We don't need parentheses at the beginning and end because there is not prefix or suffix to the alternation. From https://www.regular-expressions.info/alternation.html :

The alternation operator has the lowest precedence of all regex operators. That is, it tells the regex engine to match either everything to the left of the vertical bar, or everything to the right of the vertical bar. If you want to limit the reach of the alternation, you need to use parentheses for grouping