std::basic_regex defines several constants that govern general regex matching syntax.
These constants are duplicated from std::regex_constants:
Grammar option Effect(s)ECMAScript Use the Modified ECMAScript regular expression grammar. basic Use the basic POSIX regular expression grammar (grammar documentation). extended Use the extended POSIX regular expression grammar (grammar documentation). awk Use the regular expression grammar used by the awk utility in POSIX (grammar documentation). grep Use the regular expression grammar used by the grep utility in POSIX. This is effectively the same as the basic option with the addition of newline '\n' as an alternation separator. egrep Use the regular expression grammar used by the grep utility, with the -E option, in POSIX. This is effectively the same as the extended option with the addition of newline '\n' as an alternation separator in addition to '|'. Grammar variation Effect(s) icase Character matching should be performed without regard to case. nosubs When performing matches, all marked sub-expressions (expr) are treated as non-marking sub-expressions (?:expr). No matches are stored in the supplied std::regex_match structure and mark_count() is zero. optimize Instructs the regular expression engine to make matching faster, with the potential cost of making construction slower. For example, this might mean converting a non-deterministic FSA to a deterministic FSA. collate Character ranges of the form "[a-b]" will be locale sensitive. multiline (C++17) Specifies that ^ shall match the beginning of a line and $ shall match the end of a line, if the ECMAScript engine is selected.
At most one grammar option can be chosen out of ECMAScript, basic, extended, awk, grep, egrep. If no grammar is chosen, ECMAScript is assumed to be selected. The other options serve as variations, such that std::regex("meow", std::regex::icase) is equivalent to std::regex("meow", std::regex::ECMAScript|std::regex::icase).
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