C++ Primer, Fifth Edition

17.3.3. Using Subexpressions

A pattern in a regular expression often contains one or more subexpressions. A subexpression is a part of the pattern that itself has meaning. Regular-expression grammars typically use parentheses to denote subexpressions.

As an example, the pattern that we used to match C++ files (§ 17.3.1, p. 730) used parentheses to group the possible file extensions. Whenever we group alternatives using parentheses, we are also declaring that those alternatives form a subexpression. We can rewrite that expression so that it gives us access to the file name, which is the part of the pattern that precedes the period, as follows:

// r has two subexpressions: the first is the part of the file name before the period// the second is ...