Sometimes, within regular expressions, we want to group things together.
Doing this allows building of larger regular expressions based on smaller
components. The ()'s are used for grouping.
For example, if we want to match any string that contains abc or
def, zero or more times, surrounded by a xx on either side,
we could write the regular expression xx(abc|def)*xx. This
applies the * character to everything that is in the parentheses.
Thus we can match any strings such as xxabcxx, xxabcdefxx,
etc.