Is it true that std::regex::multiline is not present?

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No, this statement is not accurate. The std::regex::multiline flag is present in the C++ standard library, and it is used to enable multiline mode in regular expressions. This flag is used to indicate that the "^" and "$" metacharacters in the regular expression should match the beginning and end of each line, respectively, instead of only the beginning and end of the entire input string.