty string"

`x+` is just `xx*` - "x followed by zero or more x characters"

Whether this is exactly how they are implemented in any particular regex engine is an implementation detail, but formally there are very few operations which aren't just syntactic sugar.