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18.3.7 Grouping Operators (( … ) or \( … \))

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 The Alternation Operator (| or \|)) 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 (see The Back-reference Operator (\digit)).
  • 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 ‘)’.