Grouping Operators (`(' ... `)' or `\(' ... `\)')
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  A "group", also known as a "subexpression", consists of an
"open-group operator", any number of other operators, and a
"close-group operator".  Regex treats this sequence as a unit, just as
mathematics and programming languages treat a parenthesized expression
as a unit.

  Therefore, using "groups", you can:

   * delimit the argument(s) to an alternation operator (
see Alternation Operator.) or a repetition operator (
see Repetition Operators.).

   * keep track of the indices of the substring that matched a given
     group.  See Using Registers, for a precise explanation.  This
     lets you:

        * use the back-reference operator (*note Back-reference
          Operator::.).

        * use registers (see Using Registers.).

  If the syntax bit `RE_NO_BK_PARENS' is set, then `(' represents the
open-group operator and `)' represents the close-group operator;
otherwise, `\(' and `\)' do.

  If the syntax bit `RE_UNMATCHED_RIGHT_PAREN_ORD' is set and a
close-group operator has no matching open-group operator, then Regex
considers it to match `)'.