From the Regular expressions article:
In all these cases the word "brace" or "round braces" in fact means brackets: ( ). Usage of the name "brace" is less conventional; moreover, "braces" may be used as a synonym of the "curly brackets" { }, rather than round ones. In the grammar and typography areas the round brackets are also called "parentheses". I would suggest to replace the terms "braces", "round braces" in the Help with "round brackets"....Alternatives are put in round braces, and are separated by a vertical dash.
...Text parts in round braces are taken as subexpressions.
...Here $1 means the text in the first brace, and $2 the text in the second brace.
BTW, the term "square bracket" is used correctly.
See also the Wikipedia article: https://en.wikipedia.org/wiki/Bracket