There are several reasons to use a group in a regular expression:
do(g|or)
will match 'dog' and 'door') In any case, having an empty group is most probably a mistake. Either it is a leftover after refactoring and should be removed, or the actual parentheses were intended and were not escaped.
"foo()" // Noncompliant, will match only 'foo'
"foo\\(\\)" // Matches 'foo()'