* (...) what is inside the parentheses are a group - the contents of a group can be retrieved after a match has been performed, and can be matched later in the string with the ''\number'' special sequence * (?...) - This is an '''extension notation''' - Extensions usually do not create a new group; (?P<name>...) is the only exception to this rule. Following are the currently supported extensions. * List of supported extensions: (?iLmsux), (?:...), (?P<name>...), (?P=name), (?#...), (?=...), (?!...), (?<=...), (?<!...), (?(id/name)yes-pattern|no-pattern)